91 d250, 160k miles, auto, 2wd. Hopped in the truck a while back and the speedometer was bouncing all over the place. I shut it off and fired it back up hoping it would just hit the reset button and go back to normal, but the needle doesn't move at all now. Been browsing through forums for the past couple weeks and came across some similar posts that discussed how the gear reducer is known to fail. I ordered a new gear reducer and VSS but when I went to swap them out, I noticed that the truck did not have a gear reducer and the VSS was plugged directly into the tranny. Does the gear reducer not apply to the 91/auto/2wd, or did the previous owner have an incorrect setup installed? Any info would be much appreciated. Thanks in advance! Oh and some additional info, the odometer seems to be moving, just the speedo is out.

 

How many digits are on your odometer? The cable driven units only have 5, where as the electronic units have 6.

I was under the impression that 89 was the one and only year that the cummins trucks came with a cable driven speedo. 90-93 were all electronic speedos. But this is not to say that a previous owner was having issues with the electronic speedo and ditched it in favor of the old reliable cable driven unit.

If you do have a cable driven speedo, your symptoms sound like the inner drive cable has broken. Purchase a used, or new cable and install...done.

 

6 digits and the VSS has wires running to it, so its definitely electronic. I guess my main question is, to use or not to use the gear reducer when installing the new VSS?

 

Have you read this? from the sticky.
https://www.dieseltruckresource.com/...-works-198123/

 

Ohhh....I miss read your original post. You can install the VSS into the tranny first, then follow this thread I wrote up to figure out your error.

Speedo ratio adpater... - Dodge Cummins Diesel Forum

 

On 2 wheel drives there is no speed reducer the VSS goes directly onto trans . Or atleast that is what I have run across on the 2 truck's that I have seen that were 2 wheel drive .

 

Once you get it working, then you need to figure out what tire size you have, what gear ratio and then swap out the speedo gear inside the tranny. PATC has all kinds of gears available as well.

 

Is the VSS also controlled by the computer on the driver side fender?

 

That 1 I am not sure on . but don't believe it is .

 

The PCM does not control the VSS. It does use a signal from the VSS to control cruise, and maybe more? The VSS signal is basicly wired straight up to the speedo.

 

Aww, I need to look into this now.

 

Thank's Brian for the Info .

 

I remove steering column cover and there is a partially stripped red/black wire near ignition switch, any hints on where it should be connected

 

This wire thats unhooked is on the truck i just bought a 93 D250

 

I tore my vss off my truck this weekend. Hooked a drill to it and it is working fine. Wondering if the square hole is rounded at the top just enough to cause the needle to jump around? Any suggestions on how to fix that?