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Wednesday, March 4, 2009

1994 GMC Sierra Pickup, Fuel Pump Inop

I have 1994 GMC Sierra, the fuel pump will not come on when key is turned on but will work when directly connected to battery. We have checked all fuses that we know of and they are good. If you pour gas into throttle body the engine will fire and run for a few seconds. We have been trying for a week now to figure out what may be wrong even replaced the computer and fuel pump relay can you help?


Sparky said...

The easiest place to test the system is at the fuel pump relay located on the firewall on the passenger side. I recommend using a voltmeter but a test light can be used on all terminals EXCEPT the dark green w/white wire. Using a test light on this wire may damage the computer. The orange wire should have 12 volts at all times. If there is no power on this wire check the fusible links under the plastic cover next to the fuel pump relay. The red wire will not have power or ground as it is the fuel pump test wire. The gray wire is the fuel pump wire and will only have power with the system working. It goes to the fuel pump fuse located next to the relay and then from there it goes to the fuel pump.The black w/white wire is the ground and it should have a constant ground on it. It should terminate at or near the thermostat housing. Check the wires there for looseness if you do not have a ground on it. The dark green w/white wire is the signal wire from the computer and should only be checked with a volt meter. There should be 12 volts on this wire for several seconds after the ignition is turned on. I would suspect that either the orange wire does not have power on it or the ground is faulty at the thermostat housing. If it were the relay or computer the engine should have stayed running once you had oil pressure from cranking the engine by pouring fuel in the throttle body. The oil pressure switch has a parallel control circuit for the fuel pump.


Unknown said...

Sparky, can you help me?! I have a 91 silverado 5.7 with TBI. For the past couple of months I have had symptoms that all pointed to my TPS. I had issues with my idol at a stop light where the truck wanted to die but would not come to that. However, in the past few weeks when I put it in reverse it would in fact die on me or get VERY close to dying. And then yesterday I was driving on the freeway at about 65 and it died! I haven't been able to start it since. I have replaced my TPS and nothing! I have checked for cracks in my Map's hose and other hoses and can't find any. If anyone has any suggestions I would GREATLY appreciate it. Thanks in advance.

Sparky said...

Hi John,

I will need a lot more info to help you. Please email me at Does the engine turn over fine? Does the starter not work? Can you hear the fuel pump coming on? Do you have spark? Do you have injector pulse? What kind of testing tools do you have?