Back on the Bench – Someone Dropped the Ball (Drum)!

Someone Dropped the Ball (Drum)! By Rob Faucett

Most days in the field, it’s just building your transmission and sending it down the road. We’ve all seen the common problems and their causes with most transmissions, so we know where to look for concerns. And when one comes back or doesn’t work, we know where to start looking. But then there are those days when the world throws us a curveball, only for us to swing and miss.

I recently rebuilt a 6T70, which seemed to work on the initial road test. We drove it several miles. All the gears were there. Reverse was good, it held in Park, and it was almost ready to ship. Then, all of a sudden, problems with third and reverse started to occur. So, we began to discuss possible causes. It could be a solenoid in the TECHM, a valve sticking in the valve body, or a problem with the 3-5 reverse drum. I replaced the drum with a new one during the rebuild, so I assumed it was not the problem. The first step was to check the scan tool, then the pressure. No codes, and the line pressure was fine. We then removed the valve body, inspected it, and air-checked the 3-5 reverse circuit in the case.

While our diagnostician had the scan tool on it, we found the speed sensors were reading extremely high, over 40,000 RPMS (figure one, recorded after repair), but when it hit third gear, the input speed sensor went to zero. And with a 6T70, if the speed sensor drops to zero in third, we all know the snap ring came off, and the reluctor was pressed up against the side cover from fluid pressure applying the piston and locking down the reluctor. But this extremely high RPM  reading from both speed sensors threw us off and got us to look at TECHM or speed sensor issues, which caused us to waste time testing and running into dead ends.

We first checked the scan tool, and the ISS/OSS supply voltage was okay, but a defective TECHM could possibly give us false information, so we checked the outside powers and grounds. Everything looked good, as well as the charging system. So, we pulled the cover to check the speed sensor supply voltage, which was correct. Thinking of all the possible causes, the only thing we could come up with was a defective TECHM. However, the boss did not want to hear that, nor did he want to spend the money for it, program it and not fix it.

We were using a Snap-on Triton that was recently updated. Though we were dismissive of it being the problem, we waited until the next day, so I could bring my Solus in and try it. We thought it might be a software glitch in the scan tool. The following day, I came in with my Solus, thinking I would probably get the same readings. I hooked it up and took off for a quick road test. Both sensors read correctly on my Solus (figure 2, recorded after repairs). We got the Triton and reconnected it, and it was back to the 40,000 RPMs. Since we confirmed the excessive RPM was only scan tool related, we knew we needed to remove the side cover to check if the snap ring came off the drum.

When we got the side cover off, the snap ring dropped out, along with the reluctor. Some of the aluminum retaining the snap ring broke off the drum (figure 3). All I can assume is that the drum had been dropped at some point and did slight but unnoticeable damage, which with my aging eyes, doesn’t take much to get by me anymore. Once pressure was applied a few times, it all came apart and started giving us problems. And then the curve ball came at us with the scan tool software problem. In all honesty, I was sure we had a TECHM problem, and the boss would not have been happy to replace it, program it, and still have the same problem.

We got a new drum, installed it, and test-drove it for several miles to ensure we did not have the same problem reoccur. To our boss’s delight, it did what it was supposed to, and we were able to deliver the vehicle. While we knew that the problem had something to do with the 3-5-reverse drum due to the input speed sensor dropping to zero, a glitch in the scan tool software sent us off in the wrong direction. While I have recommended in the past that if something looks suspicious, grab another scan tool, even if it’s cheap. I have to admit; it’s sometimes easy to miss when life throws you that curve ball.