1985 BMW 3 SERIES Start

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

slightly by hand. Release the throttle slowly
until it reaches 0.2 to 0.6 mm from the throttle
stop. There should be continuity.
29Check the resistance between terminals 3
and 18 as the throttle is opened. There should
be continuity when the throttle switch is within
8 to 12 degrees of fully-open. If the readings
are incorrect, adjust the TPS.
30If all the resistance readings are correct
and the TPS is properly adjusted, check for
power (5 volts) at the sensor, and if necessary
trace any wiring circuit problems between the
sensor and ECU (see Chapter 12).
Adjustment
31If the adjustment is not as specified
(paragraphs 28 to 30), loosen the screws on
the TPS, and rotate the sensor into the correct
adjustment. Follow the procedure for
checking the TPS given above, and tighten
the screws when the setting is correct.
32Recheck the TPS once more; if the
readings are correct, reconnect the TPS
harness connector.
Early 535i models with automatic
transmission
Check
33First test the continuity of the TPS. Follow
paragraphs 28 to 30 and check for continuity.
34Next, test the idle position switch (see
illustration). Unplug the electrical connector
in the idle position switch harness, andconnect an ohmmeter to terminals 1 and 2.
There should be continuity. Open the throttle
slightly, and measure the resistance. There
should now be no continuity.
35Check for the correct voltage signals from
the TPS, with the throttle closed and the
ignition on. Probe the back of the TPS
connector with a voltmeter, and check for
voltage at terminal 3 (black wire) and earth.
There should be 5 volts present. Also, probe
terminal 3 (black wire) and terminal 1 (brown
wire). There should be 5 volts present here
also.
36Check for voltage at terminal 2 (yellow
wire) and terminal 1 (brown wire), and slowly
open the throttle. The voltage should increase
steadily from 0.7 volts (throttle closed) to
4.8 volts (throttle fully-open).
Adjustment
37First measure the stabilised voltage. With
the ignition on and the throttle closed,
measure the voltage between terminal 3
(black wire) and terminal 1 (brown wire). It
should be about 5 volts.
38Next, loosen the sensor mounting screws,
and connect the voltmeter to terminal 2
(yellow wire) and terminal 3 (black wire). With
the throttle fully open, rotate the switch until
there is 0.20 to 0.24 volts less than the
stabilised voltage. Note: You will need a
digital voltmeter to measure these small
changes in voltage.
39Recheck the TPS once more; if the
readings are correct, reconnect the TPS
electrical connector. It is a good idea to lock
the TPS screws with paint or thread-locking
compound.
Airflow meter
General description
40The airflow meter is located on the air
intake duct. The airflow meter measures the
amount of air entering the engine. The ECU
uses this information to control fuel delivery. A
large volume of air indicates acceleration,
while a small volume of air indicates
deceleration or idle. Refer to Chapter 4 for all
the diagnostic checks and renewal
procedures for the airflow meter.
Ignition timing sensors
41Ignition timing is electronically-controlled
on Motronic systems, and is not adjustable.
During starting, a crankshaft position sensor
relays the crankshaft position to the ECU, and
an initial baseline ignition point is determined.
Once the engine is running, the ignition point
is continually changing based on the various
input signals to the ECU. Engine speed is
signalled by a speed sensor. Early Motronic
systems have the reference sensor and the
speed sensor mounted on the bellhousing
over the flywheel. Later Motronic systems
have a single sensor (pulse sensor) mounted
over the crankshaft pulley. This sensor
functions as a speed sensor as well as a
position sensor. Refer to Chapter 5 for more
information. 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 it is not part of
the Motronic ignition system.
5 Positive crankcase
ventilation (PCV) system
1The Positive Crankcase Ventilation (PCV)
system (see illustration)reduces
hydrocarbon emissions by scavenging
crankcase vapours. It does this by circulating
blow-by gases and then re-routing them to
the intake manifold by way of the air cleaner.
2This PCV system is a sealed system. The
crankcase blow-by vapours are routed
directly to the air cleaner or air collector with
crankcase pressure behind them. The vapour
is not purged with fresh air on most models or
6•4 Engine management and emission control systems
5.2 PCV hose being removed from the
valve cover5.1 Diagram of the PCV system on the
M20 engine (others similar)4.34 Idle position switch and TPS on early
535i models with automatic transmission
4.28c . . . then check for continuity
between terminals 3 and 18 as the throttle
is opened

filtered with a flame trap like most
conventional systems. There are no
conventional PCV valves fitted on these
systems - just a hose (see illustration).
3The main components of the PCV system
are the hoses that connect the valve cover to
the throttle body or air cleaner. If abnormal
operating conditions (such as piston ring
problems) arise, the system is designed to
allow excessive amounts of blow-by gases to
flow back through the crankcase vent tube
into the intake system, to be consumed by
normal combustion. Note: Since these
models don’t use a filtering element, it’s a
good idea to check the PCV system
passageways for clogging from sludge and
combustion residue(see illustration).
6 Evaporative emissions
control (EVAP) system
2
General description
Note:This system is normally only fitted to
those vehicles equipped with a catalytic
converter.
1When the engine isn’t running, the fuel in the
fuel tank evaporates to some extent, creating
fuel vapour. The evaporative emissions control
system (see illustration)stores these fuel
vapours in a charcoal canister. When the
engine is cruising, the purge control valve is
opened slightly, and a small amount of fuel
vapour is drawn into the intake manifold and
burned. When the engine is starting cold or
idling, the purge valve prevents any vapours
from entering the intake manifold and causing
excessively-rich fuel mixture.
2Two types of purge valve are used;
electrically-operated or vacuum-operated. To
find out which type is on your vehicle, follow
the hose from the charcoal canister until you
locate the purge valve. Some are located on
the intake manifold, and others near the
charcoal canister. Look for either an electrical
connector, or vacuum lines, to the purge
valve.3A faulty EVAP system will only affect engine
driveability when the engine is warm. The
EVAP system is not usually the cause of
difficult cold starting or any other cold-running
problems.
Check
Vacuum-operated purge valve
4Remove the vacuum lines from the purge
valve, and blow into the larger valve port. It
should be closed, and not pass any air. Note:
Some models have a thermo-vacuum valve
that delays canister purging until the coolant
temperature reaches approximately 46º C.
Check this valve to make sure that vacuum is
controlled at the proper temperatures. The
valve is usually located in the intake manifold,
near the thermo-time switch and the coolant
temperature sensor.
5Disconnect the small vacuum hose from the
purge valve, and apply vacuum with a hand-
held vacuum pump. The purge valve should
be open, and air should be able to pass
through.6If the test results are unsatisfactory, renew
the purge valve.
Electrically-operated purge valve
7Disconnect any lines from the purge valve,
and (without disconnecting the electrical
connector) place it in a convenient spot for
testing.
8Check that the valve makes a “click” sound
as the ignition is switched on (see
illustration).
9If the valve does not “click”, disconnect the
valve connector, and check for power to the
valve using a test light or a voltmeter (see
illustration).
10If battery voltage is present, but the valve
does not work, renew it. If there is no voltage
present, check the Motronic control unit and
the wiring.
Canister
11Mark all the hoses for position, then
detach them from the canister.
12Slide the canister out of its mounting clip.
Engine management and emission control systems 6•5
6.1 Diagram of the EVAP system on the M10 engine (others similar)
6.9 Check for battery voltage at the
electrical connector to the purge valve6.8 When the ignition is switched on, there
should be a distinct “click” from the purge
valve
6
5.3 It’s a good idea to check for excess
residue from the crankcase vapours
circulating in the hoses and ports - this
can eventually clog the system, and cause
a pressure increase in the engine block

On some models, it will be necessary to
release the retaining clip (see illustration).
13Visually examine the canister for leakage
or damage.
14Renew the canister if you find evidence of
damage or leakage.
7 Catalytic converter
1
General description
1To reduce emissions of unburnt
hydrocarbons (HC), carbon monoxide (CO)
and oxides of nitrogen (NOx), the later
vehicles covered by this manual are equipped
with a catalytic converter (see illustration).
The converter contains a ceramic honeycomb
coated with precious metals, which speed up
the reaction between the pollutants listed
previously and the oxygen in the exhaust gas.
The pollutants are oxidised to produce water
(H
2O), nitrogen and carbon dioxide (CO2).
Check
2Visually examine the converter(s) for cracks
or damage. Make sure all nuts and bolts are
tight.
3Inspect the insulation cover (if applicable)
welded onto the converter - it should not be
loose.
Caution: If an insulation cover is
dented so that it touches the
converter housing inside,
excessive heat may be
transferred to the floor.
4Start the engine and run it at idle speed.
5Check for exhaust gas leakage from the
converter flanges. Check the body of each
converter for holes.
Component renewal
6See Chapter 4 for removal and refitting
procedures.
Precautions
7The catalytic converter is a reliable and
simple device, which needs no maintenance
in itself, but there are some facts of which an
owner should be aware, if the converter is to
function properly for its full service life.
(a) DO NOT use leaded (eg UK “4-star”)
petrol in a car equipped with a catalytic
converter - the lead will coat the precious
metals, reducing their converting
efficiency, and will eventually destroy the
converter.
(b) Always keep the ignition and fuel systems
well-maintained in accordance with the
manufacturer’s schedule, as given in
Chapter 1. In particular, ensure that the air
cleaner filter element, the fuel filter (where
fitted) and the spark plugs are renewed at
the correct interval. If the intake air/fuel
mixture is allowed to become too rich due
to neglect, unburned fuel will enter the
catalytic converter, overheating the
element and eventually destroying the
converter.
(c) If the engine develops a misfire, do not
drive the car at all (or at least as little as
possible) until the fault is cured - the
misfire will allow unburned fuel to enter
the converter, which will result in its
overheating, as noted above.
(d) DO NOT push- or tow-start the car - this
will soak the catalytic converter in
unburned fuel, causing it to overheat
when the engine does start - see (b) or (c)
above.
(e) DO NOT switch off the ignition at high
engine speeds - ie do not “blip” the
throttle immediately before switching offthe engine. If the ignition is switched off
at anything above idle speed, unburned
fuel will enter the (very hot) catalytic
converter, with the possible risk of its
igniting on the element and damaging the
converter.
(f) DO NOT use fuel or engine oil additives -
these may contain substances harmful to
the catalytic converter.
(g) DO NOT continue to use the car if the
engine burns oil to the extent of leaving a
visible trail of blue smoke - the unburned
carbon deposits will clog the converter
passages, and reduce its efficiency; in
severe cases, the element will overheat.
(h) Remember that the catalytic converter
operates at very high temperatures -
hence the heat shields on the car’s
underbody - and the casing will become
hot enough to ignite combustible
materials which brush against it. DO NOT,
therefore, park the car in dry
undergrowth, or over long grass or piles
of dead leaves.
(i) Remember that the catalytic converter is
FRAGILE - do not strike it with tools
during servicing work, and take great care
when working on the exhaust system.
Ensure that the converter is well clear of
any jacks or other lifting gear used to raise
the car, and do not drive the car over
rough ground, road humps, etc, in such a
way as to “ground” the exhaust system.
(j) In some cases, particularly when the car
is new and/or is used for stop/start
driving, a sulphurous smell (like that of
rotten eggs) may be noticed from the
exhaust. This is common to many
catalytic converter-equipped cars, and
seems to be due to the small amount of
sulphur found in some petrols reacting
with hydrogen in the exhaust, to produce
hydrogen sulphide (H
2S) gas; while this
gas is toxic, it is not produced in sufficient
amounts to be a problem. Once the car
has covered a few thousand miles, the
problem should disappear - in the
meanwhile, a change of driving style, or of
the brand of petrol used, may effect a
solution.
(k) The catalytic converter, used on a well-
maintained and well-driven car, should
last for 50 000 to 100 000 miles - from
this point on, the CO level should be
carefully checked regularly, to ensure that
the converter is still operating efficiently. If
the converter is no longer effective, it
must be renewed.
6•6 Engine management and emission control systems
7.1 Typical catalytic converter (M10
engine type shown, others similar)6.12 EVAP system charcoal canister
viewed from under the vehicle (316i model)

and cause the system to malfunction. If
the ABS wiring harness is damaged in any
way, it must be renewed.
Caution: Make sure the ignition is
turned off before unplugging or
re-making any electrical
connections.
Diagnosis and repair
If the dashboard warning light comes on
and stays on while the vehicle is in operation,
the ABS system requires attention. Although
special electronic ABS diagnostic testing
tools are necessary to properly diagnose the
system, you can perform a few preliminary
checks before taking the vehicle to a dealer
service department.
a) Check the brake fluid level in the
reservoir.
b) Verify that the electronic control unit
connectors are securely connected.
c) Check the electrical connectors at the
hydraulic control unit.
d) Check the fuses.
e) Follow the wiring harness to each front
and rear wheel, and verify that all
connections are secure and that the
wiring is undamaged.
If the above preliminary checks do not
rectify the problem, the vehicle should bediagnosed by a dealer service department.
Due to the complex nature of this system, all
actual repair work must be done by a dealer
service department.
3 Disc brake pads- renewal
2
Warning: Disc brake pads must
be renewed on both front wheels
or both rear wheels at the same
time - NEVER renew the pads on
only one wheel. Also, the dust created by
the brake system may contain asbestos,
which is harmful to your health. Never
blow it out with compressed air, and don’t
inhale any of it. An approved filtering mask
should be worn when working on the
brakes. Do not, under any circumstances,
use petroleum-based solvents to clean
brake parts. Use brake system cleaner
only! When servicing the disc brakes, use
only original-equipment or high-quality
brand-name pads.
Warning: Brake fluid is
poisonous. It is also an effective
paint stripper. Refer to the
warning at the start of Section 16.
Note:This procedure applies to both the front
and rear disc brakes.
1Remove the cap(s) from the brake fluid
reservoir, and syphon off about two-thirds of
the fluid from the reservoir. Failing to do thiscould result in the reservoir overflowing when
the caliper pistons are pressed back into their
bores.
2Loosen the wheel bolts, raise the front or
rear of the vehicle and support it securely on
axle stands.
3Remove the front or rear wheels, as
applicable. Work on one brake assembly at a
time, using the assembled brake for reference
if necessary.
4Inspect the brake disc carefully as outlined
in Section 5. If machining is necessary, follow
the information in that Section to remove the
disc, at which time the pads can be removed
from the calipers as well.
5Follow the accompanying photos,
beginning with illustration 3.5a, for the pad
removal procedure. Be sure to stay in order,
and read the caption under each illustration.
Note 1:Different types of front calipers are
used on 3 and 5-Series models. Illustrations
3.5a to 3.5e are for the front calipers on 3-
Series models.Illustrations 3.5f to 3.5m are
for the front calipers on 5-Series models.
There’s no photo sequence for rear calipers;
although slightly different in size, they’re
identical in design to the front brake calipers
used on 5-Series models.Note 2: Some
models may have different numbers and types
of anti-squeal shims and other hardware than
what is shown in this Chapter. It’s best to note
how the hardware is fitted on the vehicle
before dismantling, so you can duplicate it on
reassembly.
Braking system 9•3
3.5c Hold the guide pins while loosening
the caliper mounting bolts (3-Series)3.5b Unplug the electrical connector for
the brake pad wear sensor (3-Series)
3.5a On 3-Series models, unscrew the
caliper mounting bolts (left arrows); right
arrows point to the caliper bracket bolts,
which should only be removed if you’re
removing the brake disc
3.5f On 5-Series models, unplug the
electrical connector for the brake pad
wear sensor3.5e Remove the outer brake pad
(3-Series) - to fit the new pads, reverse the
removal procedure
3.5d Remove the caliper, brake pad wear
sensor and inner pad all at the same time
(3-Series), then refit the inner pad on the
piston and press the piston fully into the
bore with a C-clamp
9

6Be sure to inspect the wear sensor(s) (left
front wheel only, or left front and right rear
wheel). If they’re OK, transfer them from the
old pads to the new ones; if they’re worn by
abrasion, fit new sensors on the new pads.
7To fit the new pads, reverse the removal
procedure. When refitting the caliper, be sure
to tighten the mounting bolts to the torque
listed in this Chapter’s Specifications.
Warning: Check and if necessary
renew the mounting bolts on 3-
Series models whenever they are
removed. If in doubt, use new
bolts.
8After the job is completed, firmly depress
the brake pedal a few times, to bring the pads
into contact with the discs. The pedal shouldbe at normal height above the floor, and firm.
Check the level of the brake fluid, adding
some if necessary. Check carefully for leaks,
and check the operation of the brakes before
returning the vehicle to normal service.
9Avoid heavy braking as far as possible for
the first hundred miles or so until the new
pads have bedded in.
4 Disc brake caliper- removal,
overhaul and refitting
4
Warning: Dust created by the
brake system may contain
asbestos, which is harmful to
your health. Never blow it out
with compressed air, and don’t inhale any
of it. An approved filtering mask should be
worn when working on the brakes. Do not,
under any circumstances, use petroleum-
based solvents to clean brake parts. Use
brake system cleaner only!
Warning: Brake fluid is
poisonous. It is also an effective
paint stripper. Refer to the
warning at the start of Section 16.
Note:If an overhaul is indicated (usually
because of fluid leakage), explore all options
before beginning the job. Overhauled calipers
may be available on an exchange basis, which
makes this job quite easy. If you decide to
overhaul the calipers, make sure that anoverhaul kit is available before proceeding.
Always overhaul the calipers in pairs - never
overhaul just one of them.
Removal
1Loosen the wheel bolts, raise the front or
rear of the vehicle, and place it securely on
axle stands. Remove the wheel.
2If you’re just removing the caliper for
access to other components, it isn’t
necessary to detach the brake line. If you’re
removing the caliper for overhaul, disconnect
the brake line from the caliper, for preference
using a split ring (“brake”) spanner to protect
the fitting. Plug the line, to keep contaminants
out of the brake system and to prevent losing
brake fluid unnecessarily.
3Refer to Section 3 for the front or rear
caliper removal procedure - it’s part of the
brake pad renewal procedure. Note:The rear
caliper is similar in design to the front caliper
on 5-series models.
Overhaul
4On all calipers except the front calipers on
3-Series models, remove the circlip for the
dust seal (see illustration),then remove the
dust boot (see illustration). Before you
remove the piston, place a block of wood
between the piston and caliper to prevent
damage as it is removed.
5To remove the piston from the caliper,
apply compressed air to the brake fluid hose
connection on the caliper body (see
9•4 Braking system
3.5l Hang the caliper out of the way with a
piece of wire
3.5m Remove the outer brake pad - to fit
the new pads, reverse the removal
procedure
3.5k Unclip the inner brake pad from the
piston (5-Series)3.5j Remove the caliper and inner brake
pad (5-Series)
3.5i Depress the piston with a C-clamp
(5-Series)3.5h Prise off the anti-rattle spring
(5-Series)3.5g Remove the plugs for the brake
caliper mounting bolts, then remove the
bolts (5-Series)

illustration). Use only low pressure, such as
that produced by a foot pump, to ease the
piston out of its bore.
Warning: Be careful not to place
your fingers between the piston
and the caliper, as the piston
may come out with some force. If
you’re working on a front caliper of a 3-
Series model, remove the dust boot.
6Inspect the mating surfaces of the piston
and caliper bore wall. If there is any scoring,
rust, pitting or bright areas, renew the
complete caliper unit.
7If these components are in good condition,
remove the piston seal from the caliper bore
using a wooden or plastic tool (see
illustration). Metal tools may damage the
cylinder bore.
8Remove the caliper guide pins or bolts and
remove the rubber dust boots.
9Wash all the components using methylated
spirit or brake system cleaner.
10Using the correct overhaul kit for your
vehicle, reassemble the caliper as follows.
11Dip the new rubber seal in clean brake
fluid, and refit it in the lower groove in the
caliper bore, making sure it isn’t twisted.
12On all calipers except the front calipers of
3-Series models, coat the walls of the caliperbore and the piston with clean brake fluid, and
refit the piston at this time. Do not force the
piston into the bore, but make sure that it is
squarely in place, then apply firm (but not
excessive) pressure to refit it. Fit the new
rubber dust boot and the retaining ring.
13On the front calipers of 3-Series models,
coat the piston with clean brake fluid, and
stretch the new dust boot over the bottom of
the piston. Hold the piston over the caliper
bore, and insert the rubber flange of the dust
boot into the upper groove in the bore. Start
with the furthest side from you, and work your
way around towards the front until it is
completely seated. Push the piston into the
caliper bore until it is bottomed in the bore,
then seat the top of the dust boot in the
groove in the piston.
14Lubricate the sliding surfaces of the guide
pins or bolts with silicone-based grease
(usually supplied in the kit), and push them
into the caliper. Refit the dust boots.
Refitting
15Refit the caliper by reversing the removal
procedure (see Section 3).
Warning: Check and if necessary
renew the mounting bolts on 3-
Series models whenever they are
removed. If in doubt, use new bolts.16If the hose was disconnected from the
caliper, bleed the brake system (see Sec-
tion 16).
5 Brake disc- inspection,
removal and refitting
2
Note:This procedure applies to both the front
and rear brake discs. Brake discs should
always be renewed or refinished in pairs (both
front or both rear discs) even if only one is
damaged or defective.
Braking system 9•5
4.4b Remove the circlip for the dust seal
4.4a An exploded view of a typical rear caliper assembly (front calipers similar)
4.7 Remove the piston seal from the
caliper bore using a wooden or plastic tool
(metal tools may damage the
cylinder bore)
1 Caliper assembly
2 Bracket mounting bolt
3 Bleed screw
4 Dust cap
5 Anti-rattle spring
6 Guide bolt
7 Brake pad wear warning
light wire8 Cable clamp
9 Brake disc
10 Allen bolt
11 Shield
12 Bolt
13 Washer
14 Plug15 Plug
16 Caliper bracket
17 Cable clamp
18 Piston seal, piston, dust
boot and circlip
19 Guide bush repair kit
20 Brake pads4.5 With the caliper padded to catch the
piston, use low pressure compressed air
to force the piston out of its bore - make
sure your fingers are not between the
piston and the caliper
9