Need help on c8500 not starting

[Mike_IUOE]

I have a 04 GMC c8500 dump truck that I use around home. It was running great, I shut it off while I was loading it and now it cranks but wont start. Code reader show no faults, the check engine light comes on and goes out when key is turned. I was thinking fuel pressure but dont have a pressure gauge for it so I removed both lines coming off of the fuel tank, and do not get any fuel coming out of either line when the key is turned or while cranking. I dont have a wiring diagram but I pulled the connector off of the pump and I do not have power on any of the wires when the key is turned or while cranking. I checked the fuses in the under hood block and swapped the relays and still no power.

I want to make sure I'm on the right track here and there isnt some sensor or something that could cause the no power going to the fuel pump. Also, any good source to find a wiring diagram? I tried following the wires but I lose it up in the frame under the cab.

 

[mekanik]

What engine does it have?

 

[Mike_IUOE]

It has a Cat C7

 

[mekanik]

Can you find a gauge some where and plumb it into the fuel system.
It needs 65 PSI to start. If there are no engine lights coming on its probably not an electrical problem.

 

[Mike_IUOE]

It has no pressure at all. I disconnected both lines at the pump in the fuel tank. When I turn the key or crank it there is no fuel coming out of the pump, so the pump is not coming on. I checked for voltage at the pump connector and there is nothing. All the fuses were good and I swapped the relay. I followed the wiring back and found 1 connector and I don’t have voltage there either. After that I cant follow the harness

 

[mekanik]

I did not realize there was an electric pump. I had one in a Freightliner M2 it
had a gear pump mounted on the back of the high pressure oil pump on the left near the front of the engine.
The connector at the tank wouldn't be for the fuel gauge?

 

[Mike_IUOE]

This has a pump in the top of the tank that looks exactly like the pump in the GM pickups. It’s got 4 wires going to it so I’m sure at least one of them is for the gauge.

 

[Truck Shop]

Really it's better to have a new relay to swap in place-Relays get tired from heat, could end up swapping bad for bad. Your sure there is no power distribution box under the dash?

 

[DMiller]

One wire ground, one gauge the other two are dedicated pump. Should be a Oil Pressure switch to keep it running and a start side bump relay for a 10-20second starting run until oil pressure switch takes over.

 

[Mike_IUOE]

One wire ground, one gauge the other two are dedicated pump. Should be a Oil Pressure switch to keep it running and a start side bump relay for a 10-20second starting run until oil pressure switch takes over.

would that relay be in the under hood relay box? I didn’t see any fuse or relay box in the cab.

 

[Mike_IUOE]

Really it's better to have a new relay to swap in place-Relays get tired from heat, could end up swapping bad for bad. Your sure there is no power distribution box under the dash?

I swapped with the horn relay and the horn was working. It’s broke down in the middle of a field so I had to work with what I had. I didn’t see any fuse box under the dash.

 

[DMiller]

Can be nearly anywhere
Could be a circuit off the ecm.

 

[Old Doug]

Really it's better to have a new relay to swap in place-Relays get tired from heat, could end up swapping bad for bad. Your sure there is no power distribution box under the dash?

I cut the cover off a relay so i could push the points together for a test very handy i would like to build a relay with lights for a tester.

 

[Birken Vogt]

In industrial controls many relays of the same size and shape as automotive can be had with both lights and a lever so you can push the points together without taking them apart.

I wonder if there is one made with these features for the automotive style.

 

[Mike_IUOE]

I cut the cover off a relay so i could push the points together for a test very handy i would like to build a relay with lights for a tester.

I believe they make test relays similar to what you're describing. They have a rocker switch to turn them on. The Mac Tools guy around here carries them