Major Electrical gremlin
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Elder User
Joined: Aug 2015
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No, pin 80, the white black wire is the signal from the pcm to close the IDM relay. To me that means pin 80 should be a ground, but I have 11 volts there. 2 theorys on that. One, fuse box crossed inside charging that pin at the relay and probably back feeding into the pcm. Two, Pin 80 does have a charge, and the pcm closes the IDM relay by cutting the power on pin 80??? Both seem equally possible, but one means a bad pcm which is unlikely because both would be bad then, and the same issue. Its odd to me that the PCM relay has a ground, which is that diode by the way, but the idm relay does not have a ground? That's why I'm leaning toward a melted fuse box. All guesses for now until I know for sure the pcms are good. Ill check the wires too. All 50 million of them. Yikes.
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Elder User
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Umm... No. I think its obvious I know nothing about vehicle wiring. So I assume your cutting me up because I reply a lot. Sorry to offend, and exceed any limits. But I really need my truck for work, and its got a serious gremlin. So I apologize to everyone, I was just trying to get help. I'll refrain from posting until I have the solution. Guess I'm on my own.
Logistics Pro
Joined: Aug 2009
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From: Arizona
Umm... No. I think its obvious I know nothing about vehicle wiring. So I assume your cutting me up because I reply a lot. Sorry to offend, and exceed any limits. But I really need my truck for work, and its got a serious gremlin. So I apologize to everyone, I was just trying to get help. I'll refrain from posting until I have the solution. Guess I'm on my own.
An issue like this can happen to any of us, especially as these trucks get older. I have a lot of interest in your issue because by following and trying to help only helps me and anyone else following this become more educated on our trucks and how they work. You are not offending ANYONE by posting, there are no limits and I assure you there are plenty of people lurking and reading every post to see how this comes out so DO NOT stop posting. This is a BROTHERHOOD and we help each other out.
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Elder User
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The wires are the white/black wire from the pcm to the idm relay. It is the signal wire to turn on the idm. I'm not sure if its positive or negative, but mine is positive at the idm relay terminal. I would have thought it would need to be negative. That said I have a possible clue. I will double check my findings after work. What I've found is I have power to 3 of the 4 terminals on the idm relay. Pin 80 off the pcm is the wh/bk wire. When I was testing that wire. Its dead. But at the fuse box. Its hot on that terminal of the IDM relay. Again this supports my theory and hope the box is the bad link. I will double check again, but Im sure that wire was dead, and the relay was hot???Not only that, but I probed the wire on the back of the fuse box plug. Pin 4 on the c270 plug. So it cant be the pcm sending that power. And also there must be no feed back like I suspected earlier. Not sure how that can be unless that wire is broken. But it has continuity.
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Ok so I have confirmed, pcm relay terminal 87 does function and sends power to terminal 85 on the idm relay. I confirmed by squeezing a bare wire in with the relay on terminal 87 of the pcm relay and cycling the key on and off. Powers key on, Off key off. So good there. Here is what I think is the big find. Terminal 86 on the idm relay has power as soon as you turn on the key. That terminal is directly attached to the pcm via pin 80 on the pcm. It is the idm enable line. This line goes from pin 80 on the pcm to pin 4 on plug c270h on the back of the fuse panel, then to terminal 86 on the idm relay via the fuse box. I have just confirmed that key on I have 11 volts on that terminal 86 of the idm relay, BUT THERE ARE "0" VOLTS ON PIN 4 OF PLUG C270H, AND AT THE PCM PIN 80. So inside the fuse box the connection must be lost? Correct?
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Ok so I have confirmed, pcm relay terminal 87 does function and sends power to terminal 85 on the idm relay. I confirmed by squeezing a bare wire in with the relay on terminal 87 of the pcm relay and cycling the key on and off. Powers key on, Off key off. So good there. Here is what I think is the big find. Terminal 86 on the idm relay has power as soon as you turn on the key. That terminal is directly attached to the pcm via pin 80 on the pcm. It is the idm enable line. This line goes from pin 80 on the pcm to pin 4 on plug c270h on the back of the fuse panel, then to terminal 86 on the idm relay via the fuse box. I have just confirmed that key on I have 11 volts on that terminal 86 of the idm relay, BUT THERE ARE "0" VOLTS ON PIN 4 OF PLUG C270H, AND AT THE PCM PIN 80. So inside the fuse box the connection must be lost? Correct?
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Elder User
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I did confirm it with DMM. My fuse box is not loaded down though. Its all stock except for the fog lamp circuit cancel delete. My alarm is attached to the wires on the key block, but it uses next to nothing to operate.
Logistics Pro
Joined: Aug 2009
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From: Arizona
It sounds like that is it, which makes sense because from all the other measurements you have everything you need to energize that relay. You can further check the line by setting your meter to ohms or continuity and putting one lead at the PCM end of the signal wire and putting the other end at the relay terminal to see if it is open.
So before you verified continuity of the line to the pcm earlier but not all the way thru the fuse box to the relay terminal right?
So before you verified continuity of the line to the pcm earlier but not all the way thru the fuse box to the relay terminal right?
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Elder User
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That is correct. The relay and plug are about an inch apart inside the fuse box, It never even crossed my mind that I was missing that inch. I did check the wh/bk wire from pcm to plug, and it is fine. I havnt found a bad wire yet. So my fingers are crossed that putting in a new fuse box in starts it up. I'm trying to track down a fuse box from a wrecker, but might have to go to ford. What do you think they want for one?
Logistics Pro
Joined: Aug 2009
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From: Arizona
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