1985 Speedometer/Odometer Stops Working When Hot
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Instructor
Joined: Feb 2010
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From: Victor NY
1985 Speedometer/Odometer Stops Working When Hot
Got a 1985 here and the speedometer and odometer works perfect when cold but after the vehicle gets to operating temperature they both stop working. The speed sensor was replaced before I started looking at it. When plugged in, the ECU does not show a value when the speedometer stops working. Could it be the ECU needs to be replaced or a gauge cluster issue? Thanks 
Team Owner
Joined: Sep 2003
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Got a 1985 here and the speedometer and odometer works perfect when cold but after the vehicle gets to operating temperature they both stop working. The speed sensor was replaced before I started looking at it. When plugged in, the ECU does not show a value when the speedometer stops working. Could it be the ECU needs to be replaced or a gauge cluster issue? Thanks
You mention the VSS was replaced before you bought the car? Has it done this all the time since your purchase/ Is this intermittent or does it do it at every drive? How frequently?
Race Director
Joined: Apr 2002
Posts: 10,036
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From: Anaheim CA
The speedometer circuit in the dash drives the odometer, so if the speedometer is not working then the odometer won't work either.
The trick is to split the system to isolate the problem. Either the VSS is bad or the dash is bad (though it could be a bad connection between the two).
The usual test method is to use a drill to spin the VSS and then verify that the dash shows that something is happening.
If you have a multimeter (DVM), you can look at the signal coming into the dash from the VSS. It is an AC voltage. I did it this way:
The VSS signal input is pin D11 (yellow wire). The ground is on pin C15 (purple wire).
The trick is to split the system to isolate the problem. Either the VSS is bad or the dash is bad (though it could be a bad connection between the two).
The usual test method is to use a drill to spin the VSS and then verify that the dash shows that something is happening.
If you have a multimeter (DVM), you can look at the signal coming into the dash from the VSS. It is an AC voltage. I did it this way:
The VSS signal input is pin D11 (yellow wire). The ground is on pin C15 (purple wire).
