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Below is a collection of Jim Grant's Tech Tips sorted by Vehicle Make. These Tech Tips were answers to questions submitted to Jim by users over the course of many years.
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Lincoln - Mercury
  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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