Rick's Free Auto Repair Advice

Speedometer wrong – Chrysler minivan

Speedometer wrong – Chrysler minivan

Speedometer wrong–Speedometer shows speed when standing still, Dodge, Grand Caravan, Town & Country, Plymouth Voyager

If you own a Grand Caravan, Town & Country, or Plymouth Voyager and find that the speedometer is showing speed when you’re at a stop, it can only be caused by a few things. The speedometer gets its signal from the from the PCM based on digital data it gets from the transmission control module via the data bus. The transmission control module gets the speed signal from the output shaft speed sensor.

speedometer is wrong, speedometer shows speed when stopped, speed sensor

Output shaft speed sensor

One way to test the system without a scan tool is to disconnect the output speed sensor at the transmission. The transmission will go into limp mode will be stuck in second gear. But if the speedometer problem goes away, just replace the sensor and you should be back in business.

If the sensor is good, the data is being corrupted somewhere on the data bus. Either a connected module is bad or something is causing interference. The most likely candidate for interference is the alternator since it generates AC voltage prior to converting it to DC. If a diode fails, it can induce an AC ripple through the electrical system. The easiest way to test for alternator interference is to disconnect it. If the problem goes away, replace the alternator. If that doesn’t solve the problem, you’ll have to get a good wiring diagram and component location from either of the sources listed below and unplug each module from the data bus to see which one is causing the interference.


© 2012 Rick Muscoplat


Posted on by Rick Muscoplat

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