Hullo

I have a 79. the speedometer does not work the cables and gear seam fine, however the gear at the back of the trany does not seam position right. I am told bye a a guy you resores/parts vettes that the gear is just clipped in place and can walk up the shaft if the clip gives out. His description to repair it sound like a pain in the ... Has anyone here seen this befor?

Ron

Ron,
I've got the same problem I understand that the gear can slip if the clip gives. I have a question on the forum now to see how to correct. What did the man say you needed to do to fix If I hear anything I will let you know!
Mike

I have the same problem too. I bought a new cable and planned to swap it out this spring, but I heard that it might be the clip instead of the cable.

Let me know if you find out anything.
Glenn

The drive gear for the speedometer in mounted on the output shaft of the transmission. It is made out of plastic. There is a single hole that does not go all of the way through the output shaft, directly below the gear. A metal clip slides under the gear and has a tab which drops into the hole. Using the hole as a locater the clip maintains the position of the gear. To remove the gear, depress the clip, slide the gear off the clip, then lift the clip off of the shaft. These parts interlock. If the clip is not there, the gear floats on the shaft. Likewise the fact that the gear is around the shaft and slides over the clip to hold it to the shaft. It's really very simple once you look at it.
Getting to it is the problem. You need to remove the tail stock from the transmission. Remove the drive shaft. Support the trans, remove the rear mount. Disconnect the speedo cable. Remove the speedo driven gear from the housing. Remove the tailstock bolts, and pull the housing. Now you can see gear and clip, and fix it. You will likely need a new clip, and maybe a gear.
Just be sure the drive gear on the output shaft is the problem. Often the driven gear is worn, or misinstalled with the same symptoms. The driven gear is the one that comes out with the housing that the cable goes into. The cable going into the driven gear often strips the inside of the driven gear and causes this problem.
Be sure to get the correct gear. Both the drive and driven are pastic and color coded. With age the color may change a bit, so count the teeth on each gear. This is what calibrates the speedo to the final drive and tire size. The wrong gear changes the reading. Like wise, if you change the rear axle gears to a different ratio, you can change one or both of these gears to correct the speedometer reading. The Chevy parts dept or a good trans shop has the listing of what gear for what axle/tire combo.

Ken Styer

Thanks Ken,
My problem is that the speedometer will bounce or not work at all. Sometimes it works (not sure how accurate) but will bounce from the speed then down to 0. Other times it doesn't work at all. Is this a simptom of the gear slipping or do you think it just needs to be greased inside the cable?
Glenn

Needing grease will make the cable bounce. It binds, twists, and releases. It can lead to not operating, but is not the failure point. Many times lack of grease makes the cable turn harder. A cruise control unit will also cause turning stress on the cable if it starts to bind. This causes the driven gear to strip inside, where the cable enters the gear. Due to the fact this is pastic, sometimes it will slip, sometimes grab. Sometimes the driven gear will wear away due to extra stress driving the cable. You can pull it out of the trans and see the wear on the gears. They get thin or become dish shaped. Either way it needs replace. While you have it out pull the driven gear out of the housing and stick it on the end of the cable. If you can turn the gear on the cable, the inside is stripped.
Some times a worn cable can "walk" in and out of the gear causing it to lose it's grip. This is usually caused by a cable starting to twist, which makes it shorter. Many times this will strip the outer end of the inside of the driven gear.
SO... lube the cable and inspect it. Inspect the driven gear, replace if necessary. Usually the drive gear on the output shaft is okay, but can fail, and sometimes the clip comes off. With the driven gear out, try to turn or slide the drive gear with a screwdriver. Reach through the hole and push on it. Don't damage the plastic gear teeth.

I didn't metion, many of our C3's will allow the tail housing of the trans to come off without moving the trans mount. The drive shaft still needs to come out. MOST of the time the drive gear is not the problem. It's in the driven gear or cable.