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John 4/26/2010

1997 Chevrolet C3500 Silverado 8 Cyl 7.4L


Fuel Pump will not run. Not getting power

Will not start. The fuel pump is not running. Have removed fuel pump and it will run when applying power directly to the pump. Not getting power to fuel pump. Have replaced fuel pump relay in fuse box under hood.

1 Answer


HouseCallAuto 4/26/2010

Make sure that the fuse marked ECM B is not open. The PCM supplies a battery voltage signal to pin 85 (the relay is marked) when the ignition key is switched to on. The signal is there for two seconds and stops. You can check this by removing the relay and using a 12 volt automotive test light, ground the one end of the test light and hold the other end in pin 85 (this is a green / white wire if you were to look under the relay box where the wires come in to the bottom of the relay. This tests the ability of the PCM to turn the realy on. The output side of the relay supplies the same two seconds of battery voltage to the pump (when the relay is installed). There should also be battery voltage on the same pin 85 when you are cranking the engine to start it up. If you have the two seconds of battery voltage when the key is switched on but do not have battery voltage during cranking, that would mean that the relay is good but the PCM is not receiving ignition pulses from the crankshaft position sensor during cranking. Possible bad crank sensor) The PCM always supplies the two second long voltage signal when the key is switched on though. If that voltage signal is not present that would mean the PCM is no good. Bad PCM's are not common. It would be a good strategy to know if there is ignition spark also.


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