Major Electrical gremlin
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Elder User
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Trying to read the diagram correctly. Diagram 27-3 Engine controls, shows the pcm relay sending power out 87 on the relay as it should, then passing threw a fuse on its way to the pcm. But it also shows it splitting off before the fuse and going to the idm relay and in pin 86 on that relay. It then shows the pcm sending power out pin 80 as it should to pin 85 on the idm relay???????? that makes both 85 and 86 positive? One has to be negative to trip the relay??? It then shows battery to idm relay pin 30 and power when switch out pin 87 to the idm. How can all 4 terminals be positive???? Starting plug search now.
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Elder User
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Well, back to square 1. I found which plug is c270h, and found pin 4 on it. It has no signal. So. Went back to the pcm harness and checked it there. Nothing. Were did it go. It had power at one point. Now I'm back to the pcm not sending a signal. Only on this truck. Help!
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Elder User
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I cant figure this drawing out. it shows all the relays being positive all the time on all terminals. How can that work? I now cannot reproduce the power from pin 80 to the relay. Its gone. I don't know what happened, but I can't reproduce it. So now what? Two dead pcms. but only in this truck. How can that be. What a mess.
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Not knowing were to go. I'm just testing the harness for now to make sure its ok. The pin 80 wire is good all the way from pcm to fuse box plug, so is the power wire for the idm, from the fuse box to the idm. They tested good. No power, just good continuity no leaks. I need to figure out why the pcm will not send those signals to turn on fuel pump relay, idm relay, which I think are connected to the pcm relay some how. I'm so confused because I was always told that a relay is tripped by a pos and neg charging a magnet to close the points and send the power threw. But these show on the drawing that they are positive on all poles. I don't understand that, cant comprehend how it works. That said the idm relay has power to pole 30 only. rest are dead. pcm relay fires and powers pcm, but does not trip idm relay and fuel pump relay like the drawing says it does, I think. Its one of the positive on the idm relay, the other is pin 80 that's not working either. Apparently they collide in the relay coil and trip the relay some how??? Two positives colliding usually means a fire. But ok, drawing says so.
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Yes, And no. It is "ground switched" So, the power runs from source, through the load, and to the PCM where, when it is activated, its done by the PCM closing the Circuit.
So, you can have power all the way to the PCM. But, you need to ensure it's signaling closed. The easiest way is to Back-Pin, Use a Low Amp Clamp or run through your meter.
Also, I will use a Electromagnetic Compass and witness the movement when someone power the circuit up. It should pint to ground (-) when off and (+) when energized. Lust separate the wire from the bundle and hold it on the wire.
Logistics Pro
Joined: Aug 2009
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From: Arizona
Yes, And no. It is "ground switched" So, the power runs from source, through the load, and to the PCM where, when it is activated, its done by the PCM closing the Circuit.
So, you can have power all the way to the PCM. But, you need to ensure it's signaling closed. The easiest way is to Back-Pin, Use a Low Amp Clamp or run through your meter.
Also, I will use a Electromagnetic Compass and witness the movement when someone power the circuit up. It should pint to ground (-) when off and (+) when energized. Lust separate the wire from the bundle and hold it on the wire.
So, you can have power all the way to the PCM. But, you need to ensure it's signaling closed. The easiest way is to Back-Pin, Use a Low Amp Clamp or run through your meter.
Also, I will use a Electromagnetic Compass and witness the movement when someone power the circuit up. It should pint to ground (-) when off and (+) when energized. Lust separate the wire from the bundle and hold it on the wire.
In that case, since he already swapped the PCM and it worked in another truck I am thinking the PCM is not receiving what it needs to do the job. He said there is no power at IDM relay pin 86 so I am wondering about fuse #22.
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Elder User
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checking that fuse 22 again. Omg. what you said is a foreign language to me. I don't know what any of that means.
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Elder User
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Had the pcm plugged in for about 15 minutes while I was trying to get signal measures from pin 80. It got pretty hot so its getting power. Also I tested all the power to the pcm plug pins, they all have power. There are a few of them. all have power as the drawing says they should. All the grounds work too.
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I havn't probed pins 15 and 16 on the pcm plug with the pcm plugged in and key on. They should be the communications positive and negative, and only work if the pcm is plugged in. I think they should be powered. That's my next test. Maybe it needs to power some sort of communication to start sending signals?? Don't know. Guessing. Totally in the dark. Thomas seems to know his stuff, but I can't understand what hes saying. He must be too smart for me. Big words and stuff. I'm lost.
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Elder User
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don't know if fuse 22 has power. all I know is the pcm gets power threw several pins, and warms up while I'm testing. I'll take out fuse 22 to see if power goes to the terminals.. I will do the pcm relay with my test relay. Back in a few with results. Thanks for helping guys. I felt pretty alone for awhile.