PCM Signal Return - Ford Powerstroke Diesel Forum
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Old 05-23-2012, 04:24 PM
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Unhappy PCM Signal Return

My truck (2001 F350) has been having a "flickering" check engine light for a long time. Codes initially read P1316 and after buzz test read p1668. I have now replaced the IDM and am getting codes P1316, P1293, P1294, P1278, and P0269. My mechanic has chased the wiring from the IDM to the PCM and from the IDM to the injectors and has found no issue anywhere. Resistance on all 8 was between 3.0 and 3.9.

The real bizarre thing is the signal return wire is grounded out (reads continuity between the wire at the PCM block and a good ground). When I remove the PCM the wire does not show any continuity. We are thinking that perhaps the PCM is somehow internally faulted and is grounding that wire. When the truck is powered up (but not started) there is still continuity between that wire and ground. From what I am understanding that wire (grey/red) is responsible for putting out 5V (or is it 5.5V??) to a vast majority of the sensors on the drivetrain and should not be grounded. Wiring diagram shows it to be wire #91. Mechanic went so far to unplug EVERY sensor that that wire feeds and check continuity from the PCM to each sensor connector and they ALL check out.

Mechanic says if he was a betting man we would wager that replacing the PCM would make the light go away and make the engine run better that it has in a LONG time. He figures that if the PCM is not getting a signal return on that wire it is using "default" settings to operate. He is even astonished that the thing even starts let alone drives.

I am worried that dropping that kind of coin on a PCM that may or may not be the issue. I have notice that the “flickering” CES light is getting more frequent and more random.

As a side note there was also oil in my ICP and that is being changed out right now. I don’t think the oily ICP connector could/would cause the grounding of #91 but what does the Powerstroke community think? I am sorry for the long post but if this is the actual gremlin I have been chasing since I bought the truck then I cannot wait to get it wrapped up.

Last edited by Silvercrew; 05-23-2012 at 04:26 PM. Reason: Grammar and punctuation.
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Old 05-24-2012, 01:10 PM
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Follow up: Spoke to a PCM vendor and he said it is feasible for the PCM to be holding the ground on the signal return wire. He thinks the reason my truck is still running is due to the PHP chip over-riding the PCM programming. Of course my PCM happens to be a PMT1 which is one that is highly sought after making finding one in the wild all that more difficult. He is going to look through some inventory of re-sellers and see about getting one in 3-5 business days. Guess there goes the Memorial Day camping plans.

My one question still remains though. Has anyone ever heard of something like this?
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