dapo29

I have a 1970 Corvette with a M21 manual and the speedometer was jumping up and down so I replaced the speedometer cable. The problem was still there. Then the speedometer just stop working so I checked the cable connections and they were good. I also replaced the pencil gear thinking maybe it was bad, but that did not change anything. I can turn the cable where it attaches on the transmission and the speedometer will move. The speedometer gear inside the transmission is a press fit gear so I can't see that it has moved. I'm lost on what is wrong.

billschroeder5842

My guess is that your issue is in your transmission as the speedo works when you spin it independently.

Thought... have you measured the length of the steel cable that fits in the trans? If it is shorter, it may not make contact with the gear.

0Willcox Corvette

I'd hook up a drill (in reverse) to the speedo cable at the tranny and see if it turns the speedo fine without issue.... If it's jumping then I'd take a look at the speedometer and if it works without issue I'd take a look at the drive gear inside the transmission.

Willcox

dapo29

I have tried that and I do think it is the drive gear But I was hoping that it might be something else that I was not thinking of.

dapo29

Thanks for the input.

Giant_Robot

Press/turn your trip reset while you are driving and see if it starts working. I have an issue with mine, and messing with the trip reset gets it going again. ?? Why, I dunno.

oledave60

I just installed a M20 and my speedo was 10 mph slow, so I changed the driven gear and the speedo would not work. Come to find out there are two different drive gears, one is 1.76" dia and one is 1.84" dia. There is also two different sizes of driven gears and they will not interchange.

Driven gears - green, black, yellow, orange fit 1.78"

Driven gears - purple, brown, natural, blue, red, silver fit 1.84"

I had to pull my just installed trans to change the drive gear as I needed the natural gear and I had the 1.78 drive gear. The teeth would not mesh.

dapo29

I'm pulling my transmission to find out what going on inside. Thats the only thing I have left to do to find out whats wrong. Thanks for the help.