During startup sequence with ignition switch in run the "Check Engine" light fails to illuminate. The fuel pump also does not energize.
Problem was intermittent but is becoming worse.
My initial guess as to the the cause of grief was the ignition switch.
Vehicle does not have a security system.
What debugging steps should be performed to isolate this grief?
Also of note: the brake light switch has been sticking. This switch is used by the PCM for the cruise control functions. Probably no direct effect upon start up.
I have the functional schematic for the PCM which includes the supply voltages for the sensors.
It appears that in "open loop mode" the MAP, TPS and ECT are used by the PCM for determining basic fuel strategies.
I had a TPS failure on my 87 4.0L XJ resulting in a no start condition. I do not recall if it impacted the PCM setting the check engine light.
The ignition switch does not appear to be suspect, both the fuel and ASD relays have 12v on coil when switch is in run.
TPS and MAP have 5V. IAT and ECT are also reading 10k.
I disconnected all of the above individually without any change in behaviour. I also pulled the coil, distributor and CPS connections with same result.
Observed the tachometer needle bouncing while checking the sensors. This is driven off the PCM which appears to be prime suspect at this point. OEM part number is 56027
896. The check engine light is also directly connected to the PCM.
Checked coil and injectors for voltage with key in run, all good. Confirming ASD circuit is live.
I can pick up a replacement from ECM Depot for $90 delivered with 30- day warranty.
Replaced the PCM and the ZJ started on first crank.
Also note that the cruise control is functioning much better. Prior to swap setting speed took 2 or 3 presses on the set button. The PCM is also the cruise control on the 94 ZJ.
I highly recommend ECM Depot,
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