Shift Pointers
- Subject: Code P0797 – “Pressure Control Solenoid C Stuck Off”
- Unit: GM 6T75
- Vehicle Applications: GMC Acadia, Buick Enclave, Pontiac G6, Saturn Aura & Outlook
- Essential Reading: Rebuilder, Diagnostician
- Author: Pete Luban
Premature ring-land wear leads to shift problems
The 6T75 has been on the road for a little more than a year now, and we’re beginning to see some problems with this unit. The first problem we ran into was at a Miami shop that was doing factory-warranty work for a local GMC dealer. The problem vehicle was a 2007 GMC Acadia, and there were some problems with the two-wire Hall-effect speed sensors.
In this instance the output-speed sensor was open. GM also uses the 6T70/75 in the Buick Enclave, Pontiac G6, and Saturn Aura and Outlook vehicles. Ford uses its version of this jointly developed transmission in the Edge crossover sport/utility vehicle.
What actually prompted the writing of this article is the GM version and the setting of code P0797, which is defined as “Pressure Control Solenoid C Stuck Off.” Like most pressure-control-solenoid performance codes, the definition is totally misleading. This problem also causes a bind-up on the 3-4 and 4-3 shifts. The warmer the transmission gets, the worse the bind-up becomes.
It seems there are problems with ring-land wear on units built prior to June 15, 2007. The ring lands in question are in the transmission end cover (see Figure 1). The hub in the center of the end cover has four Teflon rings on it that seal the fluid circuits for the 3-5-reverse and 4-5-6 clutches.
Since the 3-5-reverse clutch is used in third gear and the 4-5-6 clutch is used in fourth gear, this would explain the shift problems occurring on the 3-4 and 4-3 shifts due to a cross leak between the 3-5-reverse and 4-5-6 clutch-apply circuits.
So why the P0797 code? My guess is that the input- and output-speed sensors are telling the TCM that the 4-5-6 clutch is slipping and – stupid computer – the TCM is commanding the solenoid to provide enough holding pressure for the clutch.
There cannot be more than 0.008 inch (0.2 millimeter) clearance between the ring land and the ring when measured with a feeler gauge (see Figure 2). Transmissions built prior to June 15, 2007, apparently had a poor surface finish on the hub in the ring-groove area, causing premature wear of the rings to occur.
If you are doing dealer work you’ll need a new hub and four new sealing rings. The end-cover gasket can be reused unless it is damaged. These parts are available through GM Service Parts Operations or probably will be supplied to you from the dealer if you’re doing a factory-warranty repair.
Part numbers should you need them are:
- Support hub 24237674
- End-cover gasket 24237724
- Sealing ring 24237428
Remember, the part number for the sealing rings is for one ring; you must order four of them.
