2nd Gen Non-Engine/Transmission strange speedometer readins

[mikeg135]

I have a buddy that is having a problem with his 99 CTD. All of this occurred at the same time.



The antilock light and brake light warning lights are now on. Also, his speedometer does not start working til he hits 30 mph. Are these related?



I was told that the speed control box is located on the rear axle. I do know that he just put a 4 inch jardine exhaust on and has it blowing almost directly on the axle since he is too lazy to get the correct pipes to finish the project. Hence, most likely damaging the speed control box if it is located on the rear axle.



But would the speed control box effect the anti lock and brake warning lights?



Mike

 

[BV]

Speed sensor

The speed sensor is plugged into the top of the rear diff and I believe that it is used for part of the anti-lock system so it could effect the light. Not sure on the brake light part.

Your might have him check, clean and put diolectric grease on the speedo wire plug in and see if that clears up part or all of it. I can't believe that all that soot and extra heat can be good for the rear diff or the sensor.

 

[fest3er]

Originally posted by mikeg135

I have a buddy that is having a problem with his 99 CTD. All of this occurred at the same time.



The antilock light and brake light warning lights are now on. Also, his speedometer does not start working til he hits 30 mph. Are these related? ...

Yup. The signal from the rear sensor goes intot he anitlock brake controller (CAB). It is a low-voltage signal. If the CAB does not receive a good signal from the sensor, it will think there is something wrong with ABS and brakes in general.



Second, the CAB feeds a speed signal to the PCM. If the CAB is not receiving a strong-enough signal from the sensor, it will not be able to tell the PCM how fast the vehicle is moving.



At low speeds, the sensor is producing a low voltage (around 1/2 volt). At higher speeds (35 and over), it is producing about 2 volts. The minimum voltage the CAB needs to read the sensor is around 0. 4 volts.



It would appear that there is some resistance in the sensor circuit; enough to reduce the signal voltage significantly, but not completely. At 30 MPH, the signal voltage rises high enough for the CAB to read it.



Check and clean all connections from the rear wheel speed sensor to the CAB (at the sensor, at the CAB, and, IIRC, at a junction somewhere in the middle). If that does not fix it, check the sensor itself, making sure it is mounted correctly. If this doesn't fix it, visit a dealer!



Fest3er