Allison TCM shift inhibit
recently I worked on a 07 F750 with a cummins 5.9 paired with Allison 2000 series. symptom was intermittent no shift, and shift inhibit light comes on. According to the TCM the reason for shift inhibit was engine speed, so checked out a few things, upon little further investigation, i noticed when this happens all the TCM calibration value goes wonky, but all the live data values are good and it happens during certain ignition cycles only. So i checked all the usual stuff, power, ground, checked for voltage spike, connector pin.I believe when the calibration goes wonky it also changed the value for engine rpm limit for shifting, maybe it wont allow shifting above 300rpm. I couldn't see that actual calibration value, so Im just guessing, but my idle rpm was around 750 in the TCM the whole time. In the end it was a bad TCM, put a new one in everything is good.
Im just wondering if anyone run into similar situation like this? just seems strange to me that a bad module will only act up during certain ignition cycles, but not all the time.
this image was taken when the calibration was off. everything is off when this happens
Yes I have seen this problem before. another thing it will do is scramble the vin in tcm on some trucks. We service a tow truck fleet and I changed a lot of TCMs for this problem. Also make sure the batteries are in good condition Pass a load test and cables are tight.
Have also replaced quite a few TCM's. They must be having a problem. The Allison dealer usually has 5 0r 6 on hand.