MalibuRacing

iTrader: (1)

What I have:
2004 4.8L
Factory Harness/PCM modified by Jesse at Wait4Me Perf.

Problem:
The fuel pump activation wire from the PCM seems to be putting out 12v as it should when read with a DMM. It does not seem to have the current capability to activate the relay. When the relay is connected to the wire, it will not activate the relay.

The engine fires right up and runs great when I activate the relay with IGN 12v to "hot wire" it. So, it's not the relay. I've checked all grounds, they're good. I even tried running a ground wire from the battery negative post to the PCM housing and to the negative side of the relay coil. It still won't activate the relay with the PCM. I even tried a "factory" relay from the underhood fuse block that I didn't use. Still no go.

Is there anything that would prevent the PCM from being able to drive the relay (other than a bad PCM)? No signal on a certain wire to the PCM? What is required for the PCM to activate the fuel pump circuit?

I'm no expert at automotive wiring, but I have been an electronic technician for 20+ years, so I have a pretty good grasp of this stuff. I'm really leaning toward a bad PCM, but sure would like to be able to verify someway before purchasing another one from Jesse.

Thanks,

oange ss

iTrader: (50)

i thought it was a ground signal ?

stealth71

iTrader: (15)

It's power, fan is ground.

Unless it's different on a 4.8L.

sandking

iTrader: (7)

Have you contacted Jesse? He should be able to tell you the problem.

jfg455

iTrader: (5)

Load test your fuel pump control circut wire. It should light up a head light bulb (like a sealed beam) or about 6 amps of current. If it doesn't then you have possibly a broken wire inside of the insulation. Usually you will have a couple of strands holding on. Enough to light a test light or ohm meter but you get a current drop when loaded with the relay. Also check and see if any of the terminal ends are spread open. People like to shove a test light lead in there and it opens up the connector and it won't make enough contact to trip the relay. If the circut checks out ok then it is the PCM.

Old Kid

Does the 4.8 truck harness have anything like a vats system or other inputs to the pcm that might effect the fuel pump output signal?

MalibuRacing

iTrader: (1)

Yeah, he was helpful. I tried the things he suggested and that's what leads me to believe that it is the PCM. But, I will try what the previous poster said and trace the wire back to the PCM to make sure it isn't damaged, etc. If the wire checks out OK, I guess I will get another PCM from Jesse. I don't feel like it would be safe for a daily driver (that my son will be driving) to connect the fuel pump to a 12v source that is connected to ignition (key on).

MalibuRacing

iTrader: (1)

Yep, it's in the PCM. I'm assuming that it's been properly disabled, because the engine runs great as long as I provide the voltage to the fuel pump.