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Chevy/GMC Allison TCM GM Duramax Troubleshooting

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If you're still having issues with your TCM after it has been refurbished. Here are some general tips to help you further troubleshoot the issue:

  • First, check that the vehicle's car battery is fully charged and getting a good connection.
  • Check the fuses.
  • Inspect the pins on both the harness plug as well as the TCM and make sure the pins are not pushed back on the harness plug. If some of the tabs are broken on the TCM plug the harness will have to be manually pushed in further towards the TCM to ensure a good connection.
  • Clean the pins with electronic cleaner on both the harness plug as well as the TCM itself and inspect for any dirty or receding pins.
  • Check the transmission fluid. If it's very dark and the filter is full of debris, it'll show trouble codes and the A/T along with the check engine light might come on. This is highly likely if the transmission fluid was never serviced and the vehicle has over 75,000 miles. 


Perform a hard battery reset with the TCM plugged back into the vehicle to clear any remaining codes and reset the TCM:

  • Remove the key from the ignition.
  • With the battery removed, to ensure there is no power, short the battery cables in the vehicle and connect the positive and negative cables together for at least 20-30 minutes.
  • Reconnect the battery cables starting with the positive cable on the battery first.

This will reboot all the computer modules including the TCM and make the vehicle relearn its own values.

It's recommended to have the vehicle idle for a few minutes then take it for a drive at least 15-20 minutes after the hard battery reset. 

It might idle rough at first and shift strangely but it should smooth out after the drive. 

If you're not confident in doing the hard battery reset, simply leave the car battery disconnected overnight. The main goal is to drain the voltage in the computer modules to reboot them. 


Troubleshoot 'No Communication' by using a multimeter, and check that there is continuity between the battery and the pins on the harness plugs. Testing the wiring harness pins for any connectivity issues:

  • Power - 10, 63, 70
  • Ground - 9, 69
  • Can Hi - 6
  • Can Lo - 27

For any communication issues, with the ignition-on, make sure that you do an ohm test between pins 6 and 27 on the harness plug for any remaining U0100 and U0101 (no communication) issues that are still present after the repair. There should be 120-ohm resistance. 

If any of these pins are not getting connectivity or resistance between the CAN lines, then check the pins for any dirt, or receding pins and check the wiring harness. 


Gear shift indicator not working properly, the vehicle won't start, or the vehicle is stuck in limp mode (3rd gear) and won't move:

  • Cleaning the connectors on the plug and doing a hard battery reset didn't help check the NSBU.
  • The NSBU (Neutral Start Back-Up) Switch is mounted to the transmission and needs to be inspected especially the connector to make sure it's getting a good connection.
  • These are notorious for failing on the 2001-2005 Allison transmission however for 2009-2015, it should still be checked if it's still in limp mode. 


Getting codes for Vehicle Output Speed Sensor and stuck in 1st gear:

  • Check the resistance on the speed sensor mounted to the Allison transmission. 
  • It should be getting 300 ohms between the pins. If it's higher or lower than that, it can throw the P0700 codes. 
  • As a result, the TCM detects a faulty signal and will not shift gears.




General Motors (GM) has an official TSB (Technical Service Bulletin) on cleaning the TCM connector and inspecting the ground of the G102 wire terminal on the engine block for P0880 and U0101/U0100 trouble codes:

This topic was modified 4 weeks ago 6 times by Ernest
This topic was modified 2 weeks ago by Ernest
Topic starter Posted : 05/04/2023 9:56 pm