Speedo Inactive
 

94 3500HD, NV4500 5-speed transmission. Cluster fuse is good. Everything in/on the truck works except the speedometer and odometer.

Where would I start to test the system? What is the common fail points? Any help would be great... Thanks!

Re: Speedo Inactive
 

Here are some steps to see where the problems is.

Check and see if a trouble 24 is stored in the ECM. If there is no speed signal getting to the ECM it should have stored this code. Let me know if you need instruction on checking for a code.

Is the brake light on the instrument panel on?

RWAL was standard equipment starting in 1988 on full size trucks. RWAL stands for REAR WHEEL ANTILOCK. If the is no speed signal getting to the RWAL module, the RWAL system should set a trouble code and turn on the Brake light on the instrument panel. Starting around 1990 the brake switch code 10 and the speed codes are “soft codes”, and can only be read with a scan tool.

If there are no computer codes or antilock brake codes, then the problem is most likely inside the instrument panel.

If there are speed related codes in the computer and anti-lock brakes then test the vehicle speed sensor.

You can connect a multimeter across the Vehicle speed sensor wires. With the meter on the A/C 200 volt scale, running the vehicle in gear, the voltages should go up with vehicle speed. For safety use long leads so you are not under the vehicle doing this test.