Hot Rods speedo not working

i have a 350 turbo that the speedo is not working .if i put a drill on the cable at the transmission the speedo works on the dash. if i take the cable off at the dash and put a drill there with the gear on the cable the gear will turn out of the transmission

 

The plastic drive gear may have broken, or the metal tab that keeps it in place has failed.

 

or the driven gear may have broken, or stripped, or have a stripped square hole in it, or parts could be missing, or the wrong ones.

Take the bolt out, remove the "bullet" sleeve that holds in the driven gear, see how it all looks

 

Do you know if it has ever worked, aka what's the history of the trans?

Chris

 

well when you have a shop rebuild your transmission you would think they would put a gear in the transmission so the spedo would work. but they did not.

 

they'd have to know what gear to put in it, eh? so it's not real surprising they left it out, the installer is supposed to deal with that.

 

thanks i did not know that. i thought they would have put one on the shaft inside the transmission .now i have to pull the tail housing to put one on

 

Find out your gear ratio, and tire diameter... TCI site used to have a calculator that would figure which driven gear you need with your drive gear, vic versa. Depending on your combo, the drive gear could be one of several tooth counts/colors.

 

What I meant was that you have to sort it out before you install the transmission. At least, I always do....

 

Wait a minute,,,,there is no gear inside the housing on the main shaft ?

Yes,,,,I would expect the shop to install that on the shaft,,,,,,you are only responsible for the driven gear inside the “bullet “ .

Tommy

 

Do you or the transmission shop still have your old, failed transmission available? You may be able to swap the drive gear out of your old transmission. Are the old and new trans both TH350s? Even if the gears may not swap between different transmission types, you may still be able to determine the tooth count of the drive gear that had, I assume, worked previously. The color of the drive gear may also give you an indication of its tooth count.

 

nope. no gear

 

If you were having the existing trans rebuilt I would expect the drive gear, if present and in good condition, to be reinstalled.

If you bought an "off the rack" rebuilt trans I wouldn't expect the gear to be there. Though it would be a nice touch if they wired a tag to the trans to inform you of the missing gear.

Did you get any printed installation instructions with the new transmission?

 

There are several drive gear options as well, so for the trans builder to fit a drive gear would be an educated guess at best.
I would like to think that they would at least ask the question, and install the gear you nominate.

 

Excellent site to figure out what's needed.
https://www.tciauto.com/speedometer-gear-calculator

 

Unless I sent the transmission in for rebuild, I have never received a transmission with a drive gear installed.

 

well

 

That's one that never crossed my mind but the one exchange transmission that I have installed came with the speedometer gear. That was on my mother's 75 Chevelle and she needed the car and I didn't have anyone local who did side jobs on Turbo 350's at the time. I think that one came out of Portland Transmission and I got it through Teds Auto Parts (long closed) in Toppenish.

 

So,,,,,when you receive the transmission back,,,,,you have to disassemble the back half and put a gear on the main shaft ?

Tommy

 

Yes, only way to get to it.

 

"disassemble back half" means removing 4 bolts and the tail housing, which is sealed with an O ring. It's not a big deal while the transmission is still on the bench or the floor.

 

When you said speedo not working, I was really afraid of seeing pictures of fat old men in little swimsuits ☹️

 

When I worked at a transmission shop many years ago, we would have them built on the shelf with no speedo gear on the shaft. We put the output shaft housing on upside down as a reminder and switch out the gear from the one being traded in.

 

Now that's the way to do it!

 

When you're having your old transmission rebuilt, they should put the old driven gear back on.

If you're buying a rebuilt trans off-the-rack they don't know which drive gear your vehicle requires, unless you maybe tell them the tooth count of the gear from your old transmission. Or swap it out yourself.

 

Yup. On a GM transmission, that's just a couple of bolts.

 

[you are never to old to learn] i went back to the shop that built the transmission.[he told me the same thing squirrel said]and he gave me the right gear and clip to hold it in place i guess the next time i should look before i install it

 

My son’s Apple Watch with a speedometer app!

 

Here's a hint .....jack the vehicle up in the back before pulling the tail shaft housing, less fluid loss...I learned that quickly while trying to get the correct output shaft gear in the last two cars I'd built.