Whenever I can, I like to get off the ATRA Hotline and do some hands-on diagnosing on a live vehicle, and there are a few shops in town where I get the chance.
A friend of mine recently had an Allison come into his shop with a customer complaint of slipping shifts after an extended drive. He drove the customer’s vehicle, and sure enough, it had a slight flare on the shift from second to third, and fourth to fifth. He also noticed the reverse engagement was rather soft. His first thought was the C3 clutches, which work for third, fifth, and reverse.
He felt like this could be a blown seal or a cracked C3 piston because he’s seen it before. He’s made the mistake of pulling the unit for this complaint only to find that the piston was fine, so he didn’t want to make that mistake again. The customer agreed to leave the vehicle, which is where my opportunity to get involved came in.
Our testing was straight forward. Line pressure was good, no codes have been set in any system, and no abnormalities with any sensors on the scan tool. We discuss the possibilities of what is causing this problem.
- Low line Pressure
- Computer hardware or software malfunctions
- Solenoid problems
- Valve body wear or other defects
- Circuit leaks that we’d check, with other testing
We decided to check the computers’ ability to properly operate the trim solenoids first. Trim solenoid “A” and “B” work together to control the quality of each shift. One solenoid controls the apply rate of one clutch, while the other controls the release of another clutch. These solenoids alternate apply and release between the shifts, as seen in figure one. The computer changes the duty cycle to the solenoids, which controls the pressure through the shift valves, and then on to a particular clutch.
Our next step was to let the vehicle sit overnight to cool off, and then drive it and monitor everything when it was working properly. This was so we could compare our testing results when it warmed up and started having the flared shifts. For most vehicles, we’d need to monitor the amperage on the solenoid wires while monitoring the scan tool. Fortunately, the scan tool we were using provided actual and desired amperage (figure 2).
He recorded the data through a few shift patterns when it was cold, to get an idea of what the solenoid commands should look like. It took a little over a half an hour to get it to start flaring on the shifts. Up until that point, it worked like a brand new transmission. He then recorded the data through a few shift patterns, saved it, and headed back to the shop to compare.
We found almost no difference in the commands to the trim solenoids from cold to hot. We were confident that it was not a computer issue, and that it was time to do further inspection. We narrowed the possible causes down to solenoids, valve body, or internal hydraulic circuit leak. We could have rushed to pull the transmission, but the oil was clean, and there wasn’t any debris in the pan. The last thing we wanted was to tear it all apart and find nothing wrong.
Though he did not have a solenoid tester, we could air check the circuits, check the valve body, and replace the solenoid if the other two were okay.
We air-checked the C3 clutch with about thirty-five psi to see if it held, and then maximum shop pressure to check it for leaks. The circuit integrity appeared to be good and held the air pressure with no leaks. He proceeded to vacuum test the trim valves and check for broken springs (figure 3). Though the springs were good, the vacuum test on the trim “A” valve did not fare so well. It only held 12 hg of vacuum, so we believed we found out the culprit.
He informed the customer of what he had found, and that it might fix it. He stated his only reservations depend on how long it had been driven like this, might have resulted in damage to the clutches. We were confident we’d found the problem and could help the customer out with a repair. The customer agreed to the repairs and understood the situation.
After the repair, the trim “A” valve held 22 hg of vacuum. We got the transmission back together and were ready for a long test drive; it worked great.
Upon returning the vehicle to the customer, he requested they call or stop by in about ten days to verify the fix. Which they did to confirm it has not had one issue, and they have put several miles on it. And they were very grateful for the money they saved. Another shop had told them it needed to be rebuilt and was going to cost a great deal more than what Mike had charged. They also stated if anyone ever needed any transmission work done, they would refer them to him for his honesty and hard work.
The following day, Mike found that the customer had left a five-star review online. He mentioned how much money he had saved them, the time he took to get it right, and his courtesy to them. These days, word of mouth is now just a click away, and reputation is worth more than gold.