Is there a typical failure area for the speedo/odo? Mine hasn't worked since I got the truck. Everything's hooked up, but not doin'.

 

Check the drive gear in the transmission - that's easy to slip off if it's ever been out and not reinstalled properly.

Also, if you don't have the cruise control, you can pull the dash, pull the speedo out, and make sure that the cable is spinning as yo go down the road. If it's not, possibly a bad speedo cable (you can use the generic replacement units in the factory housing.)

You might also have a bad speedometer.

One more thing - the plastic clip on the end of the cable can be broken, especially if a previous owner tried to manhandle out the dash cluster on a cruise-equipped 1987-1989, since that cable is short and doesn't give much. You have to release it from the cruise control first.

(By 1990, the cruise speed sensor was the VSS; previous, it was in the servo and was driven by the speedo cable.)

Last thing - if the plastic piece has come off the end of the housing, it will easily move enough to pull the cable out of the back of the speedo even if the plastic end is glued back on. Don't ask me how I know *grins*

RwP

 

Well, I know the speedo gear was replaced properly, I did it when I rebuilt the transfer case. It's either the cable, the cable adapter/sensor unit, or the speedometer itself that's the problem. Guess I can check that the cable is turning as you suggested. I just watched a guy on YouTube check his '92 Dakota sensor/cable adapter unit by turning the transmission end while watching the speedometer. Since he was alone and under the truck, he used his smart-phone to record a video of the speedometer while he turned the cable under the truck. The gauge needle moved when he turned the cable, so he discovered that his sensor/cable adapter unit was bad. Pretty clever way to see if the problem is at the tranny end, i.e. the sensor/adapter unit or the drive gear.

Also, looking at the 4WD cable on Rock Auto, it looks like there are actually two parts; an upper and a lower cable.

 

It depends. Cruise has two parts; non-cruise, only one.

For the cruise control (through 1989 only!), the cable from the transmission feeds the cruise control servo, and there's a cable from that to the speedometer.

For non-cruise trucks, it's from the transmission straight to the speedometer.

Don't be confused by RockAuto showing two; they show two from the transmission, but NOT the servo/speedo one for those with cruise.

For the "adapter/sensor" unit, that's a straight shaft through; if you're not getting a 15 code on the CEL, then that's probably working (the sensor has magnets that flip around and trigger a Hall Effect or magnetic reluctor).

As to the adapter - It only took four tries for me to get it seated and "biting" into the output shaft gear properly on my truck when I went from a 32 tooth to a 29 tooth to correct my speedo. *grins* It's still worth checking, although, again, if you're not getting a 15 code on the ECU, it's probably working fine.

RwP

 

Great info Ralph, thanks. My truck had cruise but I guess the original owner disabled it for some reason. The cable goes from the tranny to the speedo, I think. I need to look at it again to be sure there isn't a disconnect somewhere.

 

Heh. My cruise, I'm on my third servo, and keep buying spares when I find them, until I can either figure out how to rebuild one, or find someone WHO rebuilds'em.

They're getting old, they're starting to crack the diaphragm which causes a MASSIVE vacuum leak, and the electronics are going noisy. But dang it, I DO love me my cruise when I'm booking 75MPH down I-20 or I-49!

Yah, you'll just have the one long one for now.

RwP