5v parasitic load on 12v circuits from 5.0 PCM

[kenlaird]

Troubleshooting a gremlin in my 5.0 EFI harness... with motor off, 12v Run circuits are measuring 5 volts with battery connected. PCM relay is off. Looking for 1) why PCM has 5v output / what pin # could activate it? And, 2) likely intersections with 12v run circuits in the harness. The VREF line (a brown/white wire on pin 26) feeds several items with 5v.

Anybody fight something like this before? Suggested approach(es)? Thanks in advance!

 

[ared77]

Can't answer your question exactly, but would this be the +5VDC for memory, etc?

 

[kenlaird]

No, it is 5 volts for devices like the TPS (throttle position sensor) that uses a low voltage reference signal. I don't believe the PCM should be activated with the key off. There is KAPWR (keep alive power) on pin #1 that is 12v.

 

[EPB72]

https://www.supermotors.net/getfile/892730/original/eecconnectors.jpg Heres some connector pin outs.. I'd check all powers and grounds,,,and is this a new setup to you and has the harness been reworked crossed up wiring at harness connectors??

 

[DirtDonk]

Could that be one of the things that happens when you use a manual computer with an automatic harness, and vice versa?
There is a change in pin locations for manual vs auto, so just wondering if that's the case. I believe it's a ground, and can burn out a computer if it's run that way, but not sure what symptoms it would show otherwise.

Paul

 

[ared77]

I don't know much about it (as you can tell from my first answer%)) but after some looking into it, I do see the KAPWR is right off the +12 battery terminal, and I also suspect like others above said it probably is crossed connectors, wrong auto/manual harness or a couple of pinched wires. Anyway, I have this book if you need a picture of anything in particular.

 

[kenlaird]

Very helpful, EPB72. Thanks a million. I have a Cobra motor and I am pretty sure it is a '94. So your diagram covers things very well. The harness in it was not a full replacement; it's a hybrid with the old '73 harness and it has been a mess. I'd like to avoid the expense of replacing it and even if I ponied up the money for it, I'm somewhat afraid that I would have to do modifications to make it work fully. If I could just find all of the PO's bad splices and connections...%)

 

[kenlaird]

Thank you, ared77, I hope to find that book somewhere. I have Chilton's manual for Mustangs and Explorers and they have helped quite a bit. I think I'm just going to have to do some serious debugging and buzzing out under the hood and behind the dash.

 

[Broncobowsher]

It is possible the computer has an internal issue where the switched power to the internal 5V regulator is simply stuck on. Once you have ruled out the external factors, next would be internal to the ECM.

 

[kenlaird]

I'd say that is possible, Broncobowsher, but I will assume wiring connections are messed up for now given past experience. I have had this issue since I bought the vehicle. It was an automatic (C4) then and has since been converted to a manual (NP435). The problem didn't change from that (to address DirtDonk's comment). Wheeling it has flushed out some other wiring problems. I will keep cleaning it up as investigate the 5v problem.

 

[kenlaird]

Update: found that the run or start power leads (pins 37 and 57 on the PCM) for the PCM were connected to a hot at all times lead. I can easily fix that, if I can find it. Tracing further back, I found that there is a constantly hot lead that should not be hot that is tied to my single oxygen sensor (for its heater circuit) and the power leads for injectors for cylinders 5 to 8. That same hot lead was the culprit for the previously mentioned PCM power lines. It seems to be buried in back of the engine on the driver's side with the injector rails, under the intake manifold. Now for the climbing and contortion aspect of the job...

 

[EPB72]

A relay for your switched power inputs to pcm would be ideal.. also may consider just pulling intake plenum to gain better access for repair ...if you like i could get you a wiring diagram and text to you ..tomarrow.. let me know year of diagram and #to send to..

 

[kenlaird]

Update: I found the wire causing the issue - there was a hot lead connected to what I believe should have been the ground for the oxygen sensor circuit. Who knows what the PO was thinking or getting for info when that got hooked up. Now I will be pretty much doing what EPEB72 suggested, using the PCM power relay to switch the power into the PCM and the other circuits that need power for run and start modes. Many thanks to all who have contributed!