[bryany]

1978 TA, 400, ST 10 manual trans

My speedo flopped around giving no real useful information when it worked at all. As part of a recent exhaust project, I pulled the driven gear out of the tranny. It was a red, 21 tooth gear showing a fair amount of wear. I figured, great, I can replace it and everything will be good again. While taking apart the speedo cable from the trans, I found a box like gadget in the middle of the connection. My internet search showed that they weren't uncommon but also weren't universal. I cleaned everything up and reconnected things. There were a couple of times that the parts didn't seem quite right. The hold down clamp needed a 10 mm wrench to tighten it and the adapter was loose and sloppy when connected.

Thinking I had fixed the problem, I took the car out for a spin. No joy! No better function than before. Time to do some research on the internet. First I fixed the metric bolt problem with the correct 1/4" bolt. Then I used a cordless drill to spin the speedo cable while watching the dash with my phone camera. The gauge worked fine then. Winter came back and Covid 19 causing me to get busy with some other tasks but I was thinking about the problem all along.

Finally got some more usable days to work on the problem. Cleaned up the parts and put them on snug. No speedo activity at all but at least it mounted solid. I read somewhere that too tight might keep things from spinning so I had another plan to loosen the connections. The adapter still bothered me so I took it off and cleaned it up. The trans end seemed to be in good shape but the cable end was looking a little shabby. The square hole looked round where the cable end fit into it. I had bought a gear assortment and bullet off of Ebay and they showed up.

I jacked the car up, removed the gear and adapter. I'm getting pretty good at this now. I cleaned up the adapter and the only markings I found were 1.1111. I took this to mean the amount of adjustment. After removing the adapter from the mix, the gauge ran smooth and steady, just wrong by about 10%. Looking for the answer on the web led me to putting in a gear with fewer teeth and trying again. Did I mention I was getting pretty good at changing out these things? Again, the speedo ran smooth and steady and now it was almost perfectly in sync with what my GPS was telling me.

I'm marking this as fixed, now it's time to look for the next job on this project.

Conclusions: The adapter might have been original and it might not have. In either case, it was wore out and just covered up the need for the right driven gear.

Bryan

[bryany]

Car details
Super T10
3.42 rear gears
22570R15 BFG Radial TA Tires

20 tooth Blue driven gear

Speedometer is right on the money according to my GPS

[1968GTO421]

Back in the '70's I had a '75 Duster V-8 manual trans (purchased new). I had speedo problems and a mechanic at the dealership told me "off record" that the "adapter" thing midway on the speedo cable was a sensor that after spinning for X miles would turn on the check engine light so I would have the EGR cleaned and other emissions things checked. Since I had put on headers with an Edelbrock manifold, etc., the mechanic said the "adapter" was useless and to just install a speedo cable from a '70-'72 model which ran uninterupted from the trans to the speedo gauge. Did that and had no more problems. Do not know if this is applicable to your situation but it reminded me of that problem I had.

["QUICK-SILVER"]

Bryan,

Need to take a trip on a road with mile markers and compare odometer reading to markers.

The little boxes correct cable speed in smaller increments than one tooth on the gear can do.

Odo off 1/10th in ten miles wouldn't show on the speedo needle.

Clay

[bryany]

Thanks but I'll trust the GPS for accuracy of my speedo.

1/10 in 10 miles would be a non issue I think.

I am guessing the adapter was someone's attempt to correct for non stock tires at one time. It looked more cruddy than the trans and the wrong bolt was used to hold the speedo cable to the trans. An old wore out part maybe or someone's attempt to add 50 magic horsepower with a bolt on. Not some of the best shade tree work I've seen.


Bryan

[FrankieT/A]

My 78 T/A had that trans box as well from the factory. My box was 1 to 1 so i just eliminated it. I have in the garage somewhere.