1989 BMW 3 SERIES ECU

8Remove the hold-down nut or bolt and
clamp.
9Remove the distributor. Note:Do not rotate
the engine with the distributor out.
Refitting
10Before refitting the distributor, make
certain No 1 piston is still at TDC on the
compression stroke.
11Insert the distributor into the engine, with
the adjusting clamp centred over the hold-
down hole. Make allowance for the gear to
turn as the distributor is inserted.
12Refit the hold-down nut or bolt. The marks
previously made on the distributor housing,
and on the rotor and engine, should line up
before the nut or bolt is tightened.
13Refit the distributor cap.
14Connect the wiring for the distributor.
15Reconnect the spark plug HT leads.
16Reconnect the vacuum hoses as
previously marked.
17Check the ignition timing (see Section 7).
Motronic system
Removal
18Remove the cover from the distributor
(see illustration)and remove the distributor
cap (see Chapter 1).
19Using a small Allen key, remove the three
screws from the rotor (see illustration).
20Remove the rotor.
Refitting
21Refitting is the reverse of removal.
9 Ignition coil -
check and renewal
2
Caution: Do not earth the coil, as
the coil and/or impulse generator
could be damaged.
Note:On models equipped with the Motronic
system, a faulty ECU can cause the ignition
coil to become damaged. Be sure to test the
ignition coil if the engine will not start and an
ECU fault is suspected.
1Mark the wires and terminals for position,
then remove the primary circuit wires and the
HT lead from the coil.2Remove the coil assembly from its
mounting, then clean the outer casing and
check it for cracks and other damage.
3Inspect the coil primary terminals and the
coil tower terminal for corrosion. Clean them
with a wire brush if any corrosion is found.
4Check the coil primary resistance by
attaching an ohmmeter to the primary
terminals (see illustrations). Compare the
measured resistance to the Specifications
listed in this Chapter.
5Check the coil secondary resistance by
connecting one of the ohmmeter leads to one
of the primary terminals, and the other
ohmmeter lead to the coil high-tension
terminal (see illustrations). On TCI systems,
Engine electrical systems 5•5
8.19 Remove the rotor screws (arrowed)
with an Allen key, and pull the rotor off the
shaft8.18 Remove the three bolts (arrowed)
from the distributor cap8.7 Mark the relationship of the rotor to
the distributor housing (arrowed)
9.5b Using an ohmmeter, measure the
secondary resistance of the coil (later
Motronic system)9.5a Using an ohmmeter, measure the
secondary resistance of the coil (TCI
system)
9.4b Some Motronic systems use a
different type of coil. First, remove the coil
cover and . . .9.4a Using an ohmmeter, measure the
resistance between the primary terminals
of the ignition coil (TCI system shown)
9.4c . . . using an ohmmeter, measure the
resistance between the primary terminals
of the coil
5

and down - you should feel a slight drag on
the feeler gauge as it is moved if the gap is
correct. The gap must be as given in this
Chapter’s Specifications.
3To adjust the gap, it is necessary to remove
the impulse generator and the baseplate
assembly from the distributor (see illus-
tration 10.1).
4Follow paragraphs 17 to 24 in Section 10
and loosen the screws that retain the impulse
generator to the baseplate assembly.
5Carefully insert the feeler gauge and tighten
the screws.
6Refit the assembly back into the distributor
and recheck the adjustment.12 Ignition sensors (Motronic
system)- check and renewal
2
Note:Some models are equipped with a TDC
sensor mounted on the front of the engine.
This sensor is strictly for the BMW service test
unit, and is not part of the Motronic ignition
system.
Speed and position sensors
Check
1Locate the two electrical connectors for the
sensors (see illustrations). The grey
connector is for the position sensor, and the
white connector is for the speed sensor.
2Using an ohmmeter, check the resistance
between terminal 1 (yellow wire) and terminal
2 (black wire) on the sensor side of each
connector. The resistance should be 860 to
1,060 ohms.
3Also check the resistance between terminal
3 and either terminal 1 or terminal 2. The
resistance should be approximately 100 000
ohms.
4If the reading(s) are incorrect, renew the
sensor(s).
Renewal
5Remove the sensor mounting screw(s),using an Allen key where necessary, and pull
the sensor(s) from the sockets. Disconnect
the wiring from one sensor at a time - be sure
the connectors are not interchanged when
fitting new sensors. The bellhousing is marked
with a B for the position sensor (grey
connector) and D for the speed sensor (black
connector) (see illustration). Note: It is a
good idea to check the condition of the raised
pin on the flywheel while the sensors are out
of the sockets. Turn the engine by hand as
necessary to bring the pin into view.
6Tighten the sensor mounting screw(s)
securely, but be careful not to overtighten.
Pulse sensor (later models)
Check
7Locate the two electrical connectors for the
sensor (see illustrations). Disconnect the
electrical connector from the front.
8Using an ohmmeter, check resistance
between terminal 1 (yellow wire) and terminal
2 (black wire) on the sensor side of each
connector (see illustration). The resistance
should be 500 to 600 ohms.
9If the reading is incorrect, renew the sensor.
Renewal
10Remove the pulse sensor mounting bolt
using a 5 mm hex spanner (see illustration).
5•8 Engine electrical systems
12.10 The pulse sensor itself (arrowed) is
located on the timing belt cover, to one
side of the pulley (later models)12.8 The resistance of the pulse sensor
should be 500 to 600 ohms (later models)12.7b On later 5-Series models, the pulse
sensor connector (arrowed) is located next
to the valve cover
12.7a On later 3-Series models, the pulse
sensor connector (arrowed) is located next
to the 20-pin diagnostic connector
12.5 Location of the position sensor (B) and
speed sensor (D) on the bellhousing on all
Motronic systems – do not interchange the
sensors, or the engine will not start12.1b Location of the position sensor
(grey connector) (A) and the speed sensor
(black connector) (B) on Motronic systems
(later models)12.1a Location of the position sensor (grey
connector) (A) and the speed sensor (black
connector) (B) on Motronic systems (early
models)

Withdraw the sensor from its bracket and
remove it.
11When fitting the new sensor, use a brass
feeler gauge to position the tip of the sensor
the correct distance from the pulse wheel
(see illustration).
12Tighten the mounting bolt, but be careful
not to overtighten it.
13 Charging system- general
information and precautions
There are two different types of alternator
fitted on these models; Bosch and Motorola.
Also, there are three different amperage
ratings available; 65A, 80A or 90A. A stamped
serial number on the rear of the alternator will
identify the type and amperage rating.
Perform the charging system checks (see
Section 14) to diagnose any problems with the
alternator.
The voltage regulator and the alternator
brushes are mounted as a single assembly.
On Bosch alternators, this unit can be
removed from the alternator (see Section 16)
and the components serviced individually.
The alternator on all models is mounted on
the left front of the engine, and utilises a V-
belt and pulley drive system. Drivebelt tension
and battery servicing are the two primary
maintenance requirements for these systems.
See Chapter 1 for the procedures regarding
engine drivebelt checking and battery
servicing.
The ignition/no-charge warning light should
come on when the ignition key is turned to
Start, then go off immediately the engine
starts. If it remains on, there is a malfunction
in the charging system (see Section 14). Some
vehicles are also equipped with a voltmeter. If
the voltmeter indicates abnormally high or low
voltage, check the charging system (see
Section 14). Note:On models up to 1986, a
blown ignition/no-charge warning light will
prevent the alternator from charging. After
1987, a resistor is wired in parallel with the
warning light in order to allow current tobypass the light in the event of a broken circuit
(blown warning light).
Precautions
Be very careful when making electrical
circuit connections to the alternator, and note
the following:
a) When reconnecting wires to the alternator
from the battery, be sure to note the
polarity.
b) Before using arc-welding equipment to
repair any part of the vehicle, disconnect
the wires from the battery terminals and
from the alternator.
Caution: If the radio in your
vehicle is equipped with an anti-
theft system, make sure you
have the correct activation code
before disconnecting the battery. Refer to
the information on page 0-7 at the front of
this manual before detaching the cable.
Note: If, after connecting the battery, the
wrong language appears on the instrument
panel display, refer to page 0-7 for the
language resetting procedure.
c) Never start the engine with a battery
charger connected. Always disconnect
both battery cables before using a battery
charger.
d) Never disconnect cables from the battery
or from the alternator while the engine is
running.
e) The alternator is turned by an engine
drivebelt. Serious injury could result if
your hands, hair or clothes become
entangled in the belt with the engine
running.
f) Because the alternator is connected
directly to the battery, take care not to
short out the main terminal to earth.
g) Wrap a plastic bag over the alternator,
and secure it with rubber bands, before
steam-cleaning the engine.
14 Charging system- check
3
1If a malfunction occurs in the charging
circuit, don’t automatically assume that the
alternator is causing the problem. First check
the following items:
a) Check the drivebelt tension and condition
(see Chapter 1). Renew the drivebelt if it’s
worn or deteriorated.
b) Make sure the alternator mounting and
adjustment bolts are tight.
c) Inspect the alternator wiring harness and
the connectors at the alternator and
voltage regulator. They must be in good
condition and tight.
d) Check the fuses.
e) Start the engine and check the alternator
for abnormal noises (a shrieking or
squealing sound indicates a worn bearing,
but could also be due to a slipping
drivebelt - see a) above).f) Check the specific gravity of the battery
electrolyte. If it’s low, charge the battery
(doesn’t apply to maintenance-free
batteries).
g) Make sure the battery is fully-charged
(one bad cell in a battery can cause
overcharging by the alternator).
h) Disconnect the battery cables (negative
first, then positive). Inspect the battery
posts and the cable clamps for corrosion.
Clean them thoroughly if necessary (see
Chapter 1).
Caution: If the radio in your
vehicle is equipped with an anti-
theft system, make sure you
have the correct activation code
before disconnecting the battery. Refer to
the information on page 0-7 at the front of
this manual before detaching the cable.
Note: If, after connecting the battery, the
wrong language appears on the instrument
panel display, refer to page 0-7 for the
language resetting procedure.
2With the ignition off, connect a 12 volt test
light between the battery negative post and
the disconnected negative cable clamp. If the
test light does not come on, refit the cable
and proceed to paragraph 4. If the test light
comes on, there is a short (drain) in the
electrical system of the vehicle. The short
must be repaired before the charging system
can be checked. Note: Accessories which are
always on (such as the clock or the radio
station memory) must be disconnected before
performing this check.
3Disconnect the alternator wiring harness. If
the test light now goes out, the alternator is
faulty. If the light stays on, remove each fuse
in turn until the light goes out (this will tell you
which component is shorting out).
4Using a voltmeter, check the battery
voltage with the engine off. It should be
approximately 12 volts.
5Start the engine and check the battery
voltage again. It should now be approximately
14 to 15 volts.
6Turn on the headlights. The voltage should
drop, and then come back up, if the charging
system is working properly.
7If the voltage reading is more than the
specified charging voltage, renew the voltage
regulator (refer to Section 16). If the voltage is
less, the alternator diode(s), stator or rotor
may be faulty, or the voltage regulator may be
malfunctioning.
8If there is no short-circuit causing battery
drain but the battery is constantly
discharging, then either the battery itself is
defective, the alternator drivebelt is loose (see
Chapter 1), the alternator brushes are worn,
dirty or disconnected (see Section 17), the
voltage regulator is malfunctioning (see
Section 16) or the diodes, stator coil or rotor
coil are defective. Repairing or renewing the
diodes, stator coil or rotor coil is beyond the
scope of the home mechanic. Either renew
Engine electrical systems 5•9
12.11 The sensor tip should be set at 1.0 ±
0.3 mm from the pulse wheel
5

Refer to the information on page 0-7 at the
front of this manual before detaching the
cable.
Note: If, after connecting the battery, the
wrong language appears on the instrument
panel display, refer to page 0-7 for the
language resetting procedure.
1Disconnect the battery negative cable.
Bosch alternator
2Remove the voltage regulator from the back
of the alternator (see Section 16).
3Measure the length of the brushes (see
illustration). They should not be less than
6.0 mm. If any are worn past this point, renew
them all as a set. 4Also check for excessively worn slip rings
(see illustration 16.5).
5The brushes are retained either by set
screws or by solder. If you are not skilled at
soldering, it may be best to have an auto
electrician fit the new brushes. Note: Be
careful not to apply heat to the solder joint for
more than 5 seconds. If necessary, use a heat
sink to capture the excess heat. This can be
accomplished by clamping a pair of needle-
nose pliers next to the solder joint.
6On the screw type, hold the assembly in
place and refit the screws. Tighten them
evenly, a little at a time, so the holder isn’t
distorted.
7Refit the regulator assembly to the
alternator.8Reconnect the battery negative cable.
Motorola alternator
9Remove the alternator.
10The brushes are mounted under the
regulator on the rear of the alternator (see
illustration 16.7).
11Remove the mounting screws and
insulating washers, and separate the voltage
regulator and brush holder from the brush end
housing.
12Measure the length of the brushes (see
illustration 17.3). If any brush is less then
6.0 mm long, renew them all as a set.
13Make sure the brushes move smoothly in
the holder.
14Refit the brush holder/regulator. Tighten
the screws securely. Make sure the brushes
aren’t earthed.
15Refitting is the reverse of removal.
18 Starting system- general
information and precautions
The sole function of the starting system is
to turn over the engine quickly enough to
allow it to start.
The starting system consists of the battery,
the starter motor, the starter solenoid, the
ignition switch, and the wires connecting
them. The solenoid is mounted directly on the
starter motor. The starter/solenoid motor
assembly is fitted on the lower part of the
engine, next to the transmission bellhousing.
When the ignition key is turned to the Start
position, the starter solenoid is actuated
through the starter control circuit. The starter
solenoid then connects the battery to the
starter, and moves the starter pinion into
mesh with the flywheel ring gear. The battery
supplies the electrical energy to the starter
motor, which does the actual work of cranking
the engine.
The starter motor on some manual
transmission vehicles can only be operated
when the clutch pedal is depressed. On a
vehicle equipped with automatic
transmission, the starter can only be operated
Engine electrical systems 5•11
17.3 Check the brush length in the normal
rest position (spring uncoiled)
5
16.7 Exploded view of the Motorola alternator

when the transmission selector lever is in Park
or Neutral.
Always detach the battery negative cable
before working on the starting system.
19 Starter motor-
in-vehicle check
2
Note:Before diagnosing starter problems,
make sure the battery is fully charged.
1If the starter motor does not turn at all when
the switch is operated, make sure that the
gear lever is in Neutral or Park (automatic
transmission) or, where applicable, that the
clutch pedal is depressed (manual
transmission).
2Make sure that the battery is charged, and
that all cables, both at the battery and starter
solenoid terminals, are clean and secure.
3If the starter motor spins but the engine is
not cranking, the overrun clutch in the starter
motor is slipping, and the starter motor must
be renewed.
4If, when the switch is actuated, the starter
motor does not operate at all but the solenoid
clicks, then the problem lies either in the
battery, the main solenoid contacts, or the
starter motor itself (or the engine is seized).
5If the solenoid plunger cannot be heard
when the switch is actuated, the battery is
faulty, the switch is defective, the fusible link
is burned-out (the circuit is open), or the
solenoid itself is defective.
6To check the solenoid, connect a jumper
lead between the battery (+) and the ignition
switch wire terminal (the small terminal) on the
solenoid. If the starter motor now operates,
the solenoid is OK and the problem is in the
ignition switch, starter inhibitor switch
(automatic transmission models), clutch
switch (some manual transmission models), or
the wiring.
7If the starter motor still does not operate,
remove the starter/solenoid assembly for
dismantling, testing and repair.
8If the starter motor cranks the engine at an
abnormally-slow speed, first make sure that
the battery is charged, and that all terminal
connections are tight. If the engine is partially-
seized, or has the wrong viscosity oil in it, it
will crank slowly.
9Run the engine until normal operating
temperature is reached, then disconnect the
coil HT lead from the distributor cap and earth
it on the engine.
10Connect a voltmeter positive lead to the
battery positive post, and connect the
negative lead to the negative post.11Crank the engine, and take the voltmeter
readings as soon as a steady figure is
indicated. Do not allow the starter motor to
turn for more than 10 seconds at a time. A
reading of 9 volts or more, with the starter
motor turning at normal cranking speed, is
normal. If the reading is 9 volts or more but
the cranking speed is slow, the solenoid
contacts are burned, there is a bad
connection, or the starter motor itself is faulty.
If the reading is less than 9 volts and the
cranking speed is slow, the starter motor is
faulty or the battery is responsible (defective
or discharged).
20 Starter motor-
removal and refitting
1
Note:If the starter motor is defective, it should
be renewed, or taken to an auto electrical
specialist for repair. Overhaul of the starter
motor is unlikely to be a practical proposition
for the home mechanic, even if spare parts are
available. However, the solenoid can be
renewed separately (see Section 21).
Removal
Caution: If the radio in your
vehicle is equipped with an anti-
theft system, make sure you
have the correct activation code
before disconnecting the battery. Refer to
the information on page 0-7 at the front of
this manual before detaching the cable.
Note: If, after connecting the battery, the
wrong language appears on the instrument
panel display, refer to page 0-7 for the
language resetting procedure.
1Detach the battery negative cable.
2Raise the vehicle and support it securely on
axle stands.
3Clearly label the wires from the terminals onthe starter motor and solenoid, then
disconnect them. Note: On some models, it
may be necessary to remove the air cleaner
(see Chapter 4), coolant expansion tank (see
Chapter 3) and the heater hoses to gain
access to the top of the starter. Carefully label
any hoses or components that need to be
removed from the engine compartment, to
avoid confusion when reassembling.
4Unscrew the mounting bolts and detach the
starter (see illustrations).
Refitting
5Refitting is the reverse of removal.
21 Starter solenoid-
removal and refitting
1
Removal
Caution: If the radio in your
vehicle is equipped with an anti-
theft system, make sure you
have the correct activation code
before disconnecting the battery. Refer to
the information on page 0-7 at the front of
this manual before detaching the cable.
Note: If, after connecting the battery, the
wrong language appears on the instrument
panel display, refer to page 0-7 for the
language resetting procedure.
1Disconnect the battery negative cable.
2Remove the starter motor (see Section 20).
3Disconnect the cable from the solenoid to
the starter motor terminal.
4Remove the screws which secure the
solenoid to the starter motor.
5Detach the solenoid from the starter body.
6Remove the plunger and plunger spring.
Refitting
7Refitting is the reverse of removal.
5•12 Engine electrical systems
20.4b Withdrawing the starter motor from
the M40 engine20.4a Working under the vehicle, remove
the starter lower mounting bolt and nut
(arrowed)

6
Chapter 6
Engine management and emission control systems
Catalytic converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Electronic Control Unit (ECU) - removal and refitting . . . . . . . . . . . . 3
Evaporative emissions control (EVAP) system . . . . . . . . . . . . . . . . . 6
Evaporative emissions control system inspection . . . See Chapter 1
General information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Information sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Motronic engine management system self-diagnosis -
general information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Positive crankcase ventilation (PCV) system . . . . . . . . . . . . . . . . . . 5
6•1
Easy,suitable for
novice with little
experienceFairly easy,suitable
for beginner with
some experienceFairly difficult,
suitable for competent
DIY mechanic
Difficult,suitable for
experienced DIY
mechanicVery difficult,
suitable for expert
DIY or professional
Degrees of difficulty Contents
1 General information
To prevent pollution of the atmosphere
from incomplete combustion or evaporation
of the fuel, and to maintain good driveability
and fuel economy, a number of emission
control systems are used on these vehicles.
Not all of these systems are fitted to all
models, but they include the following:
Catalytic converter
Evaporative emission control (EVAP) system
Positive crankcase ventilation (PCV) system
Electronic engine management
The Sections in this Chapter include
general descriptions and checking
procedures within the scope of the home
mechanic, as well as component renewal
procedures (when possible) for each of the
systems listed above.
Before assuming that an emissions control
system is malfunctioning, check the fuel and
ignition systems carefully. The diagnosis of
some emission control devices requires
specialised tools, equipment and training. If
checking and servicing become too difficult,
or if a procedure is beyond your ability,
consult a dealer service department or other
specialist.This doesn’t mean, however, that emission
control systems are particularly difficult to
maintain and repair. You can quickly and
easily perform many checks, and do most of
the regular maintenance at home with
common tune-up and hand tools.
Pay close attention to any special
precautions outlined in this Chapter. It should
be noted that the illustrations of the various
systems may not exactly match the system
fitted on your vehicle because of
changes made by the manufacturer during
production.
2 Motronic engine management
system self-diagnosis-
general information
The Motronic engine management system
control unit (computer) has a built-in self-
diagnosis system, which detects malfunctions
in the system sensors and stores them as
fault codes in its memory. It is not possible
without dedicated test equipment to extract
these fault codes from the control unit.
However, the procedures given in Chapters 4
and 5 may be used to check individual
components and sensors of the Motronic
system. If this fails to pinpoint a fault, then the
vehicle should be taken to a BMW dealer, who
will have the necessary diagnostic
equipment to call up the fault codes from the
control unit. You will then have the
option to repair the fault yourself, or
alternatively have the fault repaired by the
BMW dealer.
3 Electronic control unit (ECU)
- removal and refitting
2
Removal
1The Electronic Control Unit (ECU) is located
either inside the passenger compartment
under the right-hand side of the facia panel on
3-Series models, or in the engine
compartment on the right-hand side on 5-
Series models (see Chapter 4).
2Disconnect the battery negative cable.
Caution: If the radio in your
vehicle is equipped with an anti-
theft system, make sure you
have the correct activation code
before disconnecting the battery. Refer to
the information on page 0-7 at the front of
this manual before detaching the cable.
Note: If, after connecting the battery, the
wrong language appears on the instrument
panel display, refer to page 0-7 for the
language resetting procedure.
3First remove the access cover on models
with the ECU on the right-hand side of the
engine compartment (see Chapter 4).
4If the ECU is located inside the vehicle,
remove the access cover on the right-hand side.
5Unplug the electrical connectors from the
ECU.
6Remove the retaining bolts from the ECU
bracket.
7Carefully remove the ECU. Note: Avoid static
electricity damage to the ECU by wearing rubber
gloves, and do not touch the connector pins.
Refitting
8Refitting is a reversal of removal.
The most frequent cause of
emission system problems is
simply a leaking vacuum hose
or loose wire, so always
check the hose and wiring connections
first.

4 Information sensors
2
Note:Refer to Chapters 4 and 5 for additional
information on the location and diagnosis of
the information sensors that are not covered in
this Section.
Coolant temperature sensor
General description
1The coolant temperature sensor (see
illustration)is a thermistor (a resistor which
varies its resistance value in accordance with
temperature changes). The change in the
resistance value regulates the amount of
voltage that can pass through the sensor. At
low temperatures, the sensor’s resistance is
high. As the sensor temperature increases, its
resistance will decrease. Any failure in this
sensor circuit will in most cases be due to a
loose or shorted-out wire; if no wiring
problems are evident, check the sensor as
described below.
Check
2To check the sensor, first check its
resistance (see illustration)when it is
completely cold (typically 2100 to 2900 ohms).
Next, start the engine and warm it up until it
reaches operating temperature. The resistance
should be lower (typically 270 to 400 ohms).
Note: If restricted access to the coolant
temperature sensor makes it difficult to attach
electrical probes to the terminals, remove the
sensor as described below, and perform the
tests in a container of heated water to simulate
the conditions.
Warning: Wait until the engine is
completely cool before beginning
this procedure.
Renewal
3To remove the sensor, depress the spring
lock, unplug the electrical connector, then
carefully unscrew the sensor. Be prepared for
some coolant spillage; to reduce this, have
the new sensor ready for fitting as quickly as
possible.Caution: Handle the coolant
sensor with care. Damage to this
sensor will affect the operation of
the entire fuel injection system.
Note: It may be necessary to drain a small
amount of coolant from the radiator before
removing the sensor.
4Before the sensor is fitted, ensure its
threads are clean, and apply a little sealant to
them.
5Refitting is the reverse of removal.
Oxygen sensor
General description
Note:Oxygen sensors are normally only fitted
to those vehicles equipped with a catalytic
converter. Most oxygen sensors are located in
the exhaust pipe, downstream from the
exhaust manifold. On 535 models, the oxygen
sensor is mounted in the catalytic converter.
The sensor’s electrical connector is located
near the bulkhead (left side) for easy access.
6The oxygen sensor, which is located in the
exhaust system (see illustration), monitors
the oxygen content of the exhaust gas. The
oxygen content in the exhaust reacts with the
oxygen sensor, to produce a voltage output
which varies from 0.1 volts (high oxygen, lean
mixture) to 0.9 volts (low oxygen, rich
mixture). The ECU constantly monitors this
variable voltage output to determine the ratio
of oxygen to fuel in the mixture. The ECU
alters the air/fuel mixture ratio by controlling
the pulse width (open time) of the fuel
injectors. A mixture ratio of 14.7 parts air to 1
part fuel is the ideal mixture ratio for
minimising exhaust emissions, thus allowing
the catalytic converter to operate at maximum
efficiency. It is this ratio of 14.7 to 1 which the
ECU and the oxygen sensor attempt to
maintain at all times.
7The oxygen sensor produces no voltage
when it is below its normal operating
temperature of about 320º C. During this initial
period before warm-up, the ECU operates in
“open-loop” mode (ie without the information
from the sensor).
8If the engine reaches normal operating
temperature and/or has been running for two
or more minutes, and if the oxygen sensor is
producing a steady signal voltage below 0.45 volts at 1500 rpm or greater, the ECU
fault code memory will be activated.
9When there is a problem with the oxygen
sensor or its circuit, the ECU operates in the
“open-loop” mode - that is, it controls fuel
delivery in accordance with a programmed
default value instead of with feedback
information from the oxygen sensor.
10The proper operation of the oxygen
sensor depends on four conditions:
a) Electrical - The low voltages generated by
the sensor depend upon good, clean
connections, which should be checked
whenever a malfunction of the sensor is
suspected or indicated.
b) Outside air supply - The sensor is
designed to allow air circulation to the
internal portion of the sensor. Whenever
the sensor is disturbed, make sure the air
passages are not restricted.
c) Proper operating temperature - The ECU
will not react to the sensor signal until the
sensor reaches approximately 320º C.
This factor must be taken into
consideration when evaluating the
performance of the sensor.
d) Unleaded fuel - The use of unleaded fuel
is essential for proper operation of the
sensor. Make sure the fuel you are using
is of this type.
11In addition to observing the above
conditions, special care must be taken
whenever the sensor is serviced.
a) The oxygen sensor has a permanently-
attached pigtail and electrical connector,
which should not be removed from the
sensor. Damage or removal of the pigtail
or electrical connector can adversely
affect operation of the sensor.
b) Grease, dirt and other contaminants
should be kept away from the electrical
connector and the louvered end of the
sensor.
c) Do not use cleaning solvents of any kind
on the oxygen sensor.
d) Do not drop or roughly handle the sensor.
e) The silicone boot must be fitted in the
correct position, to prevent the boot from
being melted and to allow the sensor to
operate properly.
6•2 Engine management and emission control systems
4.6 The oxygen sensor (arrowed) is usually
located in the exhaust pipe, downstream
from the exhaust manifold4.2 Check the resistance of the coolant
temperature sensor at different
temperatures4.1 The coolant temperature sensor
(arrowed) is usually located next to the
temperature sender unit, near the fuel
pressure regulator

Check
12Warm up the engine, and let it run at idle.
Disconnect the oxygen sensor electrical
connector, and connect the positive probe of
a voltmeter to the oxygen sensor output
connector terminal (refer to the following
table) and the negative probe to earth (see
illustrations).
Note:Most oxygen sensor electrical
connectors are located at the rear of the
engine, near the bulkhead. Look for a large
rubber boot attached to a thick wire harness.
On early 535i models, the connector for the
oxygen sensor heater circuit is under the
vehicle. Look for a small protective cover.
These models should have the updated
oxygen sensor fitted, to make access similar
to other models. Consult your dealer service
department for additional information.
13Increase and then decrease the engine
speed, and monitor the voltage.
14When the speed is increased, the voltage
should increase to 0.5 to 1.0 volts. When the
speed is decreased, the voltage should fall to
about 0 to 0.4 volts.
15Also where applicable, inspect the oxygen
sensor heater (models with multi-wire
sensors). With the ignition on, disconnect the
oxygen sensor electrical connector, and
connect a voltmeter across the terminals
designated in the chart (see below). There
should be battery voltage (approximately
12 volts).
16If the reading is not correct, check the
oxygen sensor heater relay (see Chapter 12).
If the information is not available, check the
owner’s handbook for the exact location of
the oxygen sensor heater relay. The relay
should receive battery voltage.
17If the oxygen sensor fails any of these
tests, renew it.
Renewal
Note: Because it is fitted in the exhaust
manifold, converter or pipe, which contracts
when cool, the oxygen sensor may be very
difficult to loosen when the engine is cold.
Rather than risk damage to the sensor(assuming you are planning to re-use it in
another manifold or pipe), start and run the
engine for a minute or two, then switch it off.
Be careful not to burn yourself during the
following procedure.
18Disconnect the battery negative cable.
Caution: If the radio in your
vehicle is equipped with an anti-
theft system, make sure you
have the correct activation code
before disconnecting the battery. Refer to
the information on page 0-7 at the front of
this manual before detaching the cable.
Note: If, after connecting the battery, the
wrong language appears on the instrument
panel display, refer to page 0-7 for the
language resetting procedure.
19Raise and support the vehicle.
20Disconnect the electrical connector from
the sensor.
21Carefully unscrew the sensor.
Caution: Excessive force may
damage the threads.
22A high-temperature anti-seize compound
must be used on the threads of the sensor, to
facilitate future removal. The threads of new
sensors will already be coated with this
compound, but if an old sensor is removed
and refitted, recoat the threads.23Refit the sensor and tighten it securely.
24Reconnect the electrical connector of the
pigtail lead to the main engine wiring harness.
25Lower the vehicle, and reconnect the
battery.
Oxygen Sensor Heated power
sensor type output signal supply (12V)
Unheated
(single-wire) black wire (+) Not applicable
Heated terminal 1 (+) terminals
(three-wire) 3 (+) and 2 (-)
Heated terminal 2 (+) terminals
(four-wire) 4 (+) and 3 (-)
Throttle Position Sensor (TPS)
General description
26The Throttle Position Sensor (TPS) is
located on the end of the throttle shaft on the
throttle body. By monitoring the output
voltage from the TPS, the ECU can determine
fuel delivery based on throttle valve angle
(driver demand). In this system, the TPS acts
as a switch rather than a potentiometer. One
set of throttle valve switch contacts is closed
(continuity) only at idle. A second set of
contacts closes as the engine approaches
full-throttle. Both sets of contacts are open
(no continuity) between these positions. A
broken or loose TPS can cause intermittent
bursts of fuel from the injector and an
unstable idle, because the ECU thinks the
throttle is moving.
27All models (except for early 535i models
with automatic transmission) combine the idle
and full-throttle switch; a separate idle
position switch indicates the closed-throttle
position, while the TPS is used for the full-
throttle position. On 535i models with
automatic transmission, the TPS is connected
directly to the automatic transmission control
unit. With the throttle fully open, the
transmission control unit sends the full-
throttle signal to the Motronic control unit.
All models except early 535i with
automatic transmission
Check
28Remove the electrical connector from the
TPS, and connect an ohmmeter to terminals 2
and 18 (see illustrations). Open the throttle
Engine management and emission control systems 6•3
4.12b These oxygen sensor terminal
designations are for the harness side only.
Use the corresponding terminals on the
sensor side for the testing procedures
(there are three different four-wire oxygen
sensor connectors available - don’t get
them mixed up)4.12a The oxygen sensor, once it is
warmed up (320º C), puts out a very small
voltage signal. To verify it is working,
check for voltage with a digital voltmeter
(the voltage signals usually range from
0.1 to 1.0 volt)
4.28b First check for continuity between
terminals 2 and 18 with the throttle closed
(later Motronic system shown) . . .4.28a The TPS on L-Jetronic systems is
located under the intake manifold
(terminals arrowed)
6