Back on the Bench - April - 2020

6R140 Repeat TCC Failure

Back when I started in this business, most technicians were expected to build three or more units a day. In some production facilities they had people tearing down transmissions, and others cleaning and loading onto a cart. Then loading the cart with the soft and hard parts to be passed on to the technician. Building six units a day and making them work under these circumstances was realistic.

figure 1Assembling those old transmissions didn’t take much time, and as easy as they were, we could almost do them in our sleep. Doing quality work quickly was not unheard of due to the lack of complexity compared to what we see in these cars today. These days, getting in a hurry, skipping steps, or missing the small things can lead to huge cost in comebacks. Modern transmissions are not nearly as forgiving as those oldies and require a bit more time to cover all the bases.

Recently, I came across a 6R140 that had been rebuilt by another shop. The customer stated it had been out three times and down for over a month total, and each time the shop told him the converter had failed. It would work for a week or two, then the converter would start to shudder, and set a slip code. At this point, the customer was frustrated and decided to give me a shot at it.

We test drove the vehicle and checked for engine misfire and torque converter clutch slippage. Sure enough, when we felt the shudder, you could see the slip rate fluctuate about 200rpms. It was time for some advanced diagnostics. I discussed with the customer the list of possible causes for this failure and requested some time to dig a little deeper into this problem to see what was causing it.

figure 2There are several things that cause a converter to slip, some outside the transmission, such as, sensor inputs from the engine. These directly affect the line pressure in the transmission. Low line pressure or poor response can allow the torque converter and other components to slip. Many things inside the transmission such as, the stator support bushing, input shaft, and valve body wear can also be to blame. Going through step by step diagnostics is the only way to narrow down all of these possibilities to find the cause of the problem.

My first step was to check the line pressure command on the scan tool. This should be around 0.9-1.1 amps at idle, then drop to 0.2-0 amps during a stall test. If this did not fall within specifications, I would have wanted to check engine data for any abnormalities. The pressure command responded correctly, so next I hooked up a pressure gauge (figure 1) to verify the transmission line pressure. Drive at idle was 92 psi, during the stall test the pressure rose to 218 psi, and while driving at highway speeds the pressure was 140 psi. Everything looked good on the gauge, so next step was to drop the pan, if the pan was clean, it might be worth checking the valve body for a possible in car repair. However, this one had debris in it already, so I called for the pull.

figure 3Once on the bench and disassembled, it was time to do all the standard checks. The Teflon rings on the input shaft showed no signs of deformation or cuts. The surfaces where the bushing rode had no scaring, and no cracks in the shaft. Looking at the stator support, the bushing to shaft clearance was good, no scaring on the bushings, and no damage where the input shafts seals rode. The surfaces between the stator body and pump body showed no signs of visual damage and with a straight edge across them, they appeared to be flat.

Upon inspecting the valve body, it appeared everything was in its proper place. No valves stuck or springs broke, no warpage, so next we need to check for bore or valve wear. Before we do this, it is a good idea to understand how everything works.

figure 4This valve body has three valves involved in the operation of the torque converter clutch. The TCC charge limit valve (figure 2) controls fluid flow to the torque converter from the pressure regulator during release mode. The TCC charge control and apply regulator valves (figure 3) are controlled by the TCC solenoid, to apply and release the converter clutch. Line pressure is supplied to the TCC apply regulator, fluid is then directed to the apply side of the converter clutch and the opposite side of the apply regulator to control TCC engagement. The TCC charge control valve when stroked, routes fluid to the TCC charge limit valve to stop it from controlling fluid flow to the torque converter, then it takes over the control of fluid flow to the torque converter.

Now it’s time to do some vacuum testing. I checked the areas shown in (figure 4), the TCC charge limit valve and TCC charge control valve appeared to be with in specifications. The TCC apply regulator valve was a bit low at 13 hg, I believe we have found the problem. Time to order some parts. Once the parts got here, I reamed the valve bore, installed the valve and did the vacuum test again to verify the fix. It’s always a good idea to check your work, worn or damaged reamers can lead to damage to the bore and leave you worse off than you were to begin with.

After installing the transmission and going on a long test drive, everything seemed to work. The real test however, is longevity. I asked the customer to come back for the normal ten day check, and to keep in contact with me over the next couple months. There hasn’t been the slightest problem out of the transmission since, and we have another happy customer on the road.

We have many time constraints on us when a job shows up on our bench. Whether it’s trying to get our customer back on the road in a timely fashion or getting the job in front of us out the door so we can move on to the next. Constraints such as these can lead to get in a hurry and overlook the small things. With almost every problem in a transmission, there is a cause, and it’s always best to take the time to find that cause, so your transmissions don’t end up back on the bench.