Q: My 2014 Jeep Grand Cherokee suddenly developed a clunk downshifting at a stop sign. Then, at the next stop sign, even my wife heard it. Took it to my local garage here, who I have great trust in, as they have saved me a lot of money. They used a machine to analyze it and they indeed fixed the clunk. In looking at the invoice, there was labor charge: reprogram PCM/TCM $175. Not in an upset mode, I simply inquired as to what is a “reprogram PCM/TCM $175.” He kinda stammered as if he was unable, yet not unwilling, to answer. The implication I got was that he was not the sure what the machine does, but it simply fixes the clunk, and it did! Can you give me a better idea of what happened?
J.R., Glenview, Illinois
A: Have you ever downloaded an update for one of your phone’s apps? You are quite able to use it without knowing how the app works. In the case of the Jeep, your mechanic downloaded an update for both your powertrain control module (PCM) and your transmission control module (TCM). He knew what to do without needing to know what the update modified.
