Shift Pointers
- Author: Wayne Colonna, Technical Editor
- Subject Matter: 6T40, 6L80
- Issue: Failure to communicate
Two shops’ experiences with defective TCMs
One very significant change that took place in our industry occurred when manufacturers chose to place the TCM inside the transmission. By minimizing the amount of wires going to the transmission, it saved them a considerable amount of money. However, for us in the repair and rebuild industry, this can be frustrating from both a diagnostic and cost standpoint, especially when the solenoids are integral to the TCM.
And/or when programming is essential to complete the repair correctly and efficiently. At some point in the diagnostic process, it may come down to an $800-$1,000 question: Is the TCM defective? To be as complete in the diagnosis as one’s ability and test equipment allows and still be uncertain, what an expensive gamble to take. This article will share a couple of shops’ experiences of what did turn out to be defective TCM’s should you be dealing with the same issues and feel uncertain about which direction to take.
A 6T40 in a 2011 Chevy Malibu comes in to R.C. Trans and Repair in failsafe with a hard code P0601: Transmission Control Module (TCM) Read Only Memory (ROM). This code is straightforward. Verifying power and grounds to the TECHM are the initial checks to make. Once verified, replacing the computer would be a safe repair. Such was the case with R.C. Trans and Repair. They shopped around and found an AC Delco replacement for $15 less than the price from a GM dealer (Figure 1).
They installed this computer into the transmission and had it brought to the dealer for programming. The dealer was unable to program the TECHM and returned the vehicle. Code U0101: Cannot communicate with the TCM was stored in the ECM preventing programming. The shop checked the TCM’s connection. They looked for burnt pins in both the ECM and TCM, as well as double checking power and grounds for the TCM. All looked well, no evidence of any problems with the pins, connectors, wiring, power or grounds. When they hooked up the original TECHM externally, code U0101 was no longer present in the ECM. The shop decided to spring for the extra $15 and bought a TECHM from the dealer, problem solved.
A 6L80 in a 2009 Cadillac Escalade came in to Kinderhook Transmission needing a transmission overhaul. Once done they experienced a 2-3 and 3-4 shift flare when cold only, no codes present. After it warmed up it shifted perfectly. They had conducted a fast-learn procedure, which took successfully but made no difference. The unit had come in with quite a bit of trash in it so the decision taken was to inspect the 3-5-R and 4-5-6 regulator valve and boost valve. There we no signs of these valves sticking. After pulling the unit to inspect for defective seals and/or rings of which none was located, the unit had been re-assembled installed back into the vehicle with a new TECHM (Figure 2).
The transmission shifted properly both cold and hot. In this case, defective solenoids integral to the TECHM was most likely the cause of this malfunction.
