P0748 Allison 5sd (3rd GEN TCM) NSBU (12 Pin)

james902

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Feb 9, 2021
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Hi Guys!

I've been chasing SUBJECT issue on a Customer's Truck for weeks now.

2006 Chevy C6500 7.8L (LG4), Miles 250k. Allison 2500HS LCT

New TCM via Allison (previous CRASHED wouldn't even COMM during Bench Test with Allison DOC)
Reman VB via Allison (Entire assembly, Internal Harness, Solenoids, Etc) SOL-A was replace in old VB. Then we decided to replace entire VB given same Labor etc.
New NSBU via Allison
Harness TCM to TRANS has been re-ran (new wire and PINS)

Before ALL the above was R&R Customer would get intermittent Latched faults. With SEVERAL Shops attempting to correct, to no avail.

Now I have it (it's a buddy of mine).

Checked all ING 0, B+ and GNDs. Load tested them to 45A, all passed.

DTC throws (90% of the time) cold start, when taking TRANS out of Neutral and into Reverse or any FWD Gears. KOEO & KOER. On Scan data, I can replicate TRIM SOL-A Ref from .09A to .02 when DTC is thrown. If I leave the truck running and clear DTC it stays off until I leave Neutral. I began to think there was a conflict on the NSBU to the TCM (why I replaced it) because when I unplug the NSBU Connector and back in a few times, I can then run the gear KOEO and watch Trim SOL-A Ref follow Trim SOL-A Actual. When they agree, I can literally drive it with NO problems and it will NEVER throw a DTC!!! I have tried to recreate it and cannot.

However, when I park the truck for say 6hrs+ and go back next morning, same story as above. I assume it to be heat/wiring related. OR Bad SOL-A from Allison out of the BOX. I have it happen a lot but wouldn't expect it from Allison. I have NOT checked Resistance yet across Trim-A. I will tomorrow.

Thanks for any insight folks!!!
 

james902

New member
Feb 9, 2021
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Update

Sol-A Resistance w/in SPEC A 5.5ohms

Did find NSBU GND was ran into J-2 PIN 20 (Analog GND). GM Calls out for NSBU GND Circuit 251 to be ran directly to GND-Zone 2A (Frame), Not into TCM. Changed tht.
 

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