Need help diagnosing electrical problem with fuel system...

2

24vCTD

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Jul 12, 2008
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Went to start my truck up this morning and didn't hear the FASS system running or see pressure on my FP gauge so I didn't crank it over. Bumped the key, nothing. Checked the inline fuse for the FASS system; it wasn't blown but the plastic on it was melted (I've had that fuse blow once or twice but have never been able to find the cause???). Replaced the fuse with a fresh one, nothing. I rigged up a toggle switch on the FASS system relay plug between the 2 terminals that normally go to the N.O. relay switch contacts, turned the switch on, and the FASS started running. Tested the relay; it clicked when I applied 12v to the coil terminals and I had continuity through the appropriate switch teminals. Plugged that relay back in then went into the Power Distribution block and pulled the fuel system relay in there and connected my toggle switch to the terminals in the block that go to the N.O. contacts on the relay...flipped the switch and heard something click in the injection pump so apparently this relay has nothing to do with the lift pump circuit. I've run out of places to look for the problem; can anybody explain to me where the stock lift pump/FASS system relay coil get their power from? Does this mean my PCM is shot? The wiring diagram in my Haynes book apparently doesn't cover the entire lift pump system so if anybody can provide more diagrams that would be outstanding. If it comes down to it for reliability's sake I'll permanently install a toggle switch in lieu of the relay to turn the FASS system on & off manually--I've already had to replace my VP44 once and that was certainly enough. BTW, I always check my gauges before shutting the key off and I was showing 12 psi fuel pressure so whatever quit working did it while the truck was parked overnight. Thanks in advance for any suggestions or diagrams you guys can help me out with...
 
The wiring harness for the FASS that goes to the battery should have a relay inline. You might check that out.
 
Hang on i just noticed you have a 95gph not sure if they wire up the same way as the 150 fass.
 
jktcummins said:
The wiring harness for the FASS that goes to the battery should have a relay inline. You might check that out.
Click to expand...

That was the first of the 2 relays I checked and it's fine. Applied 24v to it directly from the battery and checked continuity through the N/O contacts and it was good. Also wired a toggle switch between the switched terminals on the relay socket and FASS ran when the switch was turned on so I'm thinking the problem has to be upstream from the relay, i.e. whatever it gets its power from. PCM? or is there something in between that I could check?

Thanks
 
So your thinking that that its not getting juice from the power side. Or could the two wires that activate the pump the hooked up to the original lift pump might not be turning the pump on????? I just looked on the diagram and the 2 wires that went to the original lift pump one goes straight to the computer Yellow/White and the other goes to the Cummins BUS Black/white. NOt sure what that one is. You might check and see if you are getting any juice from the yellow/white wire. It says its a Transfer pump power comming from pin 35 on the ecm
 
Check what jkt said, the lift pump driver circuit in the ECM has been known to fail. If it goes kaput you will not get the signal voltage to activate the relays, be really easy to check, just hook the meter up to the positive side of the activation circuit and bump the starter.
 
Well, I put both the relays back in their sockets and lo and behold, dang thing is working again! I hate trying to troubleshoot intermittent problems...
 
Might have been a bad connection, and I am with you on intermittent electrical faults :bang :bang
 
i had this problem with a customers truck but i put the fass pump right to battery to run all time, and the truck still didnt start i found out it was a bad cam sensor after further diagnosing.
 
richsdieselperf said:
i had this problem with a customers truck but i put the fass pump right to battery to run all time, and the truck still didnt start i found out it was a bad cam sensor after further diagnosing.
Click to expand...

My truck runs just fine--it's the FASS system that wasn't working so I just didn't want to start it and smoke my VP44. I'm just thinking of running the FASS from a manual switch rather than relying on the (apparently unreliable) pump driver in the ECM.

Anybody here know about what a new ECM costs? That's probably going to have a huge influence on my decision...I spent about $2600 last year on my FASS and SHOVP44 when the stock lift pump and VP44 took a dump, and now I need a new dashboard for this thing too so I'm fed up with having to dump all this money into a babied truck that's only 7 years old...I'd rather be spending money on my 21-year-old Buick GN--at least most of the money spent on it so far goes to performance and appearance improvements rather than fixing actual problems all the time...:bang
 
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