Diagnosing ECU/Computer - '93 Caprice

[Pantaz]

My 1993 Caprice Classic (305 engine) died and will no longer start. I drove to a market just two miles from home, did some quick shopping, back in the car and I was just about to leave the parking lot. No hesitation, no warning, the engine just shut off. It acted like it stalled, so I put it in neutral and cranked it. Would not start. I stopped, put it in Park and cranked it. It has not started since. Yes, it has good gas -- I filled the tank a week ago, and it's only down to half.

The "Check Engine" light did not come on (it does illuminate during ignition on test). I have a cheap code scanner, Actron CP9001. It returns only Code 12 -- "OK".

I started to check the basics -- air/fuel/spark -- and noticed the throttle butterflies are dry after cranking the starter. I would expect to find some fuel residue. So, I decided to check the fuel pump relay.

The relay never energizes. The test-point next to it shows only 7 volts. The relay is marked "12 volts", so I pulled it and tested it directly on the battery. Works fine. I reinstalled it and connected a volt meter to the relay coil. Zero volts.

So... I'm guessing there is a problem with the ECU. However, I can imagine there may be other factors preventing the fuel relay to energize.

I really hate to have the car towed to Chevy for the privilege of paying $$$ just to discover it's a $90 part.

Any thoughts, recommendations, ideas?

[RABarrett]

It is necessary to check the relay first. Does it cycle briefly when the key is turned on? If the engine does not start after cranking long enough to raise oil pressure, there are other problems involved. Code 12 is not an ok signal. It is an expected code to indicate no tach signal to the ECM, which is expected when the engine is not running. I have repaired too many check engine lights with no code stored by troubleshooting the code 12 and finding a bad connection, defective module etc, all related to the tach signal. Code 12 only means that the ECM is only lacking a tach signal. It is not an all clear. Ray