Speedometer WAY off

[huafist]

If you've followed any of my other posts, you'll know already I've been rebuilding a 1991 TSi AWD as a project car for the past few months. It's together and running now. I ran it tonight and noticed a problem - the speedometer is off. WAY off. Like, if you add ~20mph to what it says, that'll be close to my actual speed. Any idea why on eath something like this would be happening?
Also, the backlighting in my instrument cluster isn't functioning. Gauges work, dummy lights work, turn signals work. Checked fuses and didn't find any problems.

Ideas?

 

[luv2rallye]

Speedo: You don't have larger diameter tires than stock do you?
Backlighting: check rheostat.

 

[indiglotrd]

pull gauge cluster out and check the bulbs. check speedo wire at tranny and see if damaged?

 

[DSMPowered]

I'd also check the sensor in the tranny.

 

[huafist]

1) Bulbs in the cluster are fine.

2) They're stock-sized rims, but even if they were friggin' 20" rims it wouldn't account for THAT much of a descrepancy in my speedo. I guess I should check the sensor in the tranny.

3) I'll check the rheostat today.


Thanks guys.

 

[92awddsm]

There is no sensor or wires in the tranny to check. All the speedo is in a 1g is a gear driven cable. Look closely at it and you will see that it has a retaing cap that unscrews kind of like bottled drink cap. Unscrew it and pull the cable out of the tranny. Check and make sure the cable spins freely. If so, pull the actual gear from the transmission. You will see a retaining bolt holding it in to the trans case. Check the gear for broken or missing teeth. If all that checks fine, the problem will probably lie in the speedometer itself.

As far as the dash lights, check the rheostat and all the grounds under the dash. Ive had the same problem twice. First it was the rheostat but the second time, it was a bad ground for the lights. I actually had no lights for the cluster, hvac controls, and for the ashtray and cigarette lighter light.

 

[huafist]

Update:

The rheostat was the issue with the cluster lighting. Fixed.

The speedo problem was related directly to the speedometer itself. I pulled one out of another cluster I have that a guy buggered up and it works fine now.