1999 DODGE RAM ESP

VEH IC LE C O M MUNIC ATIO N

1999 D odge P ic ku p R 1500
1999 ACCESSORIES & EQUIPMENT
CHRY - Vehicle Communications
Ram Pickup
IDENTIFYING VEHICLE COMMUNICATION PROBLEMS
Connect scan tool to Data Link Connector (DLC) to retrieve
messages. If scan tool message is blank, disconnect scan tool. Ensure
ground circuit has continuity at DLC terminal No. 4. Ensure 12 volts
exists at DLC terminal No. 16. Check power to DLC terminal No. 16 from
Power Distribution Center (PDC) fuse No. 12. Try another scan tool
and/or cable. If scan tool DTC or fault message is present, see
following bus fault messages list and proceed to appropriate DTC or
fault message:
* BUS (+) & BUS (-) OPEN
* BUS (+) OPEN
* BUS (-) OPEN
* BUS (+) & BUS (-) SHORTED TOGETHER
* BUS BIAS LEVEL TOO HIGH
* BUS BIAS LEVEL TOO LOW
* NO BUS BIAS
* NO RESPONSE AIR BAG CONTROL MODULE
* NO RESPONSE CENTRAL TIMER MODULE
* NO RESPONSE INSTRUMENT CLUSTER
* NO RESPONSE POWERTRAIN CONTROL MODULE
* NO RESPONSE COMPASS/MINI-TRIP SYSTEM
* NO RESPONSE RADIO
* NO TERMINATION
* NOT RECEIVING BUS MESSAGES CORRECTLY
* BUS SHORT TO 5 VOLTS
* BUS SHORT TO BATTERY
* BUS SHORT TO GROUND
BUS (+) & BUS (-) OPEN, (BUS (+) OPEN OR BUS (-) OPEN)
NOTE: For connector terminal identification and wiring diagrams,
see BODY CONTROL COMPUTER - INTRODUCTION article. Perform
VERIFICATION TEST VER-1 after each repair.
CAUTION: Always turn ignition off prior to disconnecting any module
connector.
1) Disconnect scan tool. Ensure ignition is on. Using an
external voltmeter, measure voltage between ground and DLC terminal
No. 3 (Violet/Brown wire). If voltage is not 1.8-2.3 volts, go to step\
3). If voltage is 1.8-2.3 volts, go to next step.
2) Measure voltage between ground and DLC terminal No. 11
(White/Black wire). If voltage is not 1.8-2.3 volts, go to next step.
If voltage is 1.8-2.3 volts, replace scan tool cable or scan tool.
3) Connect jumper wire between ground and DLC connector
terminal No. 11 (White/Black wire). Turn ignition off. Remove
instrument cluster. Using external ohmmeter, measure resistance
between ground and instrument cluster connector C1 terminal No. 9
(White/Black wire). If resistance is less than 5 ohms, go to next
step. If resistance is 5 ohms or more, repair open White/Black wire.
4) Disconnect jumper wire. Connect jumper wire between ground
and DLC connector terminal No. 3 (White/Black wire). Measure

more, replace instrument cluster.
NO BUS BIAS
NOTE: For connector terminal identification and wiring diagrams,
see BODY CONTROL COMPUTER - INTRODUCTION article. Perform
VERIFICATION TEST VER-1 after each repair.
CAUTION: Always turn ignition off prior to disconnecting any module
connector.
1) Disconnect scan tool. Turn ignition on. Using an external
voltmeter, measure voltage between ground and DLC terminal No. 3
(Violet/Brown wire). If voltage is not 1.8-2.3 volts, go to step 3).
If voltage is 1.8-2.3 volts, go to next step.
2) Measure voltage between ground and DLC terminal No. 11
(White/Black wire). If voltage is not 1.8-2.3 volts, go to next step.
If voltage is 1.8-2.3 volts, replace scan tool cable or scan tool.
3) Connect jumper wire between ground and DLC connector
terminal No. 11 (White/Black wire). Turn ignition off. Remove
instrument cluster. Using external ohmmeter, measure resistance
between ground and instrument cluster connector C1 terminal No. 9
(White/Black wire). If resistance is less than 5 ohms, go to next
step. If resistance is 5 ohms or more, repair open White/Black wire.
4) Disconnect jumper wire. Measure resistance between ground
and instrument cluster connector C1 terminal No. 9 (White/Black wire).\
If resistance is less than 1000 ohms, repair White/Black wire for
short to ground. If resistance is 1000 ohms or more, go to next step.
5) Connect jumper wire between ground and DLC connector
terminal No. 3 (White/Black wire). Measure resistance between ground
and instrument cluster connector C1 terminal No. 10 (Violet/Brown
wire). If resistance is less than 5 ohms, go to next step. If
resistance is 5 ohms or more, repair open Violet/Brown wire.
6) Disconnect jumper wire. Measure resistance between ground
and instrument cluster connector C1 terminal No. 10 (Violet/Brown
wire). If resistance is less than 1000 ohms, repair Violet/Brown wire
for short to ground. If resistance is 1000 ohms or more, go to next
step.
7) Turn ignition on. Using external voltmeter, measure
voltage between ground and instrument cluster connector C1 terminal
No. 2 (Dark Blue/White wire). If voltage is 9.5 volts or less, repair
open Dark Blue/White wire. If voltage is more than 9.5 volts, go to
next step.
8) Turn ignition off. Using external ohmmeter, measure
resistance between ground and instrument cluster connector C1 terminal
No. 4 (Black/Light Green wire). If resistance is 5 ohms or less, go to\
next step. If resistance is more than 5 ohms, repair open Black/Light
Green wire.
9) Measure resistance between ground and instrument cluster
connector C1 terminal No. 5 (Black/Orange wire). If resistance is 5
ohms or less, replace instrument cluster. If resistance is more than 5
ohms, repair open Black/Orange wire.
NO RESPONSE AIR BAG CONTROL MODULE
NOTE: For connector terminal identification and wiring diagrams,
see BODY CONTROL COMPUTER - INTRODUCTION article. Perform
VERIFICATION TEST VER-1 after each repair.
CAUTION: Always turn ignition off prior to disconnecting any module
connector.
1) Remove and inspect fuse No. 19 from junction block.

Junction block is on left side of instrument panel. If fuse is open,
go to next step. If fuse is okay, go to step 6).
2) Using external voltmeter, measure voltage between ground
and voltage input side of fuse No. 19 socket. If voltage is 10 volts
or less, repair open ignition switch output circuit (Dark Blue wire
between ignition switch and junction block). If voltage is more than
10 volts, go to next step.
3) Turn ignition off and wait 2 minutes. Using external
ohmmeter, measure resistance between ground and fused ignition switch
output run/start terminal (Light Green/Yellow wire) on fuse No. 19
socket. If resistance is less than 5 ohms, go to next step. If
resistance is 5 ohms or more, go to step 5).
4) Disconnect Air Bag Control Module (ACM) connector. ACM is\
located under center of instrument panel. Turn ignition on. Measure
resistance between ground and fused ignition switch output run/start
terminal (Light Green/Yellow wire) on fuse No. 19 socket. If
resistance is less than 5 ohms, repair Light Green/Yellow wire for
short to ground. Replace fuse No. 19. If resistance is 5 ohms or more,
replace air bag control module and fuse No. 19.
5) Turn ignition off and wait 2 minutes. Disconnect Air Bag
Control Module (ACM) connector. ACM is located under center of
instrument panel. Using external ohmmeter, measure resistance of Light
Green/Yellow wire between ACM connector terminal No. 14 and fused
ignition switch output run/start terminal on fuse No. 19 socket. If
resistance is less than 5 ohms, connect ACM connector. Turn ignition
on. Replace fuse No. 19 in junction block. Go to appropriate AIR BAG
SERVICE & REPAIR, DOMESTIC CARS, LIGHT TRUCKS & VANS for further
testing. If resistance is 5 ohms or more, repair open Light
Green/Yellow wire. Replace fuse No. 19.
6) Using external voltmeter, measure voltage between ground
and voltage input side of fuse No. 19 socket. If voltage is 10 volts
or less, repair open ignition switch output circuit (Dark Blue wire
between ignition switch and junction block). If voltage is more than
10 volts, go to next step.
7) Turn ignition off and wait 2 minutes. Install fuse No. 19.
Disconnect Air Bag Control Module (ACM) connector. ACM is located
under center of instrument panel. Turn ignition on. Using external
voltmeter, measure voltage between ground and ACM connector terminal
No. 14 (Light Green/Yellow wire). If voltage is 10 volts or less,
repair open Light Green/Yellow wire. If voltage is more than 10 volts,
go to next step.
8) Turn ignition off. Using external ohmmeter, measure
resistance between ground and ACM connector terminal No. 4
(Black/Pink). If resistance is 5 ohms or more, repair open Black/Pink
wire. If resistance is less than 5 ohms, go to next step.
9) Turn ignition on. Using external voltmeter, measure
voltage between ground and ACM connector terminal No. 21 (Violet/Brown
wire). If voltage is not 1.8-2.6 volts, repair open Violet/Brown wire.
If voltage is 1.8-2.6 volts, go to next step.
10) Turn ignition off. Using external ohmmeter, measure
resistance between ground and ACM connector terminal No. 4 (Black/Pink
wire). If resistance is 5 ohms or more, repair open Black/Pink wire.
If resistance is less than 5 ohms, go to next step.
11) Using external voltmeter, measure voltage between ground
and ACM connector terminal No. 22 (White/Black wire). If voltage is
not 1.8- 2.6 volts, repair open White/Black wire. If voltage is 1.8-2.
6 volts, replace ACM.
NO RESPONSE CENTRAL TIMER MODULE
NOTE: For connector terminal identification and wiring diagrams,
see BODY CONTROL COMPUTER - INTRODUCTION article. Perform
VERIFICATION TEST VER-1 after each repair.

CAUTION: Always turn ignition off prior to disconnecting any module
connector.
1) Remove and inspect fuse No. 11 from junction block.
Junction block is on left side of instrument panel. If fuse is open,
go to next step. If fuse is okay, go to step 3).
2) Using external ohmmeter, measure resistance between ground
and fused ignition switch output run/start terminal (Dark Blue/White
wire) on fuse No. 11 socket. If resistance is less than 5 ohms, repair
short to ground in Dark Blue/White wire. Replace fuse No. 11. If
resistance is 5 ohms or more, replace fuse No. 11.
3) Install fuse No. 11. Remove and inspect fuse No. 6 from
junction block. If fuse is open, next step. If fuse is okay, go to
step 5).
4) Using external ohmmeter, measure resistance between ground
and fused ignition switch output run/accy terminal (Dark Blue wire) on\
fuse No. 6 socket. If resistance is less than 5 ohms, repair short to
ground in Dark Blue wire. Replace fuse No. 6. If resistance is 5 ohms
or more, replace fuse No. 6.
5) Install fuse No. 6. Disconnect Central Timer Module (CTM)\
.
CTM is located under left side of instrument panel. Using external
ohmmeter, measure resistance between ground and CTM 18-pin connector
terminal No. 3 (Black/Orange wire). If resistance is more than 5 ohms,\
repair open Black/Orange wire. If resistance is 5 ohms or less, go to
next step.
6) Measure resistance between ground and CTM 14-pin connector
terminal No. 6 (Black/Light Green wire). If resistance is more than 5
ohms, repair open Black/Light Green wire. If resistance is 5 ohms or
less, go to next step.
7) Turn ignition on. Using scan tool, perform CCD Bus test.
Connect jumper wire between ground and CTM 18-pin connector terminal
No. 16 (Violet/Brown wire). If scan tool does not display BUS SHORT TO\
GROUND, repair open Violet/Brown wire. If scan tool displays BUS SHORT
TO GROUND, go to next step.
8) Disconnect jumper wire. Connect jumper wire between ground
and CTM 18-pin connector terminal No. 17 (White/Black wire). If scan
tool does not display BUS SHORT TO GROUND, repair open White/Black
wire. If scan tool displays BUS SHORT TO GROUND, replace CTM.
NO RESPONSE INSTRUMENT CLUSTER
NOTE: For connector terminal identification and wiring diagrams,
see BODY CONTROL COMPUTER - INTRODUCTION article. Perform
VERIFICATION TEST VER-1A after each repair.
CAUTION: Always turn ignition off prior to disconnecting any module
connector.
1) Turn ignition on. Using scan tool, select BODY CONTROL
MODULE. If scan tool displays NO RESPONSE, go to NO RESPONSE CENTRAL
TIMER MODULE. If scan tool does not display NO RESPONSE, go to next
step.
2) Using scan tool, select SYSTEM TEST. If scan tool displays
PCM ACTIVE ON THE BUS, go to next step. If scan tool does not display
PCM ACTIVE ON THE BUS, go to NO RESPONSE POWERTRAIN CONTROL MODULE.
3) Turn ignition off. Remove instrument cluster. Turn
ignition on. Using scan tool, turn on CCD bus bias under SYSTEM
MONITORS, then CCD BUS VOLTAGE. Connect jumper wire between ground and
instrument cluster connector C1 terminal No. 10 (Violet/Brown wire).
If scan tool voltage did not drop to about zero volts, repair open
Violet/Brown wire. If scan tool voltage dropped to about zero volts,
go to next step.

4) Move jumper to instrument cluster connector C1 terminal
No. 9 (White/Black wire). Monitor CCD BUS VOLTAGE. If scan tool
voltage did not drop to about zero volts, repair open White/Black
wire. If scan tool voltage did not drop to about zero volts, replace
instrument cluster.
NO RESPONSE POWERTRAIN CONTROL MODULE
NOTE: For connector terminal identification and wiring diagrams,
see BODY CONTROL COMPUTER - INTRODUCTION article. Perform
VERIFICATION TEST VER-1 after each repair.
CAUTION: Always turn ignition off prior to disconnecting any module
connector.
1) If engine does not run, see appropriate BASIC DIAGNOSTIC
PROCEDURES article in ENGINE PERFORMANCE section. If engine runs, go
to next step.
2) Turn ignition off. Disconnect Powertrain Control Module
(PCM). PCM is mounted in right side of firewall. Turn ignition on.
Connect jumper wire between ground and PCM connector C3 terminal. No.
30 (Violet/Brown wire). Using scan tool, perform CCD BUS test. If scan\
tool does not display SHORT TO GROUND, repair open Violet/Brown wire.
If scan tool displays SHORT TO GROUND, go to next step.
3) Move jumper wire to PCM connector C3 terminal No. 28
(White/Black wire). Perform CCD BUS test. If scan tool does not
display SHORT TO GROUND, repair open White/Black wire. If scan tool
displays SHORT TO GROUND, replace PCM.
NO RESPONSE COMPASS/MINI-TRIP SYSTEM
NOTE: For connector terminal identification and wiring diagrams,
see BODY CONTROL COMPUTER - INTRODUCTION article. Perform
VERIFICATION TEST VER-1 after each repair.
CAUTION: Always turn ignition off prior to disconnecting any module
connector.
1) Remove and inspect fuse No. 11 from junction block.
Junction block is on left side of instrument panel. If fuse is open,
go to next step. If fuse is okay, go to step 3).
2) Using external ohmmeter, measure resistance between ground
and fused ignition switch output run/start terminal (Dark Blue/White
wire) on fuse No. 11 socket. If resistance is less than 5 ohms, repair
Dark Blue/White wire for short to ground. Replace fuse No. 11. If
resistance is 5 ohms or more, replace fuse No. 11.
3) Reinstall fuse. Disconnect CMTC 12-pin connector. Turn
ignition on. Using external voltmeter, measure voltage between ground
and CMTC connector terminal No. 1 (Dark Blue/White wire). If voltage
is less than 9.5 volts, repair open Dark Blue/White wire. If voltage
is 9.5 volts or more, go to next step.
4) Using external voltmeter, measure voltage between ground
and CMTC connector terminal No. 5 (Pink wire). If voltage is less than\
9.5 volts, repair open Pink wire. If voltage is 9.5 volts or more, go
to next step.
5) Turn ignition off. Using external ohmmeter, measure
resistance between ground and CMTC connector terminal No. 7
(Black/Light Green wire). If resistance is 5 ohms or less, go to next
step. If resistance is more than 5 ohms, repair open Black/Light Green
wire.
6) Using scan tool, perform CCD bus test. Connect jumper wire
between ground and CMTC connector terminal No. 2 (Violet/Brown wire).
If scan tool displays BUS (+) SHORTED TO GROUND, go to next step. If

scan tool does not display BUS (+) SHORTED TO GROUND, repair open
Violet/Brown wire.
7) Move jumper wire to CMTC connector terminal No. 8
(White/Black wire). If scan tool displays BUS (+) SHORTED TO GROUND,\
replace CMTC. If scan tool does not display BUS (+) SHORTED TO GROUND,\
repair open White/Black wire.
NO RESPONSE RADIO
NOTE: For connector terminal identification and wiring diagrams,
see BODY CONTROL COMPUTER - INTRODUCTION article. Perform
VERIFICATION TEST VER-1A after each repair.
CAUTION: Always turn ignition off prior to disconnecting any module
connector.
1) If radio screen is not blank, go to step 7). If radio
screen is blank, go to next step.
2) Remove and inspect junction block fuse No. 8 (radio). If
fuse is blown, go to next step. If fuse is okay, go to step 4).
3) Turn ignition off. Disconnect radio Gray connector. Using
external ohmmeter, measure resistance between ground and radio Gray
connector terminal No. 6 (Red/White wire). If resistance is less than
5 ohms, repair Red/White wire for short to ground. Replace junction
block fuse No. 8. If resistance is 5 ohms or more, replace radio.
Replace junction block fuse No. 8.
4) Turn ignition on. Measure voltage between ground and
terminal No. 7 (Pink wire). If voltage is 10 volts or less, repair
open Pink wire. If voltage is more than 10 volts, go to next step.
5) Install fuse. Disconnect radio Gray connector. Using
external voltmeter, measure voltage between ground and radio Gray
terminal No. 6 (Red/White wire). If voltage is 10 volts or less,
repair open Red/White wire. If voltage is more than 10 volts, go to
next step.
6) Inspect ground strap and antenna for correct installation.
Repair as needed. If ground strap and antenna are okay, replace radio.
7) Disconnect 2-pin CCD BUS connector on back of radio.
Connect jumper wire between terminal No. 2 (White/Black wire) and
ground. Using scan tool, select BODY, BODY COMPUTER, then SYSTEM TEST.
Perform CCD BUS test. If scan tool does not display BUS SHORT TO
GROUND, repair open White/Black wire. If scan tool displays BUS SHORT
TO GROUND, go to next step.
8) Move jumper wire to terminal No. 1 (Violet/Brown wire).
Perform CCD BUS test. If scan tool does not display BUS SHORT TO
GROUND, repair open Violet/Brown wire. If scan tool displays BUS SHORT
TO GROUND, replace radio.
NO TERMINATION
NOTE: For connector terminal identification and wiring diagrams,
see BODY CONTROL COMPUTER - INTRODUCTION article. Perform
VERIFICATION TEST VER-1 after each repair.
CAUTION: Always turn ignition off prior to disconnecting any module
connector.
1) Disconnect Powertrain Control Module (PCM) connectors. PC\
M
is mounted in right side of firewall. Connect jumper wire between
ground and PCM connector C3 terminal No. 30 (Violet/Brown wire). Using\
scan tool, select SYSTEM MONITORS. Perform CCD BUS test. If scan tool
does not display BUS SHORT TO GROUND, repair open Violet/Brown wire.
If scan tool displays BUS SHORT TO GROUND, go to next step.
2) Move jumper wire to PCM connector C3 terminal No. 28

full load. The Kent-Moore J-39021 is such a tool, though there are
others. The Kent-Moore costs around $240 at the time of this writing
and works on many different manufacturer's systems.
The second method is to use a lab scope. Remember, a lab
scope allows you to see the regular operation of a circuit in real
time. If an injector is having an short or intermittent short, the lab
scope will show it.
Checking Available Voltage At the Injector
Verifying a fuel injector has the proper voltage to operate
correctly is good diagnostic technique. Finding an open circuit on the
feed circuit like a broken wire or connector is an accurate check with
a DVOM. Unfortunately, finding an intermittent or excessive resistance
problem with a DVOM is unreliable.
Let's explore this drawback. Remember that a voltage drop due
to excessive resistance will only occur when a circuit is operating?
Since the injector circuit is only operating for a few milliseconds at
a time, a DVOM will only see a potential fault for a few milliseconds.
The remaining 90+% of the time the unloaded injector circuit will show
normal battery voltage.
Since DVOMs update their display roughly two to five times a
second, all measurements in between are averaged. Because a potential
voltage drop is visible for such a small amount of time, it gets
"averaged out", causing you to miss it.
Only a DVOM that has a "min-max" function that checks EVERY
MILLISECOND will catch this fault consistently (if used in that mode).\
The Fluke 87 among others has this capability.
A "min-max" DVOM with a lower frequency of checking (100
millisecond) can miss the fault because it will probably check when
the injector is not on. This is especially true with current
controlled driver circuits. The Fluke 88, among others fall into this
category.
Outside of using a Fluke 87 (or equivalent) in the 1 mS "min-\
max" mode, the only way to catch a voltage drop fault is with a lab
scope. You will be able to see a voltage drop as it happens.
One final note. It is important to be aware that an injector
circuit with a solenoid resistor will always show a voltage drop when
the circuit is energized. This is somewhat obvious and normal; it is a
designed-in voltage drop. What can be unexpected is what we already
covered--a voltage drop disappears when the circuit is unloaded. The
unloaded injector circuit will show normal battery voltage at the
injector. Remember this and do not get confused.
Checking Injector On-Time With Built-In Function
Several DVOMs have a feature that allows them to measure
injector on-time (mS pulse width). While they are accurate and fast to\
hookup, they have three limitations you should be aware of:
* They only work on voltage controlled injector drivers (e.g
"Saturated Switch"), NOT on current controlled injector
drivers (e.g. "Peak & Hold").
* A few unusual conditions can cause inaccurate readings.
* Varying engine speeds can result in inaccurate readings.
Regarding the first limitation, DVOMs need a well-defined
injector pulse in order to determine when the injector turns ON and
OFF. Voltage controlled drivers provide this because of their simple
switch-like operation. They completely close the circuit for the
entire duration of the pulse. This is easy for the DVOM to interpret.
The other type of driver, the current controlled type, start
off well by completely closing the circuit (until the injector pintle
opens), but then they throttle back the voltage/current for the
duration of the pulse. The DVOM understands the beginning of the pulse

severe weakness that we will look at later). If an injector has a
fault where it occasionally skips a pulse, the meter registers it and
the reading changes accordingly.
Let's go back to figuring out dwell/duty readings by using
injector on-time specification. This is not generally practical, but
we will cover it for completeness. You NEED to know three things:
* Injector mS on-time specification.
* Engine RPM when specification is valid.
* How many times the injectors fire per crankshaft revolution.
The first two are self-explanatory. The last one may require
some research into whether it is a bank-fire type that injects every
360
of crankshaft rotation, a bank-fire that injects every 720, or
an SFI that injects every 720. Many manufacturers do not release this
data so you may have to figure it out yourself with a frequency meter.
Here are the four complete steps to convert millisecond on-
time:
1) Determine the injector pulse width and RPM it was obtained
at. Let's say the specification is for one millisecond of on-time at a
hot idle of 600 RPM.
2) Determine injector firing method for the complete 4 stroke
cycle. Let's say this is a 360
bank-fired, meaning an injector fires
each and every crankshaft revolution.
3) Determine how many times the injector will fire at the
specified engine speed (600 RPM) in a fixed time period. We will use
100 milliseconds because it is easy to use.
Six hundred crankshaft Revolutions Per Minute (RPM) divided
by 60 seconds equals 10 revolutions per second.
Multiplying 10 times .100 yields one; the crankshaft turns
one time in 100 milliseconds. With exactly one crankshaft rotation in
100 milliseconds, we know that the injector fires exactly one time.
4) Determine the ratio of injector on-time vs. off-time in
the fixed time period, then figure duty cycle and/or dwell. The
injector fires one time for a total of one millisecond in any given
100 millisecond period.
One hundred minus one equals 99. We have a 99% duty cycle. If
we wanted to know the dwell (on 6 cylinder scale), multiple 99% times
.6; this equals 59.4
dwell.
Weaknesses of Dwell/Duty Meter
The weaknesses are significant. First, there is no one-to-one
correspondence to actual mS on-time. No manufacturer releases
dwell/duty data, and it is time-consuming to convert the mS on-time
readings. Besides, there can be a large degree of error because the
conversion forces you to assume that the injector(s) are always firing\
at the same rate for the same period of time. This can be a dangerous
assumption.
Second, all level of detail is lost in the averaging process.
This is the primary weakness. You cannot see the details you need to
make a confident diagnosis.
Here is one example. Imagine a vehicle that has a faulty
injector driver that occasionally skips an injector pulse. Every
skipped pulse means that that cylinder does not fire, thus unburned O2
gets pushed into the exhaust and passes the O2 sensor. The O2 sensor
indicates lean, so the computer fattens up the mixture to compensate
for the supposed "lean" condition.
A connected dwell/duty meter would see the fattened pulse
width but would also see the skipped pulses. It would tally both and
likely come back with a reading that indicated the "pulse width" was
within specification because the rich mixture and missing pulses
offset each other.
This situation is not a far-fetched scenario. Some early GM