Expert: Roland Finston - 1/16/2012

Question
My 98 Caravan with 3.3L won't start.  Key in and turn to RUN, only oil light and Air Bag light illuminate.  No MIL. Turn key to Start, only single relay click under hood.  No starter activity.  Battery is fully charged.  Prior to this, the vehicle stalled while running, and the ASD and Fuel relays were chattering rapidly, even with the key off and removed.  While this was happening, the vehicle wouldn't start, but the starter would engage and disengage rapidly in sync with the chattering relays and flashing MIL.
A code scanner connected to the OBD port says "vehicle not responding".  Code from Instrument Cluster test is "940".  I have performed detailed electrical tests at various points so far, and don't find any faults, except that the PCM seems unresponsive.  I have verified voltage drop and continuity of ground connections at engine rear and below battery tray.  I have verified +12V from ignition RUN/START circuit to cavity 20 on PCM, and B+ 12V to pin 46 of PCM.  Also verified Ground to pin 50 and pin 10. Verified +5V at pin 61, and +8 (reading 8.9V) at pin 44. Also have verified ignition switch outputs in all positions at rear of switch while connected.  Have disconnected battery, disconnected and reconnected both PCM plugs, checked and tested all fuses and relays, and verified +12V at several circuits beyond the Power Distribution Center.  I have verified +12V to the Starter Relay on the Power Distribution Center when the key is in the START position, but the ground from the PCM is open. Also, when the key is on, the ground to the ASD and Fuel relays is open on the PCM side, but the relays have +12V to the other side of the coils.

I have access to vehicle wiring diagrams and schematics, but do not have troubleshooting flowchart to verify the PCM is functioning.  Is there a way to validate the PCM out of the vehicle? or is there a flowchart for verifying proper operation to rule-out the PCM?  I'm not sure where to look at the moment, without knowing the internal operation of the PCM, such as what requirements must be met before it pulls-down the Start Relay ground and the ASD and Fuel relays...

Any help would be appreciated.

Answer
The 940 code says that the cluster is not getting digital data from the pcm. The starter relay will close (pcm grounds the actuation coil when the P/N switch is closed as is sensed as such by the pcm. Similarly the ASD and fuel pump will activate for about 1 second by the pcm's action and then open up unless you then start cranking over the engine and the pcm sees pulsing from tha cam and crank sensors and so then grounds the relay coils of the ASD and fuel pump. So check to see if you hear the fuel pump run for a second or not when you turn the key to 'run' position.
Ideally if you could get fault codes readout from the PCM you would expect a 0600 or 0601 if the pcm were bad. The ignition key oscillation technique to get fault codes isn't available on the '98 or '99 minivan model. So a plug in reader may be the only way to see if you have one of those two pcm failure codes or not. The MIL not coming on is suspicious.
The chattering relays is not a good sign of pcm function, but there is no way to know for sure without a code readout.
I hope this has proven to be helpful.
Roland