6T75 Overheats

By Jody Carnahan, Diagnostician

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 proven to be a very reliable transmission, and when it comes down to diagnosing and repairing this unit, it’s fairly simple and straight-forward.

For the purposes of this article, we were working on a 2008 GMC Acadia which required internal repair of the transmission.  In this specific case, a reman unit was installed as the replacement.  Similar to the diagnosis and repairing of this unit, the R&R is fairly straight forward, 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 was no exception, as everything went as planned: vehicle road tested well, no codes returned, vehicle double checked and then released to 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 go by and the “all too smooth” 2008 Acadia that we thought we would never see again shows up at our door. The customer had advised 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 out that the TCM did have a code P0218 stored, which is 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 period of time to see if 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 converter clutch applied (still no slip observed), the hotter the transmission temp would get.  It finally reached 270 degrees, 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 temp to 284 degrees, 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 ruled out as a possible cause.  This led us to believe that we had some type of cooler and/or cooler flow problem.

After inspection of cooler lines, found we had no obvious kinks or anything in the lines that would restrict cooler flow.  We then checked cooler flow with a flow meter while at operating temp.  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, which 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 stated that GM engineering was working to determine the cause of the P0218 condition, and was requesting that the technician gather data on vehicles with this issue.  This information would then be used by GM engineering to find the root-cause of this concern, and develop and validate a fix.

After further examination of this document, there was a section that gave a phone number to call to get in contact with 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 advised in all of the cases they had reviewed, the fluid level was over-filled.

This information now gave us a direction to follow, but a transmission being over-filled did not seem like a likely cause for an overheat condition.  Regardless, with scan tool in hand and vehicle at operating temp (around 200 degrees), the fluid was slightly over the full mark by about ¼ inch.  Three quarts of fluid were removed from the unit, and this action dropped the fluid level reading to the full mark.

The vehicle was then taken 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 degrees, the same as what it was prior to leaving 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 causing the over-filled condition.  Lesson learned.

One last point I would like to make regarding fluid level on these units: the GM engineer advised us that the biggest issue they have 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 over-filling problem.

This is not the first time I have heard about overfilling a transmission will cause it to run hot.  I have heard of this happening on the TF-80 series transmissions, as well.  I can see this being a good training example for our R&R techs and diagnosticians, stressing the importance of using a scan tool to check fluid temp during the fill procedure.  Although there are many different things that can cause a transmission to overheat, this is one that really had me stumped.