My son is driving a 1995 Saturn wagon with approximately 110,000miles on it. The speedometer has started working intermittently. What could be causing this, loose connection or should it be replaced?
Speedometers aren't mechanically simple like way back when, where all you had to do was replace the speedometer cable, or maybe that little plastic gear in the transmission. I personally found it entertaining when the seal at the transmission leaked, causing transmission fluid to travel up the speedometer cable, under the dash, and onto the driver's feet.
Your Saturn's speedometer is fully electronic. The whole process still begins at the transmission, but now we produce an electronic speed signal by means of a vehicle speed sensor. This signal travels to the powertrain control module (the engine's master computer). From there the signal is routed to the electronic speedometer/odometer head in the instrument cluster, where it's transformed into usable data to operate the speedometer, odometer and trip odometer.
There are a few clues to look out for. If the vehicle speed sensor signal is missing because of a faulty sensor or a problem elsewhere in the circuit, the powertrain control module will illuminate the "service engine soon" indicator on the instrument panel, set the applicable trouble code in memory, and the cruise control would also be out of commission.
If the speedometer is off long enough to see if the odometer or trip odometer is working, then take note. If the speedometer's not working, but the odometer is, that confirms a failure within the instrument cluster. Other than that, it's going to require a technician with the proper tools and service information to narrow down the cause.