I've been trying to track down the reason for bouncy speedometer needle and the speedo reading ~10% too high at all speeds.

When I had the dash apart earlier this year, I checked the speedometer itself with a drill attached and at all speeds it was perfectly smooth.

When I removed the old speedo cable, there was a small kink in the inner cable, the outer was slightly melted at a point closest to the exhaust, and it didn't appear to be lubed. I lubed it, tried to straighten the kink slightly and reinstalled it. The needle still bounced, but occasionally steadied for a few seconds more than it used to.

I odered a new inner and out cable (from Corvette Central) and a new driven gear (from ebay) to correct the 10% speed discrepancy. I installed all that, being careful to run the cable away from the exhaust and lessen the bends as much as possible and lubed it with graphite. It spins freely by hand and doesn't seem to bind.

BUT...

Now the speedometer needle doesn't move at all!

I can spin the transmission end of the cable and needle moves, but the one thing I noticed is that the new inner cable isn't long enough. It should extend into the transmission driven gear at least 3/4" to 7/8". The new cable is just over 1/2" and there was wear on the very tip indicating it was stationary and the driven gear was spinning around it. So I ordered a new inner and outer cable from Deluxe Speedometer & Cable Service in Colorado that is the correct length.

However, the needle still doesn't move. So I checked the new driven gear against the old. It is identical in every dimension,except one, the tip opposite where the cable inserts is 0.03" longer than the original. Which would make the gear not align perfectly.

Do you think this mis-alignment between the drive and the driven gears enough to prevent them from meshing and not turn the cable? Any other thoughts on causes? Thanks!

 

You did not say what kind of trans you have. I can't speak for a manual but if you have a TH400 there is a possible issue you may be having. With the TH400 there are 2 housing assemblies for the driven gear. one housing is for gears with 35-39 teeth and the other is for gears with 40-45 - I may be off by a tooth on this. If you put in a new driven gear that is out of the range of the housing that may be your problem.

Pat

 

Thanks for the reply!

It's a TH400 transmission and the driven gear I installed is a 38 tooth. Original was a 36 tooth, which from the research I did should provide the 10% difference I'm hoping will correct the speedo.

 

36 & 38 tooth gears would use the same housing. just to make sure check the house to make sure it is the correct housing. if you are not the original owner of the car maybe some previous owner changed it. if i remeber correctly the housing will say right on it the range of gear teeth it takes.

Pat

 

When you speak of the housing, is this the same as what other people have referred to as the bullet?

I am quite certain the PO (aka, Bubba) might have done something to it. But then again, maybe not, somebody put lower profile tires on it, but didn't change the driven gear to match the new tire diameter.

Plus this new driven gear doesn't turn at all. It seems that at least the "original" gear partially turned it enough to make the needle bounce.

 

This housing is the part in question. Tooth counts are on the face. While you have it out, reach your finger in and make sure the drive gear is firmly attached to the output shaft. Could there be a burr in the bore stopping rotation?

Steve O.

 

Finally got back under the car today and removed the bullet/housing for the driven gear. The housing is for the 36-39 tooth gears so that shouldn't be the problem.




I also felt inside the transmission to see if the drive gear moves and it doesn't. I took a photo that shows what I think is the drive gear. It has a scalloped edge and a small hook and shim(?), does this look correct?


There was also another gear, what is this one for? I didn't see it in the picture until after I was out from under the car. Do they both connect to the driven gear? Does everything in this pic look okay, too?



I played around with the cable while it was disconnected from the speedo, that way I could spin the rear wheels and see if the inner cable turned. I could feel the cable insert into the driven gear when I pushed it in and it firmly engaged (in fact it locks into the gear pretty tightly) and as long as I kept it pushed in, the cable would not spin since the gear was engaging the transmission.

However, when I spin the rear wheels, the cable spins 1 or 2 times and stops. If I push in on the cable it will spin another time or 2 and stop again. I then disconnected the cable from the driven gear housing and I can push the driven gear in about 1/4" and it pushes back out when I turn the rear-wheels/driveshaft. Maybe this is the reason it's not reading on the speedo?

One other thing I noticed is that the driven gear bullet/housing does not seem to sit as far into the transmission as I think I remember when I first started this project. This pic is as far in as I push it in by hand. Neither of these last two pics has the driven gear mounted inside, I was wondering if the gear was keeping it from seating correctly.


This pic is after I tightened down the clamp. It looks a little crooked and is very tightly wedged in place after doing that. There is no way I can think of to get it to seat farther in. Does this look right?

Thanks for all the help!

 

The steel “gear” on the output shaft is for the governor. That’s the neutral color gear on the governor.

I am almost positively sure the speedo housing needs to go in further…Flush to the case. I’ve had to carefully pry them out with the relief and the threads. You can also spin it by tapping it with a flat head screw driver where the retainer goes. That may straighten it in the bore.

Steve O.

 

"Steel gear"? Both of the gears in the pictures inside the transmission appear to be the same material the driven gear is made of. Am I not looking at the right part?

I'll try messing with housing again tomorrow to see if I can get in further, but as I said before it is pretty tight as it is, and very difficult to take off once it's in there..like you said maybe because slightly askew

 

Which gear drives the driven gear for the speedometer? Is it the small, tapered, orange-colored gear on the left in the photo?
Does the tip of the driven gear rest against the machined area to the right of center?
It seems that between the extra length of the tip of the driven gear and the housing not sitting flush that this is the cause of the speedo not working.
What are your thoughts about grinding the tip of the driven gear so that it is shorter?

 

The small gear is the governor, driven. I’m loading a pic of the output shaft. That should help you answer your question.

Governor gear at bottom. Speedo above it

Relationship of shaft and both gears, speedo and governor

Bullet fully in its bore. Note this is the high tooth count

O-ring, rubber seal goes here


Use Assembly Lube to aid insertion. Ouch?
Lemme know if this helps.

Steve O.

 

I also found that the high count driven gear doesn’t fit in the low count bullet. There’s a bore size difference. For 32 years I’ve just followed the leader…..

Steve O.

P.S. If any of you out there would like to be the custodian of all these Chevy trans parts, lemme know. I always say if you’re near the south shore of Lake Erie…..Drop In!

 

Steve,
Thank you! Those pictures were extremely helpful! Now I think I have it, especially with the two pics you have with the tail piece removed, I can see how it all fits together and aligns.

It threw me when you said steel gear, I now see that your steel gear, is my plastic resin gear.
Is there a reason why one would have steel and the other resin? And would that cause a problem at any point?
Sincerely,
Tom

 

I’ve never seen a resin governor drive gear on the output shaft. But that doesn’t mean it isn’t a variant. You might try poking at it when you’re in there. Most of my encounters have been earlier cars and trucks. I haven’t tried to remove a governor drive gear. On this output shaft example the speedo gear is pressed on steel. No clip/retainer on the shaft. I’ve just followed……

Steve O.

 

I was just researching the resin drive gear that we discussed last and I came across a thread with someone asking about changing the drive gear and driven gear to get the correct speedometer reading for their rear-end tire combination.
Do I need to change both gears in order to make this work properly? Or is it okay to just change the driven gear?
It just seems that adding more teeth without changing the diameter would cause the spacing between each tooth to decrease requiring the drive gear to be changed as well. I just measured two the other driven gears I have and the spacing between the teeth, as best I can measure, is indeed different.
Do I have to change the drive gear?

 

Nope. You just change the driven gear for small speedo adjustments. Big jumps of rear end gear may require both gears to change. That’s what the 40 tooth bullet is for throwing more teeth in for numerically higher gears. Look at it like more steps will slow you down going upstairs( distance being the same).

Ok, so I don’t know how they take up the change in teeth makes. There again, I only just the follow the engineering, throw in a change of tooth count, and it still works.

Steve O.

 

Thanks for that confirmation, Steve, and all the help you've given me.

Happy New Year!