Other Articles - June - 2020

Looking for a Needle in a Haystack – P0747 on 9T Applications

So much of what you work on today is challenging, to say the least. Many shops lament, regarding the days when life and the transmission business were much simpler, customers were much easier to please and transmissions were much less of a challenge to repair. Sad to say, I do not think that the turbo 350 or the C6 are going to make a triumphant return any time soon.

So, what is in the mix for the future? I think it is safe to say that more speeds, CVT’s, DCT’s and Electric drive will become more commonplace in the future. A lot of you are now working on the GM 9 speed, Ford 8 speed applications. The good news with those applications is that about 40% of the parts used in their 6-speed cousins are carryover to the 8- and 9-speed applications. This makes repairs very familiar and a lot less intimidating.

With that said, the 9-speed units (RPO’s M3G, M3D, M3E, M3H, M3T, M3V, M3W) can be a challenge when it comes to diagnosis as the amount of electronics and diagnostics has increased dramatically as compared to the 6-speed applications. Let’s look at a real challenge; a code P0747, “Shift Solenoid Valve 1 Stuck On”. As I discuss in the ATRA seminar, the T87A TCM used in the 9-speed applications conducts a self-test of the solenoids when the ignition switch is turned to the off position. This is known as a “dither” test. You may hear the solenoids buzzing when the key is moved into the “off” position. This is considered “normal” on this application.

Keeping in mind that the wiring varies based on application, some 9T applications use the BCM to provide some of the voltage supply circuits to the TCM. The TCM requires voltage to allow it to conduct the solenoid test. If the BCM fails to provide the voltage to the TCM during initial power down, the TCM may set a P0747 DTC. The BCM needs to provide the voltage to the TCM for at least 15 seconds after the ignition key is cycled to the off position. If the TCM does not see the voltage supply, the TCM will be unable to complete the power down test of the solenoids, and a P0747 may set on the next power-up of the TCM. In other words, if the BCM fails to supply the voltage after key off, the TCM will not see that the solenoid tests were not successfully completed. Then on the next engine start cycle, it assumes that the reason they were not completed was that a fault was present.

As I have preached for years, understanding how the vehicle works is the best diagnostic tool in your toolbox. So let’s take a look at a typical diagnostic strategy for this concern. As I previously stated the pin location and wiring on your application may vary as compared to those listed here.

Figure 1: BCM Connector X4, terminal 22In our example, the BCM provides voltage to the TCM from BCM connector X4 terminal 22 (Figure 1).

So, if you are faced with this issue you would start your diagnosis at either the BCM or its corresponding pin at the TCM. In our example, we have chosen to check for the voltage at the BCM as it is many times easier to access on these applications. Probe the BCM connector X4 terminal 22 with your voltmeter. Cycle the ignition to the off position and note the voltage present. Battery voltage should be present on pin 22 of connector X4 for at least 15 seconds after the key was moved to the off position. If voltage was “not” present, the circuit should be checked for a short to ground. If no short to ground is present, check the terminal tension, condition, and its crimp, BCM grounds, TCM grounds. If all looks good then replace and reprogram the BCM (Figure 2).

SoFigure 2: Typical BCM location (Equinox shown) there you go, as we discuss in the seminars each week, a transmission problem that is really not a transmission problem is becoming a even more of a commonplace issue in shops across the county. We all understand that to bad, many customers do not understand this concept. Yes, we all miss the good old days when you could simply show the customer the broken gear, and they’d clearly understand what you were trying to convey. With a problem like the one we discussed, some customers will look at you like you are talking a different language as you try to explain why you need to replace the computer that controls his/her door locks and windows to repair their transmission.

As the complexity of the vehicles continues to evolve, issues like the one we discussed here will become even more prevalent. Until next time remember, “The price of success is perseverance. The price of failure comes cheaper”.