Technical Tip: Checking Ignition System
Application: 1998-2002 GM Trucks and Vans with 4.3L, 5.0L, 5.7L
and 7.4L engines
Symptom: No Start, No Spark, Stalls.
Theory: The Ignition Systems on these trucks use a Distributor,
Control Module, Crank Position Sensor, Cam Position Sensor and
Coil. There is no way to adjust the timing on this system. Twisting
the Distributor will only result in throwing off the Cam Position
adjustment. The only purpose of the Distributor now is to send
secondary voltage to the individual cylinders and house the Cam
Position Sensor. The Cam and Crank Position Sensors send
square-wave signals to the PCM. The PCM then sends a pulsed
signal to the Control Module, which triggers the negative side of the
- Cam Position Sensor: Inside Distributor.
- Crank Position Sensor: Front Engine Cover, passenger side.
- Control Module: Rear of Engine, passenger side.
- Coil: Rear of Engine, near Control Module.
The Test: First, check for both spark and injector pulse. If you are
missing both, go to Cam and Crank sensor diagnosis in the next
If you are only missing spark, start here. The Control Module has
a four-wire connector. Use a diagram for your vehicle as wire colors
sometimes vary. Terminal-A should have battery voltage key on and
cranking. Terminal-B is the control signal from the PCM and should
have a square-wave pattern with the engine cranking over. If it
see Cam and CrankSensor diagnosis in the next section. Terminal-C
should be a ground at all times. Terminal-D goes to the negative side
of the Coil and should have a primary ignition pattern with the engine
cranking. If you have the square wave on terminal-B but no primary
control on terminal-D, the Control Module is most likely defective. Be
sure to check the Coil before replacing the Module in case that?s what
killed the original one. Primary resistance should be 0.1 ohms to 0.5
ohms and secondary should be 5,000 ohms to 25,000 ohms. If you had
a good signal on terminal-D, check for battery voltage on the Coil
positive terminal. If this checks out, suspect a faulty Coil, although
would still be a good idea to check Coil resistance to be sure.
If you are missing both spark and injector pulse or had no signal
on Module terminal-B in the above tests, we?ll need to check
the Cam and Crank Position Sensors. To check the Cam Sensor,
watch the pattern on terminal-B of the sensor connector using a
Vantage or Modis with the engine cranking. This should be a clean
square-wave and show approximately 3hz to 5hz at normal cranking
speed. Terminal-C should have battery voltage and terminal-B should
be a good ground. To check the Crank Sensor, watch the pattern on
terminal-C of the sensor connector using a Vantage or Modis while
cranking the engine over. Again, this should be a clean square-wave
and show approximately 30hz to 40hz at normal cranking speed.
Terminal-A should have battery voltage and terminal-B should be a
good ground. If both sensors check out but there is still no signal on
Module terminal-B, the PCM will need further diagnosis, as it may be
The Fix: Replace any parts found defective in the above tests. Note
that if the Cam Sensor or Distributor has been replaced or disturbed,
you will need to perform a Cam Sensor Adjustment. To do this, use a
Scanner or Modis and find the ?CAM RETARD? value on display.
Specifications are ?2° to +2°. If yours is outside this range with the
engine running, loosen the Distributor clamp and twist it very
Since this value only updates on the display after the engine exceeds
1000rpm, you will need to rev the engine after each adjustment move
of the distributor. Once you get the CAM RETARD value within the
?2° to +2° range, lock the distributor down.
The Technical Tip is intended for professional automotive technicians
Please do not attempt this tip at home. Do not attempt this tip if you
a professionally trained and certified automotive mechanic.