Other Articles - May - 2020

Looking for Gremlins in a 9 Speed: When is a Transmission Problem NOT a Transmission Problem?

Those of you who attend seminars and classes that I have taught through the years have heard me “preach” about how important it is to look for issues outside the transmission before you start looking for what’s wrong inside the transmission. More times than I can count, the problem or the cause of that tough problem that a shop is trying to diagnose ends up being something outside the transmission. The problem may even be in a system that many would see no correlation with the problem the customer is having.

I have always advocated starting your diagnosis with a thorough visual inspection followed by a thorough scan of ALL the vehicle systems. Inputs, outputs, and DTC issues in systems, other than the transmission, are many times the root cause of the concern that you’re trying to solve. Sometimes when you have a transmission DTC, the cause is an input/ output, connection, or controller issue for another system, which, at the time, does not seem to be related to the DTC.

One such case is a P0747 (Control solenoid valve 1 stuck ON) on the 9T series of transmission. The GM 9T applications are available in several different versions, 9T45 (RPO’s M3G) 9T50 (RPO’s M3D, M3E, M3H) 9T60 (RPO’s M3D, M3T) 9T60 (RPO’s M3G, M3T) and the 9T65 (RPO’s M3V, M3W).

P0746 (Control solenoid valve 1 (A) stuck OFF) and P0747 (Control solenoid valve 1 (A) stuck ON) typically relate to the solenoid, valving, leakage, or other clutch mechanical issues according to the diagnostic information. According to the GM service information (Figure 1), to set a P0747, the following conditions must be met:

  • Transmission 1-2-3-4-5-6 clutch is applied
  • Battery voltage is greater than 9 volts
  • Engine speed less than 400 rpm
  • Ignition is in the ON position
  • TCM High Side Drivers 1 and 2 are ON

Now, from the criteria, you would think that this is pretty straightforward, and you and the shop manual may tend to go after the solenoid and/or valves. But what if putting a valve body and solenoid on the vehicle fails to repair the issue?

One of the things we discuss in this year’s seminar is how the T87A Transmission Control Module (TCM) controller performs a “Dither” function when the vehicle is turned off. As we discuss in the seminar, it’s common for you to hear the solenoids buzzing when the ignition is switched off. This is a “power down” testing and cleaning procedure that the TCM performs on the solenoids.

The Body Control Module (BCM) on these applications provides one of the power supplies to the TCM, which is used for TCM operation. What the service manual may have failed to tell you is that if the BCM voltage feed to the TCM is lost, the TCM cannot properly perform the solenoid dither test. That may be what is causing the DTC to set. Voltage must continue to be supplied from the BCM to TCM for a minimum of 15 seconds after the ignition is cycled off.

If voltage was lost, the vehicle was in park when the ignition is again cycled on, and the TCM detects engine RPM during crank, the P0747 will set as the TCM sees that the test was not completed. The key here is to pay attention to the details of your scan process. In this example, the DTC would not be present before you cycle the ignition to the off position. But, as soon as the ignition is cycled on, the DTC may set.

Paying attention to these details of when the DTC sets may seem like a no brainer, but you would not believe how important it can be in helping you decide which direction your diagnosis should go when it comes to dealing with lots of code-related issues.

This means to address this issue; you must check for power and ground as well as for connection issues between the BCM and TCM (Figure 2). If nothing is found, you may need to put a BCM in the vehicle to repair the problem which appeared to be a transmission related issue when you started your diagnostic process.

I am constantly reminded of how complex these vehicle systems are becoming. Yes, 90% of the issues you face are pretty straightforward, but the other 10% is the challenge that drives us to be successful! Keep in mind, just as your doctor now treats the whole person, you must treat the entire car if you want to be successful. Until next time remember, “It always seems impossible until it is done.”