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Fuel Pump Relay Not Engaging

spf72

By spf72
in Ask A GM Technician

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spf72 Enthusiast

spf72

Posted
Posted

I am new to this forum and have spent some time doing some research using the search function - while there are a lot of posts on fuel pump issues I have not been able to fix mine so here goes.

 

Vehicle is a 2000 with a 4.8 Vortec

 

I replaced my fuel pump and now -

No power coming from the relay - so no prime

changed relay (twice)

11.8 volts on power post of relay

ground post of relay tests good

Pump comes right on and truck runs when I use a jump wire in place of relay

Re-did ground at tank

 

Relay not puling in - this usally means bad ground - am I missing one?

Tried disconnecting battery to reset ecm

 

Suggestions???

 

Thanks

spf72 Enthusiast

spf72

Posted
  • Author
Posted
How did you jump across the relay ckt...did you go from a good B+ to the 87 term on the relay output or pin 30 to 87?

 

 

I went from 30 to 87 - also checked the grpund side and it's ok. It appears the issue is from the pcm either not sending a signal or a bad wire between the two - now just need to find the pcm!!

minibike132 Senior Enthusiast

minibike132

Posted
Posted

The pcm is under the hood, left hand side just behind the radiator (above the steering gear box) and you want connector 2 (the red one)terminal 9 dark green and white wire. There are no other connections in the circuit just the pcm, rear of fuse block and relay.

spf72 Enthusiast

spf72

Posted
  • Author
Posted

Thanks, I traced the wire back from the relay and have no power where it feeds the bottom of the fuse block when I turn the key over. I had read that the oil pressure switch feeds power after initial priming? I see no reference to this in my schematic.

 

If the oil switch is not part then does that mean the PCM is bad? As I said the truck runs fine if I jump out the relay so nothing else appears to be bad on the pcm. How do I check the pcm?

 

I did pull off the pcm electrical connector and everything appears fine but it is pretty hard to see much. For now I will do a continuity test on the wire - hard to believe only that wire somehow broke as it is in a bundle from the pcm to the relay.

Fortune50 Enthusiast

Fortune50

Posted
Posted
Thanks, I traced the wire back from the relay and have no power where it feeds the bottom of the fuse block when I turn the key over. I had read that the oil pressure switch feeds power after initial priming? I see no reference to this in my schematic.

 

If the oil switch is not part then does that mean the PCM is bad? As I said the truck runs fine if I jump out the relay so nothing else appears to be bad on the pcm. How do I check the pcm?

 

I did pull off the pcm electrical connector and everything appears fine but it is pretty hard to see much. For now I will do a continuity test on the wire - hard to believe only that wire somehow broke as it is in a bundle from the pcm to the relay.

Did you perform a continuity test on that wire? Was it bad? If it's not broken, then there may be a bad connection at the PCM, can you backprobe the connector at the PCM w/ the harness connected to it? It's possible I suppose that somehow the driver in the PCM for that circuit got damaged and only a new PCM will fix it...let us know what you find out.

spf72 Enthusiast

spf72

Posted
  • Author
Posted

Thanks to sparkstech I now know the prom on the pcm and I can do a continuity check - there are several green and white wires and I don't have detail on which is which.

 

Can the drivers be re-programed in the pcm? I know when you buy a new one that they need to program the vin data into it. Can the old one be reprogrammed and load the driver? Probably not worth it as they want $70 just for the program but I was wondering.

spf72 Enthusiast

spf72

Posted
  • Author
Posted

And do backprobe do not stick anything into the face of the connector.

 

 

May be a stupid question but when you say pull the gray cover off - I had "un-bolted" the conector before which did me no good. Does the top cover snap off? Thanks

minibike132 Senior Enthusiast

minibike132

Posted
Posted
May be a stupid question but when you say pull the gray cover off - I had "un-bolted" the conector before which did me no good. Does the top cover snap off? Thanks

 

If this comes up 3 times I appologize for some reason my postings are not working. I'm not sure what you mean by this did you no good, were you able to remove the connector from the pcm? The cover just snaps off there should be tabs in all four corners that snap in, possibly some in the middle too.

spf72 Enthusiast

spf72

Posted
  • Author
Posted

What I meant by doing no good was that the entire conector unplugged but I could not see where the wires went. The connector looked great with no signs of corrosion but I didn't know what pin to check. That is why I asked about the cover. It appeared it would un-snap but I didn't want to break it.

 

Anyway, I am trying it this morning and will let you know. Also, can this pcm be "re-programmed" or is it best just to replace it?

spf72 Enthusiast

spf72

Posted
  • Author
Posted

Brought the pcm to NAPA and had it tested before I bought a new one - old one tested fine except for a fuel pump code so I had them re-set it. Pluged it in and - no difference.

 

I can have them "re-flash" my pcm and try it again or put in a new one. Issue is my current one cheks out fine so I hate to buy something I don't need and I am not sure what flashing the old one will accomplish. Any ideas what drives the pcm fuel pump output??? My scematic doesn't show it. Hate to replace the pcm and then find out it is some other loose wire somewhere.

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