By now, most of us have had to deal with a CAN (Controller Area Network) communication “U” series code at one time or another. Most of the time, diagnosing one these codes is fairly straightforward, given all of the articles and technical publications that have been written regarding these problematic codes. In most cases, it just comes down to identifying the module that isn’t communicating with the other modules on the CAN bus line for whatever reason. Generally, it’s some type of lost voltage or a poor ground in the circuit wiring to the module in question. So, what happens when all your diagnostic tests and checks take you down a dead-end road, and the actual problem ends up being a component that shouldn’t have had anything to do with a communication error code? I’m sure there may be a few engine drivability techs out there who have run into the situation that I’m about to cover, but it was a first for me.
For the purposes of this article, we were working on a 2008 GMC Acadia that required internal repair of the transmission. In this specific instance, a reman unit was installed as the replacement. Similar to the diagnosis and repairing of this unit, the R&R is fairly straightforward 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.
Let’s look at an example of what I’ve discussed. Let’s say that you were working on a 2001 Volvo V70 XC AWD, with an AW55-50SN transmission, and the vehicle had been towed in because it suddenly quit moving. You discovered that the transmission had a problem in the final-drive area but you found no other issues. All the clutches were like new, and there was no obvious wear in the valve body etc. That being said, the transmission was probably operating normally just before the final-drive component failed.
In this particular situation, Dave had been working on a 2001 BMW 740i with a 5HP24 transmission. The vehicle originally came in with a complaint of leaking from the front. The unit was very low on fluid, was slipping and the fluid was burnt. The shop recommended that the transmission be overhauled because of the conditions mentioned and the mileage on the unit. Everything went normally with the rebuild, and the customer left with a properly working unit.
I received a call from one of our wholesale customers, a shop that does general-repair and transmission work on occasion. He was looking for help on a 2004 Ford Freestar that he could not get to leave his shop. In fact, the vehicle had been to several other shops. He told me that another shop had worked on the transmission recently and that it was now back in his shop.
This particular call came in just like many others: The technician had already eliminated everything he could interpret as the possible root cause of the concern. The vehicle in question was a 2006 Dodge 2500 pickup with a 5.9-liter diesel and a 48RE transmission. The owner had bought the vehicle recently and had concerns that the transmission was not shifting properly.