A bolt just 2mm too long created a problem that nearly cost a customer the price of a new 6L80 transmission.
That’s what happened with this late model Chevy truck with a 6L80 transmission. The customer said he just had it at the dealer. Well, that’s certainly going to make things a little more interesting! According to the customer, the transmission seemed to shuttle in and out of gear around 40 to 45 MPH.
The dealer told the customer his car needed a torque converter and a new Transmission Electronic Hydraulic Control Module (TEHCM), which is located on the valve body. So they installed the new torque converter and TEHCM… and the transmission was still shuttling in and out of gear. Not the result he was hoping for.
So then the dealer claimed that the only fix was to replace the transmission with a reman from General Motors. To the customer’s credit, he decided to cut his losses at the dealer and get a second opinion.
So the customer brought his car to a nearby aftermarket shop. The technician there took the car for a test drive. He noticed the vehicle had an intermittent misfire, which explained the shuttle. He also noticed that the shift into 3rd gear was a bit slow; not slipping, but it didn’t quite feel right, and there was a slight delay into reverse.
There were no codes set and he was pretty sure he wasn’t going to find any debris in the pan. After all, the dealer just installed a new TEHCM, along with the section of the valve body that comes as part of the assembly.
The tech decided to ask the customer about the shifting and engagement problems he’d noticed. The customer said he’d noticed a slight delay in reverse since the dealer worked on it. He wanted the transmission removed and inspected because he just didn’t feel comfortable with the way it was working. He was worried that, when he towed his boat on the weekends, he might get stuck.
So the technician removed the transmission and disassembled it. The clutches were worn but none were severely burnt. There was nothing visible to explain the slow shift into 3rd or the slight reverse delay. The pump stator support was the later, updated type with the non-rotating rings. The 3-5-reverse drum, clutches, and piston showed no indication of damage at all.
The only place left to look was the valve body. As the technician removed the two bolts at the side of TEHCM, it appeared to shift slightly away from the valve body (figure 1). Remember, this was a new TEHCM installed at the dealer just before it came to this shop.
What appears to have happened was the dealer’s technician tightened the bolts that connect the TEHCM to the main valve body before tightening the two side bolts (figure 2). That would explain why the side of the TEHCM wasn’t quite flush with the valve body when the bolts were loosened. The correct procedure would be to snug the two side bolts first to make sure the TEHCM was flush with the valve body.
Then the technician split the two valve body halves and inspected them. He figured maybe there was a broken spring or a checkball had worn through the plate.
That’s when he discovered a small piece of aluminum casting sitting in the #5 checkball bathtub (figure 3). Where could this possibly have come from? By following the worm track back along the valve body, he found the origin of the mystery piece of aluminum casting: It was from the area where the end of one of the two side bolts attaches the TEHCM to the valve body (figure 4). The comparison (figure 5) shows what the valve body should look like, without the aluminum casting broken off.
When the technician compared the length of the two bolts going into the side of TEHCM, he discovered one was slightly longer than the other. The dealer’s tech had installed the wrong bolt for that location. According to the factory manual, the two side bolts to the TEHCM are supposed to be 53mm (2.09”) long, while the others are 55mm (2.17”); a difference of 2mm (0.08”) (figure 6).
Factory information also has a tip that says “hand tighten only.” When the tech tried to draw the TEHCM flush with the valve body (with some added torque), the longer bolt went too far, breaking off a small piece of the casting.
The #5 checkball separates the Drive 1-6 oil circuit from 35CL REV FEED oil circuit and vice versa (figure 7). When oil entered the circuit with the transmission in reverse, it would push the piece of aluminum up to the feed hole to block the Drive 1-6 circuit instead of the checkball (figure 8). It was enough of a blockage to seal the hole so it didn’t slip in reverse, but it left enough of a leak to cause the delay.
Sometimes you overlook the smallest detail and that’s when it bites you. Just an added tip: Add a small washer to each of the two side bolts (figure 9) to help prevent this from happening to you.
See you at Expo!