GM Cavalier/Sunbird/Skyhawk/Firenza 1982-1994

Diagnosis and Testing


Diagnosis and testing procedures in this section should be used in conjunction with those in Driveability & Emissions Controls and Fuel System of this repair guide. This will enable you to diagnose problems involving all components controlled by the computer control module.

An accurate diagnosis is the first step to problem solution and repair. For several of the following steps, a HEI spark tester, tool ST 125, to ground. Use of this tool is recommended, as there is more control of the high energy spark and less chance of being shocked. If a tachometer is connected to the TACH terminal on the distributor, disconnect it before proceeding with this test.

The symptoms of a defective components within the HEI system are exactly the same as those you would encounter in a conventional system. Some of these symptoms could be:

Hard or no starting
Rough idle
Poor fuel economy
Engine misses under load or while accelerating

If you suspect a problem in your ignition system, there are certain preliminary checks which you should carry out before you begin to check the electronic portions of the system. First, it is extremely important to make sure the vehicle battery is in a good state of charge. A defective or poorly charged battery will cause the various components of the ignition system to read incorrectly when they are being tested. Second, make sure all wiring connections are clean and tight, not only at the battery, but also at the distributor cap, ignition coil and at the computer control module.

The quickest and easiest test of the ignition system is to check the secondary ignition circuit first (check for spark). If the secondary circuit checks out properly, then the engine condition is probably not the fault of the ignition system. To check the secondary ignition circuit, perform a simple spark test. Remove one of the plug wires and insert a spark tester. If a normal spark occurs, then the problem is most likely not in the ignition system. Check for fuel system problems, or fouled spark plugs.

For further diagnosis, please refer to component testing found later in this section.


  1. Check for spark at the spark plugs by attaching the HEI spark tester, tool ST 125, to one of the plug wires, grounding the HEI spark tester on the engine and cranking the starter.
  3. Check fuel system, plug wires, and spark plugs. If spark is present, the HEI system is good.
  5. If no spark occurs from EST distributor, disconnect the 4 terminal EST connector and recheck for spark. If spark is present, EST system service check should be performed.


See Figure 1

  1. Remove the rotor and pick-up coil leads from the module.
  3. Using an ohmmeter, attach one lead to the distributor base and the second lead to one of the pick-up coil terminals of the connector.
  5. The reading should be infinite. If not, the pick-up coil is defective.
  7. Attach both leads of the ohmmeter to the pick-up terminal ends of the connector.

Click image to see an enlarged view

Fig. Fig. 1: When testing the pick-up coil, the reading should be a steady value from 500-1500 ohms. If not replace the pick-up coil

  1. The reading should be a steady value between 500-1500 ohms.
  3. If the reading is not within the specifications, the pick-up coil is defective.

While testing, flex the leads to determine if wire breaks are present under the wiring insulation.


Because of the complexity of the internal circuitry of the HEI/EST module, it is recommended the module be tested with an accurate module tester.


See Figure 2

  1. Remove the switch connectors from the switch.
  3. Connect a 12 volt battery and voltmeter to the switch. Note and follow the polarity markings.
  5. With a knife blade inserted straight down and against the magnet, the voltmeter should read within 0.5 volts of battery voltage. If not, the switch is defective.

Click image to see an enlarged view

Fig. Fig. 2: Testing the Hall Effect Switch; Note that not all distributors are equipped with this switch

  1. Without the knife blade inserted against the magnet, the voltmeter should read less than 0.5 volts. If not, the switch is defective.


The HEI (EST) distributor uses a modified ignition module. This module has seven or eight terminals instead of the four used without EST. Different terminal arrangements are used, depending upon engine application.

To properly control ignition/combustion timing, the ECM (or PCM) needs to know the following information:

Crankshaft position
Engine speed (rpm)
Engine load (manifold pressure or vacuum)
Atmospheric (barometric) pressure
Engine temperature
Transmission gear position (certain models)

The EST system consists of the distributor module, ECM and its connecting wires. The distributor has four wires from the HEI module connected to a four terminal connector, which mates with a four wire connector from the ECM.

These circuits perform the following functions:

  1. Distributor reference at terminal B . This provides the ECM with rpm and crankshaft position information.
  3. Reference ground at terminal D . This wire is grounded in the distributor and makes sure the ground circuit has no voltage drop, which could affect performance. If this circuit is open, it could cause poor performance.
  5. Bypass at terminal C . At approximately 400 rpm, the ECM applies 5 volts to this circuit to switch the spark timing control from the HEI module to the ECM. An open or grounded bypass circuit will set a Code 42 and the engine will run at base timing, plus a small amount of advance built into the HEI module.
  7. EST at terminal A . This triggers the HEI module. The ECM does not know what the actual timing is, but it does know when it gets its reference signal. It then advances or retards the spark timing from that point. Therefore, if the base timing is set incorrectly, the entire spark curve will be incorrect.

An open circuit in the EST circuit will set a Code 42 and cause the engine to run on the HEI module timing. This will cause poor performance and poor fuel economy. A ground may set a Code 42, but the engine will not run.

The ECM uses information from the MAP or VAC and coolant sensors, in addition to rpm, in order to calculate spark advance as follows:

Low MAP output voltage (high VAC sensor output voltage) would require MORE spark advance.
Cold engine would require MORE spark advance.
High MAP output voltage (low VAC sensor output voltage) would require LESS spark advance.
Hot engine would require LESS spark advance.

Detonation could be caused by low MAP output (high VAC sensor output), or high resistance in the coolant sensor circuit.

Poor performance could be caused by high MAP output (low VAC sensor output) or low resistance in the coolant sensor circuit. Refer to the individual component tests in Driveability & Emissions Controls .

When the systems is operating on the HEI module with no voltage in the bypass line, the HEI module grounds the EST signal. The ECM expects to sense no voltage on the EST line during this condition. If it senses voltage, it sets Code 42 and will not go into the EST mode.

When the rpm for EST is reached (approximately 400 rpm), the ECM applies 5 volts to the bypass line and the EST should no longer be grounded in the HEI module, so the EST voltage should be varying.

If the bypass line is open, the HEI module will not switch to the EST mode, so the EST voltage will be low and Code 42 will be set.

If the EST line is grounded, the HEI module will switch to the EST, but because the line is grounded, there will be no EST signal and the engine will not operate. A Code 42 may or may not be set.


The Electronic Spark Control (ESC) operates in conjunction with the Electronic Spark Timing (EST) system and modifies (retards) the spark advance when detonation occurs. The retard mode is held for approximately 20 seconds after which the spark control will again revert to the Electronic Spark Timing (EST) system. There are three basic components of the Electronic Spark Control (ESC) system.

The Electronic Spark Control (ESC) sensor detects the presence (or absence) and intensity of the detonation by the vibration characteristics of the engine. The output is an electrical signal that goes to the controller. A sensor failure would allow no spark retard.

The distributor is an HEI/EST unit with an electronic module, modified so it can respond to the ESC controller signal. This command is delayed when detonation is occurring, thus providing the level of spark retard required. The amount of spark retard is a function of the degree of detonation.

The Electronic Spark Control (ESC) controller processes the sensor signal into a command signal to the distributor, to adjust the spark timing. The process is continuous, so that the presence of detonation is monitored and controlled. The controller is a hard wired signal processor and amplifier which operates from 6-16 volts. Controller failure would be no ignition, no retard or full retard. The controller has no memory storage.

Should a Code 43 be set in the ECM memory, it would indicate that the ESC system retard signal has been sensed by the ECM for too long a period of time. When voltage at terminal L of the ECM is low, spark timing is retarded. Normal voltage in the non-retarded mode is approximately 7.5 volts or more.