PCM breakout help
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Senior User
Joined: Mar 2003
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From: Western Kentucky
PCM breakout help
My TC is not engaging. I had it tested at a transmission shop and he said the transmission is good, it is not getting a signal to lock the TC. He suggested I put a breakout box on my truck. I have one on it now, but not sure what I am looking for. I think pin 54 is the TC and it is showing 12+ volts. Anyone know what I should be looking for or how to test?
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Senior User
Joined: Mar 2003
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From: Western Kentucky
Found my TC pins and checked OK. I can drive my truck and ground the v/y wire and the TC locks, so my wiring is good. Must be a sensor somewhere. I have already replaced the brake kickout switch on the master cylinder and the sensor on the pedal. My speedometer is working OK, but the VSS is also a possibility. My cruise is also not work, so they are probably related.
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Senior User
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From: Western Kentucky
Found that my PCM thinks my brakes are applied. I can unplug the brake switches and it doesn't change. I can put an Ohm meter on the brake pressure switch and hit the brakes and watch the voltage change. My brake lights all work fine. So, I am down to tracing wires. Not sure what else to check.
Post Fiend
Joined: May 2009
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From: Pea Ridge, AR
Greg, IIRC, did you have an issue with losing the GPR signal from PCM? Apologies if it wasn't you.
Do you happen to have LED tail lights? There use to be issues with TC lockup without load resistors.
If you are viewing live data through the PID on the PCM, are you seeing brakes applied I assume even if they are not? Could be as simple as the pressure switch. I do not have the manual with me to know correct voltages for the switch.
Do you happen to have LED tail lights? There use to be issues with TC lockup without load resistors.
If you are viewing live data through the PID on the PCM, are you seeing brakes applied I assume even if they are not? Could be as simple as the pressure switch. I do not have the manual with me to know correct voltages for the switch.
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From: Western Kentucky
No GPR signal problem. No LED tail lights. I have add on cab lights. I also have 4 & 7 prong trailer plugs on my bumper and in my bed for my gooseneck. I didn't think about unplugged those to see if they could be shorted.
I replaced the brake pressure switch when this started.
I replaced the brake pressure switch when this started.
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From: Western Kentucky
I went to the parts store and they sold me one that was supposed to be right, but the new one came with a pigtail to connect the round switch to the rectangular plug on the truck.
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Pulled my brake controller and trailer lights without any change. So tomorrow I am going to put the breakout box back on the PCM and test the brake input to the PCM. I will then start tracing wires. I must have a short somewhere.
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Thread Starter
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Senior User
Joined: Mar 2003
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From: Western Kentucky
Pulled the PCM and plugged the breakout box in while scanning the ODB II port. Brakes showing off. Plugged the PCM back in the truck, brakes showing on. Pulled it out again and brakes showing off. Plugged it in the truck again this time being very careful that the plugged went in even. Brakes showing off. Went for a test ride and TC worked perfect and cruise worked again. After is sit for about an hour, they have quit working again. I know it is in the connection with the PCM. I have pulled the wiring harness pretty good looking for a short, but none found. Could it be just a bad PCM? It is just a year old. Just old enough for the warranty to run out.
Post Fiend
Joined: Nov 2007
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From: Payson, AZ
Someone posted about having a loose connector pin in a PCM. The pin would move back instead of fully seating when the PCM connector was pused on. Pin 31 is for the brake pressure switch. See if that pin moves when you press on it. Worth a check.
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From: Western Kentucky
Actually pin 91 is the one I am dealing with. I checked the pin when it was out and it seemed tight. I won't get a chance to pull it again until this weekend. It may get to the point it will be easier to just put another PCM in it.
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From: Western Kentucky
Had to go to town last night to buy an alternator for my wife's van. I drove my truck and the TC and cruised worked perfectly. I hope it continues, but I suspect the PCM probably has some intermittent problem since this all started when I replaced it last year.
Post Fiend
Joined: Nov 2007
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From: Payson, AZ
Glad all is working now. If problem returns check every pin for any movement. There might be more then one that is loose.