1992 Isuzu Trooper code 42

EFI -1992 Isuzu Trooper

ELECTRONIC SPARK TIMING (EST)
Spark advance curve is totally controlled by Electronic Spark Timing (EST) circuitry of Electronic Control Module (ECM). No vacuum or centrifugal advance mechanisms are used in this system. ECM monitors crankshaft position, engine RPM, engine load, atmospheric (barometric) and manifold pressure, engine detonation, engine temperature and transmission gear position. ECM uses this data to compute desired spark advance.
When engine is cranking, ignition module sends a reference (RPM) signal to ECM on reference wire of 4-wire EST connector. When engine speed exceeds 450 RPM, ECM applies 5 volts on by-pass wire to switch ignition timing control to ECM.

NOTE: Reference signal also triggers fuel injection system. If ECM does not receive a reference signal during cranking, vehicle will not start.
ECM expects to see no voltage on EST line when timing is controlled by ignition module (less than 450 RPM). If ECM senses voltage on this line, it will set a Code 42 and it will not go into EST mode. While system is running on ignition module, signal on EST wire will be grounded through ignition module. When engine speed is greater than 450 RPM, voltage will be applied to by-pass wire. EST voltage will vary as EST is no longer grounded to ignition module.
If by-pass wire is open or grounded, ignition module will not go into EST mode. Instead, EST voltage will be low and a Code 42 will set. If EST line is grounded, ignition module will switch to EST but, there will be no EST signal and Code 42 will set. If by-pass signal is missing because of ECM failure or open circuits, engine will run at base timing plus a small amount of RPM advance built into ignition module.

CHECK EGR VALVE
remove and clean make sure its not sticking open
also check coolant temp sensor
good luck

By any chance do you have the pin outs for the ECM and the ignition module? Also, do you think the crank angle sensor could be a problem here? How do we test the coolant temp sensor apart from the needle reads cold when cold and hot when hot. I am aware of the est system but need some procedures to diagnose it.

Thanks Rob

Check EGR
coolant sensor
you can use an ohm meter for the test
or scanner tool to read the values
I will try and get you the wiring

Ok cool and thanks for the quick response times