P4 ACM Not Communicating

Dagan Williams 24 posts since Dec 12, 2019
P4 ACM Not Communicating Nov 3, 2020 1:48 PM
VIN# LE8526
PT MODULES INSTALLED- ACM, MCM, TCM, CPC, IPPC
This unit came in originally for truck shutting down intermittently with CEL and SEL illuminating, but were
unable to duplicate and not being warranty they took it to fix it themselves. Few weeks later came back with
same issue.
Codes (CPC) 520222/12, (MCM) 1231/9. Only the ACM not communicating all other modules are, have closely
inspected all harnesses, replaced bad battery draining system voltage, found ACM to have power, ground,
ignition, and data at module, installed test ACM and no fix, installed test CPC and still no fix, checked internal
resistance in ACM and CPC and both are 120 ohms.
Voltage readings back probed ACM while not communicating:
Battery Voltage- 12.10v
ACM Bat- 12.10v (all legs)
ACM IGN- 12.10v
ACM PTCAN- 3.10v Hi 2.00v Lo
All checked to ACM ground (all legs) and load tested.
PT can resistance with all modules connected ACM not communicating- 60ohms
Attached is a screenshot of codes, 2000/9 was created by unplugging CPC
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Jon Cecil 887 posts since Nov 25, 2014
Re: P4 ACM Not Communicating Nov 3, 2020 4:27 PM
Those voltages are not what I usually see on PT-CAN wiring. They might be caused by the ACM not
communicating though. Are you back probing the connector to get those voltage readings?
ghg17 ACM one box harness rub thru from lack of friction tape Intermittent 625/9 and 1231/9

Dagan Williams 24 posts since Dec 12, 2019
Re: P4 ACM Not Communicating Nov 3, 2020 4:33 PM
Those readings were taken from back probing at the ACM and back probing at the star point to verify those
readings across the entire CAN. This truck has and updated harness with tape and loom with no rub points in
it.

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P4 ACM Not Communicating

Jon Cecil 887 posts since Nov 25, 2014
Re: P4 ACM Not Communicating Nov 3, 2020 4:50 PM
Do you know what the customer tried repairing? This might narrow your search down.

Dagan Williams 24 posts since Dec 12, 2019
Re: P4 ACM Not Communicating Nov 4, 2020 12:38 PM
The bracket at the air compressor has been removed, but no signs if a repair being made or new tap being in
that area. the only thing I can see they've done is pulled the MCM 21cav loose and cut a short section of tape
out, but never taped back over it and no rubs in that area.

Chris Draft 3 posts since Sep 13, 2020
Re: P4 ACM Not Communicating Nov 4, 2020 12:44 AM
I believe your acm pt can voltage to be off. Is your resistance 120 ohms at the 21 pin connector at the acm?
Are you familiar with the wiring issues at the air compressor bracket. Usually you get pt can heartbeat codes
but I would remove the bracket off of the air compressor bracket and inspect there. There should be a recall for
that, it is so common. 54-330 is a service bulletin. I also use a headlight to load test my powers and grounds
there.

Dagan Williams 24 posts since Dec 12, 2019
Re: P4 ACM Not Communicating Nov 4, 2020 12:36 PM
Bracket has already been removed and harness inspected in that area and found no rubs, also used a head
light to load test ACM power, ground and ignition on each leg of power and ground while shaking down the
harness an watching for a flicker and all held load fine, ACM internal resistance from pin 10 & 20 on the 21cav
connector are 120 ohms.

Geoff Ambrose 13 posts since Jun 26, 2020
Re: P4 ACM Not Communicating Nov 4, 2020 2:18 PM
Remove the PT can star point connector and the ACM. Check the resistance of the ACM leg of Data link to
confirm you have the right one. Then load test that leg of the datalink with sealed beam headlamp. I have seen
multiple issues just like this where the wires were either shorted to ground intermittently, rubbed and green,
as well as poor crimps. including the 76 pin bulkhead connector pin crimped on to a wire that never had the
insulation removed. Hope you find it.

Dagan Williams 24 posts since Dec 12, 2019
Re: P4 ACM Not Communicating Nov 4, 2020 2:24 PM

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P4 ACM Not Communicating

Think you're right. Funny shows somewhat good voltage, at least nothing crazy off but when running an overlay
from the ACM to the starpoint it starts communicating. Somethings corrupting it just narrowing down what
section.

Geoff Ambrose 13 posts since Jun 26, 2020
Re: P4 ACM Not Communicating Nov 4, 2020 2:30 PM
Awesome, pay very close attention to the 76 pin. The pins like to push back often on BOTH sides of the
connector or bad crimps. Tell your old man it's time to start hanging Christmas lights.

Dagan Williams 24 posts since Dec 12, 2019
Re: P4 ACM Not Communicating Nov 5, 2020 12:38 PM
Pins in the bulkhead were all good. Got I narrowed down to between the 18cav above the one-box and the
bulkhead connector so when new terminals come in will bee c running an overlay for the CAN. I'll tell him
you're patiently waiting for them haha.

Kyle Siebert 4,101 posts since Nov 14, 2014
Re: P4 ACM Not Communicating Nov 4, 2020 4:09 PM
I don’t think it’s been mentioned. But check the two connectors at the top of the one box. For
me it’s always a PTCAN terminal lock broken when they jammed the green terminal secondary
lock in. This causes the terminal to float in and out of the connector about a 1/4”. Id be checking
the ignition and PTCAN terminals closely here on the vehicle side.
ATTACHED PHOTO. You can see the top row right side is for PTCAN. Has a broken terminal lock.
Could look fine when you remove the connector. That’s why a pull test needs to be done.
Terminal lock piece is missing so it was assembled that way.
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Kyle Siebert 4,101 posts since Nov 14, 2014
Re: P4 ACM Not Communicating Nov 4, 2020 4:11 PM
I’ve seen this about 5 times now.

Dagan Williams 24 posts since Dec 12, 2019
Re: P4 ACM Not Communicating Nov 5, 2020 12:35 PM
I suspected this issue there too, but I removed the green lock and tug tested as well as visually inspected the
locking tab and removed both CAN terminals and checked for a good crimp. Running a loose CAN overlay
from this connector to the bulkhead I was able to get the ACM communicating so waiting on terminals to install

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P4 ACM Not Communicating

the permanent overlay. It's doing some weird stuff I think there might be corrosion or something corrupting the
signal because whenever the temporary overlay was removed the ACM was still communicating, but struggling
to say online and intermittent PT CAN comm codes kept flashing active/inactive till I hooked the overlay back
up.

Dagan Williams 24 posts since Dec 12, 2019
Re: P4 ACM Not Communicating Nov 10, 2020 6:41 PM
UPDATE:
Ran permanent overlay for the PT-CAN from the bulkhead to the 18cav on top of the one-box and the Issue
never duplicated after that point. Truck was released a few days ago and has not returned.

Chaz Trimble 1,360 posts since Feb 2, 2015
Re: P4 ACM Not Communicating Nov 11, 2020 2:54 AM
Other good spots to check for are the bracket at the rear of the compressor and on top of the transmission.
Harness will be zip tied to the bracket on the low edge. Eventually standoffs will be used liberally. Glad to hear
you got it goin.

Vincent Goodwin 6 posts since Jun 26, 2020
Re: P4 ACM Not Communicating Nov 12, 2020 3:22 PM
also check PT can wiring near the bellhousing we find a lot of issues there.

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