bnold

Ok noticed fuel pressure was too low so I check out the voltage going to my LP. Was 8.88V . Thought this was weird. Thought maybe the LP was bad and was causing some sort of short pulling voltage down to 8.88. Pulled the wiring from the pump still 8.88 at the plug no load. I spliced in a direct 12V from the battery to the LP and low and behold she fires right up bring Fuel Pressure up to 14psi. What is causing the voltage at the plug to only be 8.88?? Fuse, relay, what could it be. Truck is down and its my DD... Please shed your insight...

Thanks
Brian

bnold

Anybody?? Could I just run new wiring using a relay directly from the battery?? Where could I pull signal from?? Something that is only on when engine is on?? What size fuse for the LP??

infidel

I'm not 24 valve expert but thought the voltage at the pump was limited by the PCM until after the engine starts because higher pressures make for harder starts.
If you have 8.8 while running then you have a problem

bnold

Huh well when I do the bump starter to build pressure trick the pump doesnt run. If I run 12v directly to the pump it runs like a champ. Doesn't run period on 8.8V

bnold

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Rauschbo

If you start it using 12V, and remove this jumper wire, what does the original source wire then read, does it go up to 12V or stay at 8.8? If it stays low, you misght trace it back to where the voltage drops, perhaps at the relay?

bnold

Will try that this morning thanks for the response

XLR8R

Strange... I wouldn't think a few psi difference in the LP output should affect the hundreds or thousands of psi the VP44 develops?

Kemble

I wonder how much of a pressure difference there is at 8V compared to 12V ?

4x4dually

Is your lift pump moved back to frame or is it in the factory position? Sounds like a wire has rubbed next to something metal that is creating a voltage divider between the pump and ground. Check the wiring for shorts or melted spots. It would either be a short or the output of the ECM or whatever controls the pump. Sorry I'm just learning about the pumps myself, but there could be a short that pulls just enought current to drop the voltage and not blow a fuse. I have an electrical diagram for the 2001 truck if that would help you trouble shoot, let me know. PM me with your email and I'll send you the pages when I can. Good luck.

4x4dually

Just read the trouble shooting section for your pump, everything is about PSI, nothing about testing voltage or what it should be. I'll keep looking and find the electrical diagram.

4x4dually

Here's the diagram for your pump. To my knowledge, the PSI is controlled by duty cycle, not voltage for starting and junk. I can't find anything about what the ECM voltage should be, but I'd bet it will be 12V. Anything less would burn up the pump. I just got back from lunch and needed something to do for a minute. Hope this helps.

Geico266

I'm sure infedel is right. The voltage prior to start up drops or quits all togeather. Check the PSI only, voltage will drive you nuts.

bnold

I found the culprit... After reading these posts and some other threads I found out about the ECM doing its pulsing thing to keep pressure down at startup. That would explain why my cheapo volt meter I am using couldn't keep up with the pulsing and only read 8.88.

Still had the problem of why the pump wouldn't run off of the harness. I have recently installed a vulcan kit that relocated it down by the tank. I traced through the harness and found a break inside the wire itself?? Not the connections, positive of that. actually cut them all off and tested again. If I put the smallest amount of pulling force on the wire the continuity was lost.

I ran a new harness and secured everything well. Seems to be working fine now. Pressure at idle is around 14-15psi. Thanks to all that gave their insight, we will see if any other problems arise...

4x4dually

Good to hear you got it! Dang wires.