I had a 2007 Dodge 3500 diesel in here today with some automatic transmission issues. The customer said that under very light throttle, and very intermittently, it would surge and hunt between gears. It had over 100,00 miles on it, so we did a transmission service, removing the transmission pan to replace the filter, and refilling it with new synthetic fluid. While this seemed to have fixed the problem, I wanted to make sure that we weren’t missing anything, so I hooked up my scan tool to check for any pending codes in the transmission module. It turns out that the CAN-BUS system in the truck wasn’t communicating properly due to an aftermarket sound system installed in the truck.
I know what you’re asking- the what?? The CAN-BUS system is found on most newer cars, and it links (communicates) all of the modules in the car together. These days, cars can have over 25 modules, or computers, inside them that control everything from the engine and transmission, to the doors, windows, radio, dash cluster, and alarm system. If any of these can’t talk to the others, it can cause any number of problems. In this instance, a new radio was causing a transmission problem!
Problems like these are why it is so important that I have all of the most up-to-date scan tools and software here at The Garage. To work on late-model cars, its no longer enough to have wiring diagrams and service manuals with a set of tools :-).
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