Possible lift pump problem???

[Nilao]

Does the relay complete the circuit to ground? I thought one of the terminals should always be hot and one only hot when the ecm commands the pump on. If he has no power to the supply terminal then I would think there is a supply issue.

[tree98]

(06-13-2017 )Nilao Wrote:  Does the relay complete the circuit to ground? I thought one of the terminals should always be hot and one only hot when the ecm commands the pump on. If he has no power to the supply terminal then I would think there is a supply issue.

Exactly what I was thinking

[Hammerhead]

(06-13-2017 )tree98 Wrote:  I checked the wiring today, I'm getting 12V to pin 85 of the relay for about a minute after I turn the key on then it goes out. No power to any other pins. Shouldn't there be a constant 12V to one of the relay pins?

Sorry, I didn't read your post properly.
Quickserve procedure 005-045 Fuel Lift Pump for diagnostics.

Yes, there should be a "Supply Line" (constant +12v), a "Signal Line" (timed +12v), and a "Return Line" (to chassis ground). The other two are the circuit to and from the lift pump and fuse circuit itself.
If you ONLY have +12v power to pin #85, then YES you have a power supply issue. Trace and check the wiring harness for damage.

Here is the wiring diagram and the connector for you.

[tree98]

So now do I just start cutting all the wire loom off the harness looking for a broken wire? Could I just run a 12V constant from the battery box and call it a day?

[Hammerhead]

(06-14-2017 )tree98 Wrote:  So now do I just start cutting all the wire loom off the harness looking for a broken wire? Could I just run a 12V constant from the battery box and call it a day?

The loom is "split loom", or should be, so you can peel it away. Unless the loom is damaged, the likelihood of internal wire damage inside is quite low. Follow it along looking for obvious damage. The problem is probably at a plug, a bend, or a tie/securement point. Checking the cab connection would be where I would start if you don't find any external loom damage.

Bypassing the wiring harness is a lazy way of fixing the issue. Let's just for prognostications say you do, and your truck starts on fire?
And the reason for the fire was a rub through of the main wiring harness on the power supply line and it starts melting the loom, that then ignites some oil residue and POOF, she's gone! All because you could have found a simple issue that is really a PITA to locate and fix?
I think if you put some honest effort into finding it, you will find the issue.

[Rawze]

(06-14-2017 )Hammerhead Wrote:  

(06-14-2017 )tree98 Wrote:  So now do I just start cutting all the wire loom off the harness looking for a broken wire? Could I just run a 12V constant from the battery box and call it a day?

The loom is "split loom", or should be, so you can peel it away. Unless the loom is damaged, the likelihood of internal wire damage inside is quite low. Follow it along looking for obvious damage. The problem is probably at a plug, a bend, or a tie/securement point. Checking the cab connection would be where I would start if you don't find any external loom damage.

Bypassing the wiring harness is a lazy way of fixing the issue. Let's just for prognostications say you do, and your truck starts on fire?
And the reason for the fire was a rub through of the main wiring harness on the power supply line and it starts melting the loom, that then ignites some oil residue and POOF, she's gone! All because you could have found a simple issue that is really a PITA to locate and fix?
I think if you put some honest effort into finding it, you will find the issue.

Very good answer. Personally, I would like to know the exact point of failure. EVERY TIME I have had a wiring issue where a wire failed,.. there were about 3 or 4 more in that same exact spot about to go too.

[Hammerhead]

(06-15-2017 )Rawze Wrote:  Very good answer. Personally, I would like to know the exact point of failure. EVERY TIME I have had a wiring issue where a wire failed,.. there were about 3 or 4 more in that same exact spot about to go too.

Thanks, and I hear ya on the "3 or 4 more about to go too", it's rare that only one wire is compromised.

I would like to know the point of failure as well. Particularly as he has stated in this thread...

(06-11-2017 )tree98 Wrote:  07 kenworth T800

...and I have an '09 KW T800. Sorry tree98, I missed that part earlier, maybe this will help point you in a direction.
Oh, and the whole "POOF there she goes" thing is real. T800's used to have a stigma of this from several that did burn. The harness is known to have issues on the drivers side in the rear of the engine lower/bottom of cab area.

[stevesparts]

When my lift pump stopped working on my 870 it was a plug just under the ecm, about the size of a pack of gum. One of the "teks" broke it and just stuck it together and it had come undone. i found it by accident when i set my had on it while stuck in a rest area in WY.
and that was after i had change out the pump( I had a spare one ) put some tape on it to hold it tight and all was good.
before it would work sometimes and not at other times. but after that it always came on like it should. hope that helps you out.

[Hammerhead]

(06-15-2017 )stevesparts Wrote:  When my lift pump stopped working on my 870 it was a plug just under the ecm, about the size of a pack of gum. One of the "teks" broke it and just stuck it together and it had come undone. i found it by accident when i set my had on it while stuck in a rest area in WY.
and that was after i had change out the pump( I had a spare one ) put some tape on it to hold it tight and all was good.
before it would work sometimes and not at other times. but after that it always came on like it should. hope that helps you out.

Sometimes it's the stupidest of things, ain't it?