Transmission Speedometer Gear

[Joe L.]

Re: Transmission Speedometer Gear

Terry-------


I am completely unaware of any 18-21 tooth driven gears being compatible with the 1.76" drive gears. If I recall correctly (I'm in Italy right now and can't check my references), the part numbers 3860341 through 3860344 are old part numbers replaced by the 3987918 through 3987921.

The 3860xxx driven gears were released about 1965. The 39879xx were released about late 1970.

[Gerald C.]

Re: Transmission Speedometer Gear

Guys,

I'm going to wait until my 21 tooth red gear comes in from Corvette Central and try it. Otherwise, would it be that difficult to take out the tail housing to see what size gear I have and change it if necessary??? I know it's a lot of work, but I need to get this resolved.

Thanks,

Jerry

[Patrick B.]

Re: Transmission Speedometer Gear

Joe,

Thanks for this information. Making the assumption I have the smaller 1.76" gear, what tooth count would you recommend? The speedometer is not registering at all. It does when the cable is driven by a drill, so I know it's in the tranny/plastic speedometer gear set-up.

Thanks,
Jerry

If you recently replaced your speedo cable with a Paragon reproduction, the problem may be that the inner cable it too short and does not engage the gear properly. Had this problem with a 67.

[Joe L.]

Re: Transmission Speedometer Gear

Guys,

I'm going to wait until my 21 tooth red gear comes in from Corvette Central and try it. Otherwise, would it be that difficult to take out the tail housing to see what size gear I have and change it if necessary??? I know it's a lot of work, but I need to get this resolved.

Thanks,

Jerry

Jerry------


I don't think it's practical to try to remove the extension (tail) housing with the transmission in the car. I think you'd pretty much have to remove the transmission. At least, I would not attempt to do it with the transmission installed in the car.

However, I really don't think you have a problem with the installed drive gear. Through 1967 these gears were of steel construction. They are not going to wear or otherwise deteriorate against a nylon driven gear. The only thing that can really happen to them is to get displaced enough on the mainshaft so that they don't properly mesh with the driven gear. However, that situation is HIGHLY unlikely as the drive gear is a heavy interference fit on the mainshaft.

I think that Patrick's suggestion is a good one. You may have a problem with the speedometer cable inner shaft not properly engaging the driven gear.

[Gerald C.]

Re: Transmission Speedometer Gear

The cable is new and it was purchased from LICC. Part # 37-14A. I would guess that the cable comes out of the cable shaft from the TOP! What should the overall length be?

Thanks

[Joe L.]

Re: Transmission Speedometer Gear

The cable is new and it was purchased from LICC. Part # 37-14A. I would guess that the cable comes out of the cable shaft from the TOP! What should the overall length be?

Thanks

Jerry------

All Corvette inner speedometer cables are removed from the sheath at the instrument end.

I do not know off-hand what the length of the correct cable for your application should be. However, you should be able to tell if there is enough length at the transmission end to engage the driven gear. As I recall, there should be about an inch of cable protruding beyond the knurled nut that attaches the cable sheath to the speedometer gear fitting.

[Patrick B.]

Re: Transmission Speedometer Gear

Jerry —- I compared the how much the repro cable protruded from the sleeve to the original GM speedo cable and the repro was 0.4 inch less. I bought a cable repair kit and made a new inner cable 0.4 inch longer and put it in the repro sheath which was already installed in the car which I did not want to remove. Now it works fine.

I found the problem by putting the driven gear and its fitting into a spare tailhousing I had and attaching the speedo cable to it while under the car. I could turn the gear easily without having movement on the speedometer which convinced me the POS repro cable assembly was not engaging the gear. I would have put the whole original GM assembly back in, but I didn’t want to screw around behind the dash any more than necessary.

[Gerald C.]

Re: Transmission Speedometer Gear

Patrick,

I used the red gear in the fitting out of the car and it spun the speedometer and tripometer by hand, so the cable is engaging with the gear. I'm waiting for a new red 21 tooth gear to come in and I'll try that and measure its diameter as compared to the used one I have. My other option will go with a larger diameter gear which is the 22 tooth. More to follow!

Thanks
Jerry

[Patrick B.]

Re: Transmission Speedometer Gear

Patrick,

I used the red gear in the fitting out of the car and it spun the speedometer and tripometer by hand, so the cable is engaging with the gear. I'm waiting for a new red 21 tooth gear to come in and I'll try that and measure its diameter as compared to the used one I have. My other option will go with a larger diameter gear which is the 22 tooth. More to follow!

Thanks
Jerry

Jerry— Did you have the plastic gear in a tailshaft housing with the cable attached to the gear fitting? I was not merely testing the fit of the cable in the square hole in the gear, but the ability of the cable to reach the square hole when the cable sheath is screwed onto the fitting that the plastic gear rides in. If not, at least take the gear fitting out of the transmission and with the speedo cable screwed onto the fitting, see if the gear still engaging the cable. Bend the cable sheath to approximate its installed position since the amount of cable sticking out of the sheath can change with bending.

[Gerald C.]

Re: Transmission Speedometer Gear

Patrick,

I wasn't clear enough in my explanation, sorry. I installed the red gear into the fitting and then screwed the fitting into the cable casing to replicate the actual operation. It did work! So let's see how things work with a new red gear (whenever it arrives!)

Thanks

[Donald O.]

Re: Transmission Speedometer Gear

Jerry,

I went through the same cab;e problem with 67's. Here is the link.

https://www.forums.ncrs.org/showthread.php?123958-Shout-out-for-a-job-well-done

[Gerald C.]

Re: Transmission Speedometer Gear

Don,

I tested the cable in the fitting out of the tranny. I spun the red gear and the speedometer and tripometer moved. When I put the fitting back in the gear did not move which told me it was engaged. I even tried a 22 tooth this morning and still no movement. The driven gear in the tranny appears to be lined up correctly in the hole. I'm at a loss at this point and it looks like my only option is to pull the tranny and start over!

[Richard G.]

Re: Transmission Speedometer Gear

I would remove the speedometer cable and perform these tests;
I would connect the cable to the transmission and jack up the rear wheels and run the car. Run the cable through the car door so you will be able to see the end of the cable and what it's doing. Put a little finger pressure on the cable insert to see if you can stop it. If that doesn't stop the cable the issue has to be in the length of the cable or the speedometer head. It could be something silly like the speedometer gear loose on the output shaft. I doubt it as the fit always seemed excessive to me. However, this procedure will eliminate most issues. To test is the other direction I use a cordless drill and turn it from the transmission end with it connected to the speedometer head. A 1000 cable rpm is 60mph so over-speeding the head isn't typically an issue. Use a magnifier to inspect the cable ends and both ends where the cable inserts. Sometimes I just can't see the details without magnification.
I feel your frustration. I wouldn't pull the transmission just yet.

[Gerald C.]

Re: Transmission Speedometer Gear

Richard,

I did the drill test and the speedometer and tripometer worked so I know the speedometer head is working. I'll try the second test with another speedo cable hooked to the tranny with the car jacked up and see if there is movement in the cable directly from the tranny.

Thanks

[Donald O.]

Re: Transmission Speedometer Gear

Jerry
no need to run the car in gear on jack stands.
with the cable attached to the transmission, see if you can spin the cable in either direction, just make sure you simulate ALL of the bends as it would be wen installed. I’m going to guess and say you can when you shouldn’t be able to. Time for a new correct cable, you did post that your current cable is new didn’t you?
Don