GM S10/S15/Sonoma Pick-Ups 1982-1993 Repair Manual

Diagnosis & Testing


Perform the following EST performance check and the ignition system check (earlier in this section) before attempting to diagnose EST system failures.


The EST system will usually set a Code 42 when a fault is detected in the system. Please refer to the ECM self-diagnostic information in section 4 of this manual for further information on how to retrieve diagnostic codes from the ECM memory.

Except 2.5L Engine

The ignition timing should change if the Set Timing connector is disengaged. To check EST operation, check and record the engine timing at 2000 rpm with Set Timing connector engaged as normal. Then, disengage the Set Timing connector and recheck the engine timing at 2000 rpm. If the EST system is operating, the timing should be different at the same rpm when checked with and without the timing connector engaged.

2.5L Engine

The ECM will set a specified timing valve when the ALDL diagnostic terminal is grounded. To check EST operation, first check and record ignition timing at 2000 rpm and the diagnostic terminal NOT grounded. Ground the diagnostic terminal of the ALDL connector ( Refer toself-diagnostics in Chapter 4 for more information on the ALDL) using a jumper wire and recheck the ignition timing. If the EST system is operating, ignition timing should be different at the same rpm when checked with and without the ALDL diagnostic connector grounded.


The 2.5L distributor is equipped with a Hall Effect switch which provides a voltage signal to the ECM to tell it which cylinder will fire next

  1. Disconnect and remove the Hall Effect switch from the distributor assembly.
  3. Noting the polarity marked on the switch, connect a 12 volt battery and a voltmeter as shown in the illustration.
  5. Insert a thin bladed tool against the magnet as shown.
  7. The voltmeter reading should be less than 0.5 volts WITHOUT the blade against the magnet. Replace the switch if readings are above 0.5 volts.
  9. With the blade against magnet, voltage readings should be within 0.5 volts of battery voltage. Replace the switch if not within specification.