Did you check the ground going to the the fuel pump rely from the pcm? I mentioned this before and didn't get an answer. If you have 12V on one side of the relay the pcm gives it a ground and the relay clicks on. No ground at the relay it doesn't click on. You can fold over some thin wire on the relay terminal and put back in for testing. One lead of the meter on the wire and the other on battery positive.

 

I stuck a prob into 11 and 12 of the pcm connector and then scratched the internal body of the truck for exposed metal, and each pin showed nothing on the multimeter (which means its good ground). Is that what you mean?

 

Yep, still, I would check where they actually bolt to the body, and give 'em a good cleaning.

 

You mean pin 85? Yes that is on the same circuit with pin 85 of the asd relay too, and continuity is showing good readings for ground. When truck is running, this pin then shows ~1volt (i think thats the pin) as opposed to 12v when the key is in ignition-only phase. But yes the pcm should do its thing, which then tells the relay what to do, so im unsure why it decides to sometimes work and not work. The pcm has that connector right above it for diagnostics (i think). Would a shop have a tool to plug into this and get any relevant readings? idk, doesnt seem there would be any readings since i already tested a pcm swap...

 

Im sure things got lost so here’s my recap: truck will intermittently have the FP (and ASD?) relay not send 12v out pin 87 to the source (in this case, fuel pump) . The issue is under the hood. When everything works as intended, the FP relay double clicks in ignition-only phase, and primes the pump for 1-2seconds. However, for an unknown reason, there will be no clicking sometimes, which means pin 87 will not send out 12v to the output source. I can wait 2-5seconds in between turning the key, or, I can do it rapidly...but...it makes no difference. In working order, I should be able to do both of those and have 100% success rate each time in terms of hearing the FP prime/have 12v go out through pin 87. That's the issue, here's where we are:

• Swapped out a new Crankshaft sensor, even though it isn't relevant in the pre-ignition phase in terms of the relays clicking or not. No change.
• I swapped on a working pcm from another 5.9L with the same serial number on it. Nothing changed, so I can rule out its not PCM related?
• Testing each pin on the ASD Relay when the relay double clicks as intended (ignition phase only): all pins show constant 12V, besides pin 87, which will only show a brief 12V upon turning they key.
• Testing each pin on the ASD Relay when the relay DOESN’T double click (ignition phase only): all pins still show 12V, besides pin 87, which didn’t even show anything at all, even upon turning the key. I had already knew that though, since the relay didn’t click.
• Testing each pin on the FP Relay when the relay double click as intended (ignition phase only): all pins show constant 12V, besides pin 87, which will only show a brief 12V upon turning they key.
• Testing each pin on the FP Relay when the relay DOESN’T double click (ignition phase only): all pins still show 12V, besides pin 87, which didn’t even show anything at all, even upon turning the key. I had already knew that though, since the relay didn’t click.
• Testing FP and ASD pins when the truck is running. All pins show 12-13v, besides pins 85, which now shows ~1V since the relays are making normal internal connections inorder for the truck to be running.
• Testing pin 85 of the relays. I put on probe on pin 85 and one probe on the female-side connector of pin 51 of the pcm, and it shows good readings. I tested pin 57 as well; still good readings for continuity
• Tested pcm pin 11 & 12 for good ground. One probe goes to connector pin, one connector went to exposed metal of the body. Showed normal results for both pins
• While someone held the multimeter when i did the continuity test, I wiggled wires around the underside of the fuse box, as well as those connectors mounted to the body on drivers side engine bay, right below the rubber rain divergent (the ones that carry the load of pcm wires). Lots of wiggling of wires and nothing changed on the multimeter’s screen

-When it is hot out (makes things soften such as wires and connectors) I will do the multimeter & wiggle test again, but I don’t expect anything.

-There's also a ground to the left of the power steering, bolted to the frame I think. Its solid, but I will take that off and clean it and reinstall it.


Any other input from anybody? Grounds to test, clean..etc? Sensors (well, those are irrelevant in this phase i think)? Idk..but i need to figure this out

 

I'll look for sure, but the pins showed good ground. Is this the big ground that is right beside/near the power steering? I will add that to my list. Anywhere else? I dont think my manual says where all the body's grounds go (well, or i couldnt find the right page)

 

Yeah, should be on one of the studs for the p/s bracket. There is another on the body, in front of the body, WAY down there..... but, that's mostly for interior stuff.

 

I don't have the diagram in front of me but the coil side of the relay(ground from pcm) And you need check for ground at the relay not continuity.