Odometer drive and driven gear source? - Chevelle Tech
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post #1 of 8 (permalink) Old Jun 9th, 19, 3:09 PM Thread Starter
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Eric
 
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Odometer drive and driven gear source?

My odometer on the 66 was working fine, we watched it reach 77999.9 and thought it would roll over but it just stopped there and hasn't moved since. The speedometer is still functioning so I'm thinking that there may be an issue with the odometer drive and driven gears within the speedometer. Correct me if I'm wrong but I think you have to pull the whole dash to get the speedometer out, I don't want to have the car down long so I looked on-line for the parts to fix it but came up empty. Are these available? I saw videos of the insides of the speedo and they aren't very complicated.
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post #2 of 8 (permalink) Old Jun 9th, 19, 5:47 PM
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Re: Odometer drive and driven gear source?

The gears aren't in the speedometer they are in the transmission. Speedometers don't often fail, the failure is most likely in the speedometer cable, the inner cable probably broke. Changing the cable is easy enough, be sure to grease it up well. You can get replacements at any number of Chevelle parts vendors. The length may differ between automatic and manual. You typically don't have to change the entire cable, just the inner cable.

If you do need a speedometer driven gear, they are easily obtained on ebay. But you need to know how many teeth it should have and the color (the gears are color coded) to get the right one. To get it out, just follow the cable to the tailshaft of the transmission, remove the cable and then one small bolt and a locking tab and just pull the metal "bullet" out and the gear should come with it. You can then inspect it for damage and get the color and tooth count if you need to. The drive gear is a whole different problem as it is inside the transmission mounted on the output shaft, but they are metal while the driven gear is plastic so if anything fails between the two it will be the driven gear not the drive gear.

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post #3 of 8 (permalink) Old Jun 9th, 19, 6:58 PM
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Re: Odometer drive and driven gear source?

Quote:
The speedometer is still functioning so I'm thinking that there may be an issue with the odometer drive and driven gears within the speedometer.
Actually, there are some small regular gears along with worm gears that are up in the odometer area. I wouldn't be surprised if most all GM odometers in years of the same era couldn't be easily swapped into the 66 Chevelle dash.

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post #4 of 8 (permalink) Old Jun 9th, 19, 8:03 PM Thread Starter
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Re: Odometer drive and driven gear source?

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Originally Posted by dyno jonn View Post
Actually, there are some small regular gears along with worm gears that are up in the odometer area. I wouldn't be surprised if most all GM odometers in years of the same era couldn't be easily swapped into the 66 Chevelle dash.
Yup, those are the gears I'm referring to. The speedometer itself is still working so it's not the cable or in the transmission.
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post #5 of 8 (permalink) Old Jun 10th, 19, 8:59 AM
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Re: Odometer drive and driven gear source?

Itís time to remove the speedometer and send it out to be rebuilt. Expensive. Or just forget about clocking mileage. Or if you want to drop some real $$$, time to go for a digital solution? In any case, the problem is with the gears inside the speedometer housing. 50+ years isnít bad for them to have done their thing.
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post #6 of 8 (permalink) Old Jun 10th, 19, 11:01 AM
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Re: Odometer drive and driven gear source?

I totally missed the statement about the speedometer still working.

There is a drive gear that turns the odometer. It is held in place by a tiny metal clip near the number drums. Sometimes the gear can wear out or break but often it just falls loose of the clip and no longer engages with the number drums. If this is the issue, it is an easy fix once you get the speedometer out and in your hands. It could also be that the drums jammed and that's a whole different matter. The gears driving the drums are a lot more complicated but not impossible to work with. You can pull the drum set out of the speedometer itself and work on it separately if you are careful to not let it fall apart. Push come to shove, you can replace the number drum set which you can scavenge from another speedometer. Setting the mileage to your current mileage isn't all that difficult.

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post #7 of 8 (permalink) Old Jun 11th, 19, 11:04 PM
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Re: Odometer drive and driven gear source?

or... you could just buy a repro and replace the entire speedo assembly...

I am not sure on the quality of the repros, but maybe someone else has installed one...

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post #8 of 8 (permalink) Old Jun 12th, 19, 10:06 AM
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Re: Odometer drive and driven gear source?

You'll have to disassemble the speedo head to find the problem. Might be the clip that keeps everything together is out of place. Whatever it is, when you have the tumbler in your hands DO NOT try to clean the numbers, the paint used is ultra soluble and even clean water will wipe off the numbers.
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