P1690 and VP44 Wiring Questions

[niesenjb]

Hi guys, I’ve been troubleshooting my P1690 code using Mopar1973man’s forum and found myself stuck on step 7. The procedure says to check resistance between FPCM pin 8 and ground. My problem is that i’m not seeing any continuity here. I do however have good resistance from FPCM pin 8 to ECM pin 18 (which was an earlier step in the procedure). Looking through the wiring diagrams I don’t see how FPCM pin 8 gets ground. Should I really be getting ground from pin 8? Any input  for where this ground gets picked up is appreciated.

[niesenjb]

Giving this a bump. Hopefully someone has an answer 

[IBMobile]

On 8/26/2024 at 9:03 AM, niesenjb said:

I don’t see how FPCM pin 8 gets ground.

It's not supposed to be grounded.    That test is to see if the wire is shorted to ground.

[Mopar1973Man]

Basically you need to have both ends open then ohm test from pin to pin and greater than 5 ohms is a fail. Second test is pin to ground with both unplugged. There should be infinite ohms to pass but if you see ohms then you have a short to ground.

[niesenjb]

Thanks that makes a lot more sense now. Is it safe to assume my VP is bad? I’ve done all the tests and switched the ECM with another. My symptoms are intermittent low power. If I clear the codes the truck will run fine until the CEL comes back and the truck goes back to having limited power, however it always starts and runs fine in general. I also have code P0602 but I think that’s because I haven’t gotten the new ECM programmed yet. That code never showed up the the original ECM.

 

Not sure if this is related, but the truck does have another issue regarding ABS. ABS and brake light stay on while driving. When I try to read the ABS codes with my scanner it’s says connection lost and the truck will throw NO BUS on the cluster aswell as a P1698 code. This only happens after I attempt to read the codes.

[Mopar1973Man]

Special tools are required to read ABS codes. 2nd gen truck you need a DRBIII tools or equivalent tool to read those codes.

[Doubletrouble]

 Did the abs/brake light come on after you swapped the ecm? If so it could be that the ecm your using now doesn't have the proper tire size and gear ratio programmed in it. Mine did this when I first bought the truck, a buddy has the snap on fancy scanner and was able to change these settings. Been fine ever since.

[niesenjb]

Opening this back up because i’m still having issues. Since the last post i’ve found and repaired a couple wires coming from the FPCM harness. Only change is now my CEL stays on all the time, but the symptoms are still intermittent. 

 

Was driving today and it seems like it might be getting worse. Occasionally when the truck is acting up and has “low power” I would keep my foot in the pedal and the power would come back to normal while accelerating. I could then drive normally for a few minutes and it would go back to low power.

 

I had someone look at the truck for a few days and apparently they looked at the rpm signals from the cmp and vp44 and they matched fine.