- Subject: Transmission overheats
- Unit: GM 6T75
- Vehicle Application: 2008 GMC Acadia
- Author: Jody Carnahan
- Essential Reading: Rebuilder, Shop Owner, Center Manager, Diagnostician, R & R
The GMC Acadia, Buick Enclave and Saturn Outlook are starting to become a common presence in our shops. These vehicles use the new GM/Ford co-designed six-speed unit, the 6T70/6T75. This has proved to be a very reliable transmission, and when it comes down to diagnosing and repairing this unit it’s fairly simple and straightforward.
For the purposes of this article, we were working on a 2008 GMC Acadia that required internal repair of the transmission. In this specific instance, a reman unit was installed as the replacement. Similar to the diagnosis and repairing of this unit, the R&R is fairly straightforward also. Paying attention to things like cracked flex plates, reprogramming of the TCM, performing Fast Learn and getting the transmission to the correct fluid level are important items to check for a clean installation.
This particular vehicle and transmission installation were no exception, as everything went as planned: The vehicle road-tested well, no codes returned, the vehicle was double-checked and then released to the customer. It was one of those jobs where you stand back and say, “Man, if every job that came into the shop went like clockwork like this one did, how easy would that be?”
Well, three months and a couple thousand miles went by and the “all too smooth” 2008 Acadia that we thought we would never see again showed up at our door. The customer had told us that the malfunction indicator lamp (MIL) was on, in addition to the “Transmission Hot Idle Engine” message that was displayed in the driver information center (DIC), but the vehicle had no drivability concerns. Upon further inspection, we found that the TCM did have a code P0218 stored, a code for transmission over-temp.
At this point, we cleared the code and went for a road test. At first, everything appeared to be operating normally. The converter clutch was applying with no slip concerns, and fluid-temperature readings were fairly normal. It was then decided that we needed to drive the vehicle for a longer time to see whether it developed any slip concerns, especially with the converter clutch.
Our second road test came in with different results: The longer we drove it with the converter clutch applied (still no slip observed), the higher the transmission temperature would get. It finally reached 270°, and this is the point when the “Transmission Hot Idle Engine” displayed on the DIC. We could not get the P0218 code to set, but we also could not get the transmission temperature to 284°, which is the threshold at which the code sets.
Back at the shop, it was time to dig into the over-temp situation. Having previously road-tested the vehicle and knowing that the transmission did not have any torque-converter-clutch slip issues, we decided that a mechanical issue could likely be ruled out as a possible cause. This led us to believe that we had some type of cooler and/or cooler-flow problem.
When we inspected the cooler lines, we found no obvious kinks or anything in the lines that would restrict cooler flow. We then checked cooler flow with a flow meter with the transmission at operating temperature. Unfortunately, everything was checking out normally. Now what?
It was time to start searching through our technical resources for some help. We came across a GM document, reference # PIE0063, that contained some engineering information regarding the 2007-2008 Buick Enclave, GMC Acadia and Saturn Outlook vehicles equipped with the 6T70 and setting P0218 and/or P0634 codes. This was not a TSB, but more of an informational document issued by General Motors.
The document said GM engineering was working to determine the cause of the P0218 condition and was asking that technicians gather data on vehicles with this issue. GM engineering then would use this information to find the root cause of this concern and develop and validate a fix.
After further examination of this document, we found a section that gave a phone number to call to contact a GM engineer. We called the number, and to my surprise an engineer actually answered the call. We asked about the findings of this PIE document, and he said that in all the cases they had reviewed the transmission was overfilled.
This information now gave us a direction to follow, but a transmission being overfilled did not seem like a likely cause for an overheat condition. Regardless, with scan tool in hand and the vehicle at operating temp (around 200°), the fluid was slightly over the full mark, by about a quarter of an inch. We removed three quarts of fluid from the unit, and this action dropped the fluid-level reading to the full mark.
We then took the vehicle for long a road test using the same route as before, and under the same operating conditions. The temperature never got any higher then 200°, the same as it was before we left for the road test. Everything was now normal and the concern was rectified. We later determined that the transmission fluid was not being checked at the correct operating temperature, and this was causing the overfilled condition. Lesson learned.
One last point regarding fluid level on these units: The GM engineer told us that the biggest issue is with the dipstick itself. The design makes it very difficult to get an accurate fluid-level reading. This, in combination with the fact that the new DEX VI fluid is very sensitive to temperature changes, makes these transmissions an easy target for the overfilling problem.
This is not the first time I have heard that overfilling a transmission will cause it to run hot. I have also heard of this happening on the TF-80-series transmissions. I can see this being a good training example for our R&R technicians and diagnosticians, stressing the importance of using a scan tool to check fluid temperature during the fill procedure. Although many different things can cause a transmission to overheat, this is one that really had me stumped.
Jody Carnahan, warranty director for Certified Transmission, has been with the company for more than 25 years.