Jim Grant's Tech Tips
'91
Dodge Caravan, Speedometer Jumps
Q:
I recently bought a ‘91 Dodge Caravan with a bad transmission. I
had a Chrysler rebuilt transmission installed. When driving down the road
at 30 MPH the speedometer will start jumping around sometimes will peg
out at the maximum 100 MPH on the dashboard. The engine will start jerking
at that point. Then the speedometer will settle down and the vehicle will
run smoothly. The tachometer will remain steady even though the speedometer
is jumping around. The transmission shop did not think that the rebuilt
transmission would be the cause. The OBD did not show any errors. When
the technician checked the dashboard, the speedometer would only go to
the half way point(50MPH) instead of the full range(100MPH). What should
I be looking for? Sensors, gauges, body control computer? Wiring? The
problem is intermittent and seems to be affected by bumps.
A:
If I’m not mistaken your vehicle doesn’t have a speedometer
cable. It uses a signal generator in the transmission that creates an
electrical signal in relationship to vehicle speed called a Vehicle Speed
Sensor (VSS). This information is processed by the vehicle’s computer
for engine and transmission management and provides the signal for the
body computer that controls the speedometer. If the VSS is failing it
will not always set a code in the computer. Why? It all depends on how
it is failing (lying). Also you have to know that the computer in many
vehicles have a programmed speed limiter. The computer’s program
uses vehicle speed as an input and if a specified speed is seen by the
computer it will cut the fuel injectors to prevent the vehicle from going
too fast. Knowing this, and the fact that your engine starts running rough
when the speedometer flips out at 100 plus MPH, I’d say fuel cut
out is kicking in. For this to happen it is likely that the Vehicle speed
Sensor is on its way out. Before it quits it’s going you make your
life difficult. Have your technician plug into the vehicle computer and
monitor the vehicle speed sensor signal. It’s likely your problem
is an intermittently lying Vehicle Speed Sensor.
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