No fuel pump prime pulse

[Eds]

I am looking for suggestions on how to troubleshoot for a no fuel pump prime pulse issue. Up until now, the system has functioned flawlessly for nearly 10 years.

Recently I started experiencing an intermittent engine starting issue. At initial engine startup when the ignition switch was turned on, I would periodically not get a fuel pump prime pulse. The problem has now progressed to no prime pulse. I checked all of the hardware such as FP relay, wiring and the ECM as I have a known good spare. In doing this I also verified that the ECM is not producing the prime pulse signal.

The engine is a Chevy MPFI V6 being controlled by a 1227727 ECM, $A1. I suspect VATS in the program may in some way be contributing to the issue although the ECM does not have any provisions for VATS. The $A1 program does have fault codes for VATS, but they are all unchecked and therefore should be turned off.

Any thoughts on how to remedy the problem?

[WASyL]

If prime signal is not being sent then ECM/PCM is shot

Best regards

[dave w]

Most ECM's are programmed to turn on fuel pump relay for about 3 seconds, initial key on / engine off from the ignition switch. The ECM is looking for a Distributor Reference Pulse (DRP) to turn on the fuel pump relay after about 3 seconds. Typically the green wire with a white stripe at the fuel pump relay is the wire from the ECM that "turns on" the fuel pump relay.

Most OBD1 EFI system have an oil pressure switch that will turn on the fuel pump, once the engine has about 5 PSI of oil pressure.

I once did an experiment with removing the fuel pump relay to see if the engine would start and run? About 20 seconds of cranking the engine started and continued to run until the ignition switch was turned off.

If the vehicle was in my shop, I'd unplug the relay and test the green wire with white stripe to see if the computer sends 12v with ignition switch on / engine off. No 12v on fuel pump relay green wire with white stripe ignition on / engine off = no good computer.

[stew86MCSS396]

...don't dismiss a broken wire either.

[Eds]

If prime signal is not being sent then ECM/PCM is shot

Best regards

Thanks. You confirmed my thoughts.

[Eds]

Most ECM's are programmed to turn on fuel pump relay for about 3 seconds, initial key on / engine off from the ignition switch. The ECM is looking for a Distributor Reference Pulse (DRP) to turn on the fuel pump relay after about 3 seconds. Typically the green wire with a white stripe at the fuel pump relay is the wire from the ECM that "turns on" the fuel pump relay.

Most OBD1 EFI system have an oil pressure switch that will turn on the fuel pump, once the engine has about 5 PSI of oil pressure.

I once did an experiment with removing the fuel pump relay to see if the engine would start and run? About 20 seconds of cranking the engine started and continued to run until the ignition switch was turned off.

If the vehicle was in my shop, I'd unplug the relay and test the green wire with white stripe to see if the computer sends 12v with ignition switch on / engine off. No 12v on fuel pump relay green wire with white stripe ignition on / engine off = no good computer.

You confirmed my understanding as to how the system function. Thank You.

[Eds]

...don't dismiss a broken wire either.

Your thoughts were the same as mine and where I began trouble shooting. Thanks

[tayto]

the ECMs can be repaired, it is a common issue and usually just a transistor that pops. I had a '8746 repaired by a local electronics place many years ago.