Aug 312016
 

2002 Dodge Ram 1500My speedometer will not work. And my abs and brake light is on. I asked a local mechanic. And he said to change the speed sensor on the transmission. I did that. Still not working. Changed the speed sensor on the rear tandem still not working. Any ideas?


RESPONSE

The ABS and Brake light can come on for several reasons. Make sure the brake fluid level is full. This can make both light illuminate and may be a separate issue from the speedometer. There are two speed sensor s on the transmission. The Input Speed Sensor is on the left front of the transmission and the Output speed sensor is on the left rear of the transmission. The rear wheel speed sensor is in the Differential housing. and there are to front wheel speed sensor located inside the front hub bearings. The hub bearings are common failure parts. The front speed sensor failure would also cause the ABS and brake light to illuminate.

Speedometer Operation

The speedometer gives an indication to the vehicle operator of the vehicle road speed. This gauge is controlled by the instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster from the Powertrain Control Module (PCM) over the Programmable Communications Interface (PCI) data bus. The speedometer is an air core magnetic unit that receives battery current on the instrument cluster electronic circuit board through the fused ignition switch output (run-start) circuit whenever the ignition switch is in the On or Start positions. The cluster is programmed to move the gauge needle back to the low end of the scale after the ignition switch is turned to the Off position. The instrument cluster circuitry controls the gauge needle position and provides the following features:

  • Vehicle Speed Message – Each time the cluster receives a vehicle speed message from the PCM it will calculate the correct vehicle speed reading and position the gauge needle at that relative speed position on the gauge scale. The cluster will receive a new vehicle speed message and re-position the gauge pointer accordingly about every 88 milliseconds. The gauge needle will continue to be positioned at the actual vehicle speed position on the gauge scale until the ignition switch is turned to the Off position.
  • Communication Error – If the cluster fails to receive a speedometer message, it will hold the gauge needle at the last indication for about three seconds, or until the ignition switch is turned to the Off position, whichever occurs first. After three seconds, the gauge needle will return to the left end of the gauge scale.
  • Actuator Test – Each time the cluster is put through the actuator test, the speedometer needle will be swept to several calibration points on the gauge scale in a prescribed sequence in order to confirm the functionality of the gauge and the cluster control circuitry.

The PCM continually monitors the vehicle speed sensor to determine the vehicle road speed. The PCM then sends the proper vehicle speed messages to the instrument cluster. For further diagnosis of the speedometer or the instrument cluster circuitry that controls the gauge. For proper diagnosis of the vehicle speed sensor, the PCM, the PCI data bus, or the electronic message inputs to the instrument cluster that control the speedometer, a DRBIII® scan tool is required.

 

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