Car is a 1941 sedan with straight 8 and hydramatic transmission. It is now drivable and from the beginning I noticed that the speedometer needle was bouncing around a lot, between 10-20 MPH (car not ready to drive fast yet). So I decided to repair it and these are the steps I followed:

1) Removed the cable from the transmission and from the instrument cluster
2) Cable is in perfect shape. It measures 61 and 1/8 inches, which is the recommended length for Hydramatic
3) Lubed it and reinstalled. When tested, not bouncing any more but as soon as it hit 30 MPH it got stuck there
4) Removed the instrument cluster and removed the speedometer gauge. Needle is fine, nothing on the cluster or gauges that offers any obstacle to movement of the needle.
5) Cleaned the speedometer and oiled it by removing the small bronze plug and felp
6) Hooked the speedometer gauge to a power drill with variable speed and increased progressively until the needle get to 60 MPH with absolutely no trouble,
7) Lubed the cable again and connected to the transmission first and to the instrument cluster second
8) Test drove and at low speed - about 10-20 MPH - the needle moved freely and appeared problem was resolved but when I reached 30 MPH it got stuck again

I think the speedometer gauge is fine. The power drill test seems good evidence. For some reason when connected to the car the cable is not driving the gauge. I start suspecting the problem is at the connection with the transmission but don't know how to test that. Based on the shop manual there is a gear deep inside the transmission (driven metal gear) but there are no steps how to get there. Removing transmission is totally out of my skills.

Those who have worked under the dash of this vintage know how painful it is. All the metal edges are sharp and there is no room even to breathe well. I am not anxious to go there again to remove the speedometer but will see what suggestions you have.

Thank you, Manuel

 

Manuel, from the test you did I would suspect that the speedometer cluster works properly. As you said, it's probably a faulty drive gear in the transmission. FWIW, I had a 90 Buick Estate Wagon with a broken speedometer. It was the drive gear in the 2004R transmission. By that time they were made of plastic so it was a common failure. You could always install an aftermarket speedometer...

 

After it got stuck at 30 MPH, what did you do to get it "unstuck"?

 

You can hook your drill to the transmission end of the cable to see if its anything but the transmission.

 

Nothing. After parking and turning the engine off the needle slowly returned to zero. Happened several times before. Don't know if that is a clue?

 

Yes, good idea. I did it today and despite high speed of the power tool the needle did not move. This is very unexpected because when I connected the power drill directly into the speedometer gauge on the bench it worked perfectly well. Although the speedo cable appears intact, has the correct square shape at the ends and is of the correct length, this simple test suggests the cable is not engaging the inside mechanism of the speedometer gauge. Would you recommend replacing the cable? or any other ideas?

 

Was the drill turning the cable in the same direction as the transmission did?

 

Try to loose the nut/fitting that is screwed to the speedometer a few turns, have proven in some cases to help.
Not sure but I suspect that the inner wire can be a little to long and pushes into the speedo.

My two cents,

/ Andreas

 

Have you removed the cable from the cable sheath? I've seen cables break internally and give the impression that they are connected when they are not. sometimes a broken cable snags itself back together temporarily. You should be able to remove the cable in one piece. Don't worry, if its not broken, you can always thread it back in the sheath. It should go without trouble. That's the best way to lubricate it too.

 

This^^^^^

 

Aha!! You are so smart, thank you!. I removed the cable from the transmission today and hooked the power drill again and tested the response when spinning first clockwise and then counterclockwise. Have a nice video showing how the speedometer needle reaches 60 MPH very steady when the drill is spinning counterclockwise. I totally missed it the first time.


Yes, I did and I measured it and lubed it on the bench as described initially and is in perfect shape.

At this stage it seems I have a problem at the transmission end more likely.

 

Thank you Andreas. I will follow your advise. It seems logical that if the inner wire is too long it may create excessive friction at the end that sticks the cable? Easy to try. Will see what happens but unfortunately now I have car engine troubles and it is not running so can not test the speedometer for now.

 

The attached video confirms that all works well from the gauge to the end of the speedometer cable. Something not right at the transmission end. More work to do.

 

Is there anything inside the face that the needle could be hanging up on when it hits 30? There is a speedometer repair shop in Richardson Texas that will rebuild your speedometer head. Zepco is the name. They have a website.

 

Some speedometers have a stop gear that will not let them go far in reverse so as to keep some one from running the speedometer back to change the miles on a car Don't know if that is a issue with this car but something to keep in mind....Tedd

 

Thank you all. You were right edzolz. Speedometer issue finally solved. Now that my engine is running perfectly the speedo could be tested again. The problem was so ridiculous simple that it is embarrasing. The metal plate behing the needle had a slight excessive curve so that the needle got stopped at around 30 MPH. When testing with the power drill, the RPM and torque was so much that it passed the slight obstacle and the needle went up. But when driving, the needle had no power to overcome the barrier and stopped there. That was the explanation that the mechanical shop owner gave me when he checked and discovered the problem, and it makes sense to me. Happy with result. Thank you all for your insight and ideas.