Question: I have a 2004 Ford F-150 and the speedometer quit working, but the odometer still works. I heard that disconnecting the battery would fix it. It did, but only for a couple of days. Does that mean something?
Answer: Let's go over the speed signal path. It begins at the antilock brake speed sensors (there's one at each wheel). These signals are received by the powertrain control module, which uses the data to tweak engine and transmission control functions. The PCM sends a transformed signal of the speed data to the instrument cluster. There it's used to run both the speedometer and odometer. The fact that the odometer works, and the "Check Engine" and/or ABS warning lights have not come on, tells us the speed signal is okay. Therefore, the problem is within the cluster assembly.
You were in the ballpark by disconnecting the battery as a first try. Many automotive modules -- not always, but sometimes -- can be rebooted in this manner to repair problems. But let's take it a step further. A lot of electronic control modules, such as the one inside the instrument cluster, use capacitors to store energy. Simply disconnecting the battery won't always achieve a true reboot, due to this residual voltage. Disconnect the battery again. This time, turn the ignition switch on, and short the positive and negative battery cables together (needle-nose vise grips or a jumper wire will do the trick). Let it sit for about 30 minutes. This will totally drain the residual voltage from the module's capacitors, and produce an all-out restart. If the speedo works, and stays working, you're in the clear. If not, replace the instrument cluster.
If you have a technical question regarding your pickup, SUV, or van, feel free to contact Alex, a master technician with the National Institute for Automotive Service Excellence. Send a letter to him in care of Truck Trend Garage, 831 S. Douglas Street, El Segundo, CA 90245, or e-mail us at firstname.lastname@example.org. Please include the VIN with your question. Due to the volume of questions received every month, we cannot guarantee that everyone's question will be personally answered or will appear in the magazine.