M1031 speedometer woes

[Glockfan]

So I just replaced my very defunct speedometer with a working one with about the same mileage. The speedometer is now intermittent. Sometimes it works, sometimes it doesn't. Can anyone offer up any potential causes for this? What have I missed?

 

[dependable]

The only thing that comes to mind is to check that the cable is fully engaged with back of speedometer and spinning freely. You can spray some lube into the cable to be sure it is not dry. Otherwise, the new head may be bad also.

You might be able to take it apart and fix or lube the head, have done this on another make of vehicle, but have never bothered with a Chevy, as the heads are so available.

 

[Barrman]

Used speedometer swapping was a sport I played for many years. 2 of my 3 daily driven square body trucks now have steady and accurate speedometers. We broke down and bought a new LMC unit for the other one. It has never been steady or accurate.

I got to where I would chuck the speedometer onto a drill and test them out before taking the dash apart, again. 3 in 1 oil worked good for me. So has silicone spray. Gun oil was a hit or miss thing in my experience.

Of course, the cable and cable housing has to be good along with the plastic gears inside the t-case. I had one of the gears that the cable slides into round off inside and it gave me a sometimes working and sometimes not working gauge. The reduction box is not the most precise thing in the world either.

Since you changed the gauge, that is probably your issue. However, any or several of the parts in the chain could have decided now was a good time to break and it just happened to be when you did the swap. Check them all is my advice.

 

[royalflush55]

Remove the cable from the speedometer, drop it down and remove the cable completely from the cable housing. Clean it really good with some spray lube to remove all the old grease. Sometimes there is a dry hard blob in the cable housing that will come out when you remove the cable. After getting it all cleaned up lube the cable with some white engine assembly grease. Reinstall into cable housing and reattach to speedometer. Cable should turn freely now. If speedo still fluctuates it is in the speedo itself.

 

[Sharecropper]

Joe the previous posts could all describe a solution, however I believe the problem could be a dry gear shaft where the speedo cable attaches to the transfer case. Inside the transfer case there is a plastic "drive" gear around the output shaft which "drives" a plastic "driven" gear mounted in a removable sleeve where the bottom of the speedo cable screws onto. The plastic "driven" gear has an integral plastic shaft which extends through the removable sleeve and couples with the end of the speedo cable. This shaft does not receive lubrication from the interior of the transfer case and, after more than 30 years of neglect, probably needs greasing. Simple fix, just unscrew the speedo cable and back-out the bolt that holds the removable sleeve to the transfer case. Pull out the removable sleeve and you will see the "driven" gear and shaft, and how the thing works. I'd be willing to bet it is dry, so all you need to do is to pull the gear & shaft out of the sleeve and grease it, and then reinstall everything in reverse. If the plastic shaft is indeed dry, it may also be dis-figured and require replacement. The "driven" gear / shafts are readily available on our favorite auction site and are available with varying tooth counts based on axle ratio, transmission, etc. The different tooth counts are identifiable by different colors of plastic. I can't remember which color/tooth count our CUCVs came with, but once you get it out you will see the color. If your speedometer is not displaying an accurate / actual speed, you may want to change the tooth count of the "driven" gear. All of this is explained here - http://www.73-87.com/7387garage/drivetrain/speedo.htm

Hope this helps.