I’ve run into this problem twice recently involving T5 transmissions. On my 83 and a friend’s 84, the speedometer is several MPH slow in addition to the needle bouncing. On the 84 I tried changing out the driven gear as the teeth were worn. It helped slightly. I can actually keep the cable at the speedo end from turning while the car is moving forward, although the gears mesh fine in reverse.

On mine, I had just finished rebuilding the transmission. The speedo reads about ½ what it should along with a slight bouncing. The drive gear is properly located along with the clip.

I can’t fathom how the gears can suddenly start slipping. It acts like the spacing between the tail shaft and speedo gears has somehow increased. I don’t see how this could happen as the end of the speedo gear is solidly located in the tail housing and the steel adapter holds the entire gear in place. The tail shaft bushing was replaced, BTW.

Has anybody run into this before?

 

Sounds like the drive gear clip broke

 

Often if the speedo is jumpy and/or noisy, the fault is that the cable and housing are gummed up with 20 odd years of crud. You might try cleaning the later. Info below:

https://www.thirdgen.org/forums/inte...eedo-woes.html

I neglected to mention that some of the cables are two piece.

JamesC

 

I suppose thats possible. But the clip is new and the gear is pretty tight on the shaft. I didn't have any trouble installing it.

 

It may be the speedo itself. I know on mine it would freak out on occasion and jump all over and also make a squeaking sound. I took the cluster out and dripped some 3in1 oil on the back of the speedo and it worked great after that.

 

James and Kevin,

The only problem I see with your suggestions is that it was working fine before I tore into the tranny. I must have changed something in there. Since it's happening on both cars, I am hoping there might be a similar or common cause.

I'm sure it's slipping because I can't see how the speed could be considerably low. That and the fact I can easily hold the other car's cable from turning. I'm sure my problem is identical to that one.

I guess the first thing I'll do is look inside the hole and see what the drive gear is looking like.

 

I had an issue on one of my t-5s with the driven gear.
The cable could slip too far into the driven gear and slide out of the speedometer head. It would work fine for a while, start jumping around and then just stop working.
I finally put a small piece of something (don't remember just what) into the driven gear so the cable couldn't slide in as far and that has cured the problem.
I'm not sure that this is your problem but it is an easy thing to try.

 

Thanks for everyone's suggestions!

I finally got around to checking the gears. Everything looks fine except two things:

Before I removed the adapter/housing it seemed to wobble a bit. Also, I assumed I would find some sort of locating device inside the hole on the tailshaft to positively locate the end of the gear so the housing wouldn't have to. There isn't anything like that. I can see some machine work in that area but it doesn't look like it would be able to support the end of the gear. It almost looks like there should be a stud, maybe 1/8" dia that would go inside the hole at the end of the gear.

Is there supposed to be such a locating pin/stud? The fact that I don't remember the housing wobbling like it does now makes me think that there is.

Also, I did "fix" the problem by tying the cable down to the crossmember which tilted the gear upwards to preload the gear into the drive gear. Works great now, but that's not a permanent fix.