Speedo not working

[Unactive1]

On my 88 gmc c1500 SLE the speedo doesnt work at all. It rarely works and it seems to only work when its cold temp outside. After couple minutes it just stops working. This summer it wouldnt hardly work unless it was a cool morning or if it just decided to work for a minute or two. This is the sensor type it has in it. This is the gear style, correct? The reading i have done on here points to grounding/wire issue. Trans shifts and operates fine. I did notice when speedo stops working when going down road it feels like it down shifts. Where would be best place to start diagnosing the issue ?

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[DerekTheGreat]

The easiest and most productive way to go about troubleshooting this is to acquire a scan tool. I use a Snap-On MT2500. A scanner will let you know if the computer is seeing a speed signal, which it probably is if the Service Engine Soon light isn't on. (The signal goes from the VSS to the cluster and then the ECM) That'll then help point you in the right direction. I will say the "moonie" clusters on these are common failure points. So if you are one of those who don't want to buy a scanner for x, y & z reason, you could try and get a known "good" cluster and drop in place of your existing unit to see if the problem goes away. If it does, swap the speedometer & odometer over to your cluster in order to retain speedometer accuracy.

 

[Unactive1]

Ok thank you for your suggestion. I will end up getting a scan tool eventually because none of the part stores round here have obd1 scan tools. They only sell the cheapies. I can deal without the speedo atm. Got many leaks on engine, trans and rear wheel brake cylinders to address first. I will have to investigate this issue as i get time to and post what i find and dont find. I forgot to note that the plug on vss is soaked with tranny fluid, dripping soaked.

 

[someotherguy]

The problem is almost surely in your gauge cluster. On 1988-1991 the speed sensor buffer circuitry is part of the cluster's circuit board. When they start failing, you can loose speedometer function, and also torque converter lockup control. That's why it feels like it downshifts - you'll get a couple hundred RPM rise as the torque converter unlocks.

I've seen people destroy their dashboards whacking it to get the cluster working again. That tells me the likely cause is either a bad connection at the socket (oxidation on the pins) either where the truck's harness plugs into the cluster, or where the speedometer plugs into its pins in the cluster - or possibly failed solder joints in the cluster.

You could start easy; pull the cluster and spray DeOxit contact cleaner in the socket and on the pins, reinstall. See if it improves. If not, carefully disassemble the cluster so you can pull the speedometer away from the circuit board, and spray the DeOxit in the pins where the speedometer connects. If that doesn't do it, you're looking at re-soldering the joints on that circuit board, which is easy, if you can do basic soldering work. If not.. find someone who does.

As far as your speed sensor goes, you can test it with a voltmeter. Check the factory service manual for more info - links included for free downloads in the sticky posts at the top of the "Engines" section. The leak can be cured by replacing the seal. If the sensor is working, no need to replace that..

Richard

 

[Unactive1]

The problem is almost surely in your gauge cluster. On 1988-1991 the speed sensor buffer circuitry is part of the cluster's circuit board. When they start failing, you can loose speedometer function, and also torque converter lockup control. That's why it feels like it downshifts - you'll get a couple hundred RPM rise as the torque converter unlocks.

I've seen people destroy their dashboards whacking it to get the cluster working again. That tells me the likely cause is either a bad connection at the socket (oxidation on the pins) either where the truck's harness plugs into the cluster, or where the speedometer plugs into its pins in the cluster - or possibly failed solder joints in the cluster.

You could start easy; pull the cluster and spray DeOxit contact cleaner in the socket and on the pins, reinstall. See if it improves. If not, carefully disassemble the cluster so you can pull the speedometer away from the circuit board, and spray the DeOxit in the pins where the speedometer connects. If that doesn't do it, you're looking at re-soldering the joints on that circuit board, which is easy, if you can do basic soldering work. If not.. find someone who does.

As far as your speed sensor goes, you can test it with a voltmeter. Check the factory service manual for more info - links included for free downloads in the sticky posts at the top of the "Engines" section. The leak can be cured by replacing the seal. If the sensor is working, no need to replace that..

Richard

LoL at the smackin the dash. Already tried the Fonz touch, didnt work will check into what you mentioned. Maybe that'll get it goin for me.

 

[DerekTheGreat]

Ok thank you for your suggestion. I will end up getting a scan tool eventually because none of the part stores round here have obd1 scan tools. They only sell the cheapies. I can deal without the speedo atm. Got many leaks on engine, trans and rear wheel brake cylinders to address first. I will have to investigate this issue as i get time to and post what i find and dont find. I forgot to note that the plug on vss is soaked with tranny fluid, dripping soaked.

I'd never buy one of these new, I don't have that kind of money. Here's an MT2500 you could follow on eBay, the price is currently right. Looks to have the cartridges and connectors you'd need for GM.

Snap-On MT-2500

EDIT: I see that the ad says both power on, but power off. They both probably work, or you could get this one to go with them. Still not a bad deal, as I paid $300 for my MT2500 with case, accessories and cables some ten years ago.

 

[Unactive1]

I'd never buy one of these new, I don't have that kind of money. Here's an MT2500 you could follow on eBay, the price is currently right. Looks to have the cartridges and connectors you'd need for GM.

Snap-On MT-2500

Says " for parts or not working" sketchy. Thanks tho. I'll get a used one when i do get me one.

 

[DerekTheGreat]

That's to protect themselves against schittbags who like to buy something, use it/rob it of parts and then immediately return it.

 

[Unactive1]

Update 1:

While driving to the store this morning, i just decided to smack the hell out of the column dash area (not enough coffee ) and I'll be damned it started working until i got to the store and cut it off. Got out of store and back on the road, not working again. Gave it a good bump couple times and started again. So the issue should be in dash some where i would think. Bout to pull some stuff apart and inspect things ...

 

[someotherguy]

Yep, same thing I've seen on multiple 1988-1991 models. Just don't get in the habit of whacking the dash as you'll break it easily and I have to tell you it's really not enjoyable driving around a truck with a busted-up dash. It's hard to find a good used one, often expensive when you do, and a replacement new one will cost you $400+. Installation isn't terribly fun, either.

Richard