speedo quit working. Bought new cable and plastic gear and it still does not work. I can turn cable with drill and the needle works fine on the speedometer, I guess the gear inside the transmission is broke? How
can I fix the gear in side the transmission? thanks

 

have it replaced is the only way I know, No repair that I have ever heard of. The gear is plastic and when its done its done

 

The gear in the transmission is metal on a four speed. We need more information like what year and trans would help. Did it cut the teeth off the new gear?

 

1969 m21 4 speed. it did not cut or break the teeth. Could it be something else? Do I have to pull the transmission to fix it..

 

The gear can wear over time and start to slip. On my 73 4-speed it was plastic. It is not very difficult to replace. A Chiltons or Hanyes manual will show you how step by step.

 

I have been fighting the same problem. The teeth on the plastic gear showed some minimal wear. I could feel the worm gear inside the tranny and it felt fine. However, after getting everthing back together it wouldn't work or if it did it would only work for maybe a few turns of the tires. My last repair seems to be holding for now. Here's what I did: With the rearend up in the air, car in neutral, speedo cable disconnected. Turn the rear wheels while feeling the speedo gear turn inside the housing. I used the technique of pressing my finger against the part where the cable is inserted, I suppose you could also use a small screwdriver just be careful not to strip the hole. I had to twist (finesse) the housing to get the speedo gear to mesh correctly because I could feel it slipping off of the worm gear inside the tranny. The speedo gear housing seemed to hold it tighter on one side than it did the other. Once I felt the speedo gear turn with the wheels and continue to turn with the pressure I applied with my finger held against the speedo gear, tighten the retainer, carefully twist on the cable, careful not to turn the speedo gear housing and its working. I can't say the problem is fixed but I can say the speedo is working. I sure hope this helps. While I'm at it I want to say thank you to all that contribute to this forum. Without this forum the fate of our corvettes would be compromised. Or at least mine would. Thanks

 

markhperry
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What is the original rear end ratio you have?
There are to diameters of the plastic gear, I had this problem in the past but I need to find on the computer where all the data about it.

OK I found it.

In the transmission Muncie 4-spd :
3.70-4.56 rear end ratios used a 7 tooth driving gear.
3.08-3.55 gear ratios used an 8 tooth driving gear.

for the 7 tooth driving gear you need the larg diameter of plastic drive gear (where the teeth) is 0.87inch (22mm).
for the 8 tooth driving gear you need the small diameter of plastic drive gear (where the teeth) is 0.80inch (20mm).

Avner

 

On a four speed the housing will wear where the shaft of the plastic gear goes through. A new housing may fix the problem. I still think the gear inside is metal but others have said plastic.

 

thanks i will try those solution..
get back later.

 

should I be able to turn the worm gear in the tranny with my hand with it in first on the ground?

 

I agree with Lyle. A new housing may fix the problem. As far as turning the worm gear...I don't think you can turn it by hand. I was actually rotating the tires and I could feel it turning inside the tranny with my finger. Good luck.

 

Inside the gear is metal and called drive gear.
outside plastic called driven gear.

 

The gear is pressed on the shaft if you can turn it with your finger it is slipping and will not drive the plastic gear. If you turn the main shaft it will move.

 

The 1969 and 1970 cars had a plastic DRIVE gear. This is not the pencil DRIVEN gear. The most common problem is the gear simply pops off the clip. These gears are a very sloppy fit on the mainshaft. A tight cable bend or a slight bind causes the gear to pop off the shaft.

It only came in 8 tooth and the replacement press on steel gear is GM part # 3708144.

 

Do I have to pull tranny to replace drive gear inside tail shaft?

 

Is there an easy way to get that tail off the tranny to replace that drive gear?

 

Sorry, the trans has to come out. The tail supports the reverse Idler shaft which most times is stuck to it.

 

Ive got the same problem on my 81 automatic. Mine worked but had a very jumpy needle. I replaced the lower cable (cruise) and now have nothing. I've got to pull the driven gear to color match it and check it for wear, but for now. My $150 GPS works just fine as a speedometer and probably more accurate. But I do want the odometer working also.