1990 GMC suburban speedometer not working

[jesdude7789]

I bought a 1990 suburban a couple months ago and the speedometer was working when I got it but it has already gone south on me. When I am driving, it sometimes jumps to 65mph and stays still no matter how fast i'm going and then it well peg all the way up to the max sometimes and then back to 65 and then back to zero (or any combination of that). SOMETIMES it does work correctly but probably for only about 10% of the time that I am driving it and then it will mess up again. It's very random. I already checked the wiring on the sensor in the transmission tailshaft and everything looks good and went ahead and cleaned the contacts anyway and put it back together. PLUS, the odometer keeps working anyway so I'm thinking that the speedo needle motor itself is the problem. Anybody had this problem and found a definite solution?? I have read many other threads on this issue but haven't found any where the OP fixed it and lets you know how...
Is there a component on the circuit board the speedo is attached to that I can check with a multimeter and replace if it's bad?

[Ski-me]

Does your cruise control work and hold steadily? Did you follow the twisted pair all the way up to be sure it wasn't melted/shorted out somewhere?

Those twisted wires go up to a DRAC unit, which then interprets that signal into pulses for the speedo, cruise, computer and ABS. I don't recall where the DRAC box is "factory wise" but I think it comes in on the passenger side. Whitish clear box if I recall correctly.

You might be able to go out and get a replacement in the junkyard but each DRAC is set up for a specific tire/axle ratio combo.

[Ski-me]

If that's all good, you could get another speedo and if it's important, a speedo shop can dial in the identical mileage that your current speedo indicates.

Also, you can get a junkyard DRAC and then add in a 7 pin dip switch. Then you can flick a few switches to get the speedo dialed in correctly.

[jesdude7789]

I don't know if the cruise will work because the rod that goes from the cruise module diaphragm to the throttle linkage is missing. I do know that the odometer seems pretty accurate as I have driven a few times and watched it roll over without any issues even though the Speedo is at zero. I didn't follow the twisted pair of wires and I haven't located the drac yet. The previous owner did disconnect the ABS computer box by the master cylinder though. He told me it was throwing codes and I haven't looked into that yet. The brakes work fine otherwise. Is there a way to test the drac? I know I won't find this in the junkyard here in Houston. They hardly hang on to vehicles from the 90s. I would have to use eBay probably.

[Ski-me]

If the odometer is working than I would guess it's more the Gage itself. I just saw one in our junkyard a few days ago. If the brake light was throwing codes, it may be the black control box mounted on master cylinder

[jesdude7789]

I have a couple other things to add...

The overdrive of this transmission is also having issues and slipping on me. I haven't been using this as a daily driver though so I haven't messed with it too much but a few weekends ago I did adjust the TV cable to see if that would help because I know the previous owner tinkered with it and probably didn't set it up right.

I was driving it along with the shifter in OVERDRIVE and then i felt it slipping and so I let off the gas until i was around 40-45 mph and shifted into 3rd manually. It did shift with an acceptable firmness and at the same time, the speedo needle jumped up and started working properly! On another occasion I was in DRIVE because i knew there was no point in putting it in OVERDRIVE and this was when I was in a neighborhood and the speedo was not working again. I came to a stop and then I started accelerating and made my turn to the LEFT and the speedo jumped up and started working this time as well. Either way, it only lasts for so long and then i'll look down at some point many minutes later and the speedo is dead again. It has also worked until i got to my destination and then the next time i started it back up it would be dead again as well.

Also, I know that it's important not to use OVERDRIVE when towing a heavy load or trying to haul ass but how long can I expect the transmission to last if OVERDRIVE is slipping and I keep it in DRIVE all the time? If 3RD is not slipping on me then do I have to worry about burning the trans up?

[Ski-me]

I would number one, be sure your TV cable is adjusted correctly. I've heard that it can hurt the tranny pretty quickly if it isn't set up properly. I'd take it to a tranny shop or do some research on how to get it done right.

The computer recieves an inpulse on the speed which would control the torque converter lockout engagement (I believe). However, if your tranny is slipping, then that could also be something else. I don't know a lot about trannys.

You could start by trying to locate a junkyard with a used DRAC (about $16 here) and a new gauge ($7 here). If no change, then you need to start looking elsewhere.

With it cutting in and out, maybe the wire got melted on the exhaust. I do know the twisted wire from sensor runs very close to the muffler/exhaust piping (believe me, I know....)

[jesdude7789]

I will definitely try to take a look at those wires today after work as well as re-adjust the TV cable. I believe it's too "loose" still.

[72MARIO]

I have had the exact same problems you have. The abs light on and bouncing/non working speedometer yet functional odometer. I picked up a used cluster as I think its the speedometer drive be motor broken on my dash. This trucks has 294000 km on it.

[jesdude7789]

Well thanks to Ski-me, The speedometer is working! He helped me with finding a good used speedo and that was the fix. The speedo needle drive was just worn out in mine. The "new" one works great! He also sent me a DRAC module with the switch mod so that will be very handy when I change my smaller than stock tires out for something larger. There are still other things to fix like suspension issues and cracks in the frame from the rear shocks but the most important things are good to go and now I have been enjoying it a lot more

[Ski-me]

Great news! So glad you got that fixed. I also sent you the cruise control arm that hooks up to your TBI. Just a shot to get it working but could be something else. Hey, it was worth grabbing while I was out there.

I was hoping the speedo would do it but was ready to go get the sensor on the t-case/tranny if necessary. Pretty simple sensor though.

Awesome to hear it worked for you!