Gents -- I've got a manual 4-speed transmission (believed to be an M21). My speedometer mysteriously stopped working about a week ago. I pulled into the garage on Day X and it was working and the next time I pulled it out of the garage for a test drive, it was no longer working. I figure that the cable has snapped but I've not delved into it completely yet as I wanted to research things (manuals, AIM, this forum, etc.) in terms of how much of a PITA it is to remove and replace. I've read a fair amount about C3 speedometers and even watched some videos but I've not come across what I've got. I'm going to include some pictures here to see if any of you can help me out.


Here's a close up shot. The short stub coming out of the transmission comes down on the passenger side of the transmission at about a 45 degree angle. It then makes a 90 degree turn to come towards the front of the car and that's where the speedo cable is spun on.


Here's the same view (from the front of the car, looking rearward), just a little further away than the close up above. You can see the speedo cable coming forward and terminating at the crazy 90-degree contraption.


One more view (from the front looking rearward), even a little further away than the middle picture. You can see the speedo cable coming forward and running through a retaining clamp.

I'm baffled by what I've got. Perhaps it's some sort of "converter" or "adaptor" as I do recall reading something along the way that mentioned these? I'm also baffled by how the "converter/adaptor" is actually held in place on the transmission case because there is no fork-shaped clamp nor does there appear to even be a place (threaded hole) where a fork-shaped clamp would be bolted in place. Maybe it's threaded into the transmission case itself? Or maybe it's pressed in?

Once I figure out what I've got, I believe that the approach to removing the cable (assuming it's the cable that is snapped) involves keeping the car up on jack stands as it is now and also supporting the transmission with another hydraulic jack. Then, unbolting the pieces that help support the transmission and which are bolted to the cross member that is right there. I would also need to undo the exhaust clamps that are holding the exhaust pipes to the exhaust clamp bracket which is also held in place by the transmission support brackets and bolts. I have to remove that main transmission support bracket in order to gain any sort of access to the speedo cable and/or the mystery "converter/adapter".

I'm hoping that one or more of you have encountered this same contraption and can give me some guidance into what I've got and what my repair approach should be. THANKS, as always, for the contributions that you guys provide.

 

I haven't seen one exactly like that but I'd say it's just an old 90deg adapter used to keep the cable from making that sharp turn into the transmission. Frequently seen on the tach drive distributors that aren't clocked the "right" way
M

 

Here's some of more or less the same thing
https://www.corvetteforum.com/forums...ter-cable.html

M

 

My 73 with a manual transmission had a 90 degree adapter that looked like yours. It broke and I purchased a new one from a Corvette Parts supplier, I believe they come from the same manufacturer. Also the adapter can be used on the distributor cable.

 

Use a large adjustable pliers to loosen the collar where it is attached to the tranny. You should not have to remove any supports or mess with the transmission. Once the adapter is removed connect a electric drill to the speed cable (under the car). Run the drill in REVERSE and have someone look at the speedo needle. Run the drill at a slow speed. If the needle moves, the rather adapter needs to be replaced.

 

Thanks but based on what I have seen (and tried), there is absolutely zero chance of getting a pair of pliers in there to remove the adaptor from the tranny without undoing the tranny support plate I was mentioning in my original post. But I will look again. At least I’ve heard enough from you guys to know that this 90 degree contraption is not uncommon and I can definitely understand how it would relieve stress on the cable.

It sounds like you’re saying that the adaptor screws into a very short fitting coming out of the transmission. Is that correct? It’s hard for me to tell due to the road grime but it could be a knurled collar at the end of the adaptor closest to the transmission. That would make the most sense to me.

Should I expect any fluids to come out of the short fitting after I remove the adaptor?

 

I'd be surprised you can't get it off without touching anything else, It sticks in just a little ways, there might be a dribble of fluid that comes out but shouldn't be much
A set of channel lock pliers should do the trick. Maybe remove the cable from it first to get at it better
M

 

Honestly, I tried to put a pair of channel locks on the speedometer cable's knurled collar (where it screws onto the front of the adaptor) last night, hoping against hope that it was maybe just loose and that's why the speedometer had stopped working. No luck. It is barely even possible for me to touch the knurled collar of the cable with the channel locks, let alone try to turn it. I'm not kidding. If you look closely, you may see some shinier parts on the knurled collar in my earlier pictures -- that's where the jaws of the channel locks were at least able to scrape off a little road grime. As for getting to the knurled collar that fits onto the tranny, that's even more impossible.

Also, are you saying that the adapter itself sticks into the transmission case? If so, what holds it in place (keeps it from just falling out)? There is nothing that I can see; check the close up picture that I posted earlier -- it's staying in there by sheer magic right now. I was under the impression based on what I see in my picture and what I've seen online that the adapter has a knurled collar, too, that screws onto something sticking out of the transmission.

While I truly like your positive attitude (fueled by the fact that you apparently did this yourself before), I can only wish that I could have the same success. I'm at a total loss as to how you managed to do it without removing the bracket(s) I was describing earlier. The adapter itself and the fitting on the side of the tranny to which it connects are directly above the support bracket(s) I mention. The passenger side exhaust runs right under there, too, as you might see in the photos posted earlier. Very cramped conditions.

If you lived nearby, I'd invite you over to give it a crack and if you managed to get it, I'd happily buy you a 6-pack of your favorite beer or a bottle of your favorite whiskey/bourbon/whatever.

 

I know this is an automatic but the end of the cable should be the same as it is here

The part that sticks into the transmission just mimics the end of the cable so there's only the inside part (the cable itself) and the next part goes against the end of the transmission fitting, the thin knurled nut holds it all in tight
Here's what's under the adapter you have, the inner part has the square hole that the end of the cable would go into, the outside of the cable locates against that little shoulder and the end face. So hardly anything goes into the trans at all.




The cable would have this knurled collar on the end like this



The adapter mounts the same way
M

 

Do you have an AIM or parts catalog for your vehicle?
I'm sure there is an illustration showing the parts for the adapter and how they are assembled.

 

here's one with the passenger side speedo hookup



Maybe you can get at the bolt that holds the entire piece in and then get it out where you can work on it
M

 

Years ago, a friend of mine had a speedometer cable on his Chevelle break and it made a lot of noise you could hear inside the car.

 

In this link it shows the bullet than would come out if you remove that smaller bolt and retainer tab
https://www.corvetteforum.com/forums...-question.html
M

 

Look in a Corvette Suppliers Catalog for a picture of an adapter so you know what you are dealing with. You could look for a “ Slip-pliers” that has a 90 degree turn. Harbor Freight has a large needle nose pliers with a 90 degree turn. You are correct about the knurled collar that is what is holding the adapter to the tranny.

 

Thanks, everyone, for the continued discussion and the pictures. What I haven't been able to see so far on my car... and maybe it's because of the ANGLE at which I've been looking at things... is the little forked plate (retainer tab) that holds the bullet into the trans and, which in turn, is held down by a bolt that screws into the transmission case. This shows clearly in Mooser's photo. If you look at my first close up photo, you can clearly see the groove that's machined into the bullet but there's no evidence of the retainer tab. Hence my comment about "magic" earlier. I've watched other videos so I felt that I knew what I should be seeing but I end up seeing this adaptor and no (visible) retainer tab. This could all simply be due to the fact that I've been looking at it from the front of the vehicle and not from the rear (I know... what was I thinking, right?). I do have an AIM but, honestly, have not been able to find anything about the speedometer cable connection to the transmission. I have the 1973 service manual, too, but I've not seen this specifically noted. Plus, the AIM is usually a bigger help in knowing what all the little parts are for any specific thing. On top of all that, as we all know, there's no telling what any previous owner may have done or forgotten to do.

I'll check things a bit more tomorrow if I have the chance -- focusing on trying to look at things from a different angle (from the rear of the car) to see if things come more into focus. If I don't get a chance to do so tomorrow, it'll be many days before I can get back to it as we're going to have "grandchildren watching duty" for 5 days.

 

Hi. I wondered if you have tried to look, or if possible felt up at the back of the speedometer to be sure the cable hasn't become disconnected at that end. Lots of stuff back there and isn't the easiest to see but may be worth a look. Good luck.

 

I briefly considered that, too, but in the end, I pushed forward and managed to get the adaptor off. It was tough and I question whether I’ll be able to get a new one ON but the ones I’ve seen online seem slightly smaller than the one I had so I’ve got hope. There was nearly no clearance to work it off because it came into contact with the underside of the car as I extracted it from the side of the transmission and that limited how far it could actually move. With a fair amount of wriggling around, I managed to get both my hands into that area from 2 different directions and used my left hand to keep the drive key (the ‘pin’ that connects the driven gear to the adaptor) fully seated in the transmission, thereby minimizing how much of that drive key was still protruding. I fear it’s going to be tough to get a new one in but if push comes to shove, I can remove the transmission support as I mentioned before to give me more work room.

There was one anxious moment where I could not get my right hand out from between the exhaust pipe and the transmission. I got it in there… why won’t it come out? I eventually just gritted my teeth and pulled firmly and yanked it free. *phew*. I was afraid for a moment that I was going to have to chew my arm off to get out.

I took the entire adaptor-plus-cable out as a unit and then disconnected the cable once the whole thing was out of the tight location it was in. And, as you may have already figured out from this story, the cable itself is just fine; spinning it with a cordless drill registered on the speedometer as I hoped. The adaptor isn’t so fine. Spinning the drive key in it by hand would sometimes turn the end that drives the speedo cable and sometimes wouldn’t. So, it’s clearly shot.

I’m hoping to take advantage of this situation and put in an adaptor which has a gearing correction in it. The current speedo reads about 8% higher than it should, likely due to a gear change the previous owner made at the rear end. I found an adaptor supplier that has over 100 ratio-changing adaptors (both “slow down” as well as “speed up” ratios). Still waiting to hear back from them on pricing. And, yes, I know that I could change the driven gear that’s in the side of the transmission. And, depending on the cost of the adaptor I’m looking at, I may go that route. A basic 90-degree adaptor with 1:1 gearing is running around $50 based on what I’ve seen online.

 

I have most of the Corvette Supplier Catalogs and have never seen a 90 degree adapter that would change the gear ratio. If your speedo has never been refurbished that could be the reason for the inaccurate reading. Trust me, you don’t want to remove the speedo because it’s a few mph off. In my 5 years of owning my 73, removing and then reinstalling the speedo/tach was the most awful repair that I have done! My speedo and tach head died, and so I didn’t have any choice, the repair was close to performing Open Heart Surgery. My neighbors are still talking about all the yelling and screaming coming from my garage.

 

Could you see it if you had a mechanic’s mirror?
I have one that has a small mirror on the end of expandable handle.
The mirror rotates and the handle extends about 18”.

 

I think you may have misunderstood. I don't have any need to access the speedometer in the dash. Yes, I can understand how the trauma from doing that could still be affecting your everyday life. I hope to never have a reason to get in there, trust me.

Here is the link to the speedometer company that offers all of those gear-ratio-adjusting adapters (which fit right onto the side of the transmission, just like the one I removed did): https://speedometercablesusa.com/
When you get to their page, click on the "Gear Box Adapters" image and you'll be taken to the 250 different ratio gear box adapters (I'm only interested in the 90-degree ones) they offer. Assuming that the cost isn't astronomical for one of these, I'll be killing 2 birds with one stone: replacing my no-longer-functional 90-degree adapter and also correcting the speed shown on my speedometer.