Hi all

Once again, toughest thing about working on any car is figuring out what's causing the problem in the first place.

The speedometer on my '77 (automatic w\cruise control) has always been jumpy since I bought the car in 2012 but recently it stopped working altogether. Today I removed the cable from the speedometer to the transducer and the speedometer moved when I spun the inside wire. That, to me at least, would take the speedometer being bad as to why the speedo is not working.

I pulled the gear drive out of the transmisson (it's red) and it looks fine. Then I removed the lower speedometer cable and it spun as well.

So - what's not working? Is the speedometer cable so worn out it won't move the speedometer? Is the transducer bad? All I know is that my speedometer is not work. At all.

Thanks in advance for any and all insight.

 

Just looking at the gears may not be good enough. One of the common failure modes is the recessed square-drive socket in the speedo drive gear (in trans) is that one 'corner' of that socket is cracked and the cable can no longer be driven by it. Eliminate ALL of the simple fixes between the transmission and the speedo before diving into the area of the speedometer. Also, the 'signal' can get lost inside the transducer head. You can remove the #2 cable from the transducer and use a portable drill to drive the cable and see if you get any reading on the speedo. Drive with slow rpm and see if you get ANY movement in the speedometer. Check in both directions to make sure you are turning the correct rotation for the speedo.

 

Thanks for the reply!

I did what you recommended and pulled the transducer and it appears fine. I attached the lower cable and spun everything with the gear.

Speedometer spins when I spin it via the upper cable - i accidentally yanked it out of the speedo so now I have to get into the dash to hook it back up. Lovely. But thoughts on what might be going wrong? Is it the big thing inside the transmission perhaps?

 

It is unlikely to be the speedo 'drive' gear that mounts around the trans output shaft...unless it lost the split-pin that keeps it from rotating on the shaft. More likely, it is the trans "driven" gear which you can see when you remove the cable. That is where the trouble most likely is to be found. The square socket being damaged or cracked so that the cable will just spin inside it, gear teeth sheered off, etc. You can remove the retaining bolt and retainer from the speedo 'bullet' [that holds the driven gear] and just pull the bullet out of the trans tailshaft housing. It has two O-rings: a large one on the OD of the bullet and a small one on the shaft of the driven gear (to prevent oil leaks around the shaft). These may need to be replaced, if you already have some leakage there....or if the seals appear to be hard and shrunken. That would be a DO GOOD move to prevent future leakage when you go back together.
Anyway, then you can check out the driven gear in detail and look inside to see the condition of the drive gear. Also use a plastic tool to see if that drive gear will rotate on the tailshaft. It should not.
The two areas that I think are probably candidates are the trans driven gear and the transducer. There is a clutch inside that unit; and if it doesn't have much grip left, it will not have enough uumph to make the upper cable turn. A possibility is that the upper cable is binding up in its sheath and presenting so much resistance to the transducer that the clutch MUST slip. So check out how easily the upper cable turns the speedometer (after you get it hooked back up ). Good luck.

 

Well...upon further review the driven gear does rotate on the tail shaft. Now what? Drop the drive shaft and remove the back end of the transmission to replace it I assume? Seems like a lot just for the speedometer and it would seem not out of my skill set wheel house...

Thanks again for your insight.

 

One more thing...the gear inside my transmission is green yet the gear I pulled out is red. They should both be the same color, no? This could this explain why my speedo was so jumpy?

Like I said, I always expect the worst with this car. Poor thing has been bubba'd up the ying yang.

 

Colors of the gears do not match. The color is a code for cross-referencing the number of teeth on the gears. (Google Speedometer World)

There is a metal clip that locks the drive gear onto the output shaft. And as I recall, there will be some cussing involved to get the new gear AND clip into position.
After the driveshaft is out of the way, the four tailshaft housing bolts can be reached with a long, 3/8 drive extension. Very little ATF will come out.
You will need a floor jack to raise the tranny, ever so slightly, and unbolt the tranny mount.

Because this is a afternoon job, I would highly recommend two other "while I am at it" jobs.
Change out the Tailshaft Housing gasket. (giant O ring)
Change out the output shaft seal, while the housing is off.
Both of those parts are dirt cheap, but not fun to swap out later.

So you will likely need:
Tailshaft housing gasket
Tailshaft output seal.
Drive gear of your ratio choice.
Drive gear retaining clip
And maybe a tailshaft thrust bearing. It supports the driveshaft yoke.

 

Or, you can spend $30 on a head's up speed display that uses GPS to provide speed numbers. The right way to do it is to pull the driveshaft and the tail housing. But that's a lot of work for a speedo signal that a $30 GPS device will provide.

 

Odometer is rather handy.

 

Thank you so much for all this information. Really great. Seems like a pretty straight forward job.

One last question - curious, when I remove the four bolts that hold the tail housing to the back of the transmission, will the drive shaft yoke slide off the transmission output shaft? Yes, I will have the drive shaft already out.

When I dropped my driveshaft when I was replacing my differential I noticed the yoke on the transmission slid back and forth so I could move the drive shaft out - I didn't futz with it enough to determine whether or not it would slide out of the transmission.

 

The driveshaft yoke stays with the driveshaft. Disconnect the rear gear yoke, push D/S forward into the trans as far as it will go, lift rear end of the driveshaft and move to the side a bit, then pull D/S out of trans as you move rearward with D/S above the halfshaft. Once removed from trans, just drop it away from the car.
You will need to support the transmission, somehow, as you will need to remove the trans mount under the tail shaft. If you choose to lift it by the oil pan, make sure you use a support board or plate that covers the whole pan bottom (need that support area to keep from caving the pan). There is no risk to the pan if you have the whole pan area supported.

 

Red 1977, this isn't a horrible job. It sounds like more work than it is. Do not mess with all the nuts/bolts at the tranny end yoke. Just remove the diffy end of the driveshaft and slip the whole thing rearwards, yoke & all. Much easier, less work. Then decide if you need a shop to press in/out new U-Joints

The most frustrating part to me was the metal clip locking the drive gear. If you go to Speedometer World site, I believe they have pictures of what you are up against and parts needed.