Speedometer bouncing

whb52ford

whb52ford

Senior Member
I have a 31 Tudor with F150 transmission. Everything was working fine till I decided I wanted to replace my speedometer. I received a different rebuilt speedometer back and I put a new cable from the Model A Medic in. It was fine for a couple hundred miles then the speedometer started jumping from 0 to 45 and back. Then it quit altogether. I thought the hole in the back of the speedometer has rounded out because the new speedometer cable rounded. I sent the speedometer back and he sent me another speedometer. I put the old cable back in because it looked more square than the one from the Model a Medic. I pulled the speedometer cable out, wiped it down with brake fluid. I ran a rag through the casing and put speedometer graphite on the cable. Hooked it up and speedometer read 0. I pulled the turtle and the gear was severely worn. I replaced the gear. While apart, I put a drill on the transmission and how the speedometer cable and ran it. I couldn’t see if the speedometer was working, but it did register 2/10 of a mile. I put it together and took it for a test drive. It bounces from 0 to 45 and back again. That is telling me that the cable is binding someplace. I’m stumped. Could the cable be too short and if so, where could I get a longer cable?
 
How long is the cable that you are using? How is it routed?

It sounds like the cable is binding somewhere, like there is a kink in the housing somewhere. Bouncing 0 to 45 is huge.

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Bouncing can be caused by two things. The speedometer is sticking or the cable is kinked and binding. Since the speedometers were rebuilt it is likely not the speedometer. Replace the cable but route it with generous radii. If a place along the cable has too tight of a radius it can cause binding. The rounding of the cable end at the speedometer concerns me and that would usually indicate a problem with the speedometer. The odometer will work even if the speedometer is bouncing, so have someone look at the speedometer while you turn the cable with the drill motor.
 
Take the car for a ride, with speedo disconnected, you need a view of the end of the cable as you drive. Does it turn evenly? If yes hold the end lightly to feel if it still turns evenly? If yes, your cable may be too long. Getting jammed in the speedo.
 
The T170 transmission is longer than even the later Ford semi synchronized transmissions. The drive shaft and torque tube were likely shortened to get it in there. here used to be a lot of speedometer shops back in the day but not so many now. They would make a cable what ever length needed with a flex housing to suit. As far as I know, there is only one place here in San Antonio that still fabs up cable assemblies. Bend radius is limited on the cable assemblies so it's best to try to get one that has long slow curves from turtle to speedo.

The speedometers have a magnet shaft that is driven by the transmission. The rotating magnet fits closely to the inside of a drag cup that is attached to the needle or number disc in a model A. If that drag cup makes contact with the magnet then it will be as jumpy as a grasshopper in a chicken pen. The support bearings are what keep the magnet and cup from sticking or rubbing. If the speedometer works smoothly when driven with a drill or what have you, it should be good to go
 
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Could someone explain to me more about the driving gear on the drive shaft moving forward. When I stick a screw driver in the hole from the turtle, I can push the drive gear all the way forware and back. Probably more than an inch movement. Is that the problem?
 
I also have a 150 trans.

2 things you could try

1: add some cable-ease to the entire inner shaft and try again. It is a very slippery, greasy graphite.

2: After you add the cable-ease hook up a electric drill on the cable that goes into the turtle and run the drill backwards to test the speedometer.

If this works the speedo, and then it doesn't work when hooked up, it's a good guess the cable is too short.
 
whb52ford said:
Could someone explain to me more about the driving gear on the drive shaft moving forward. When I stick a screw driver in the hole from the turtle, I can push the drive gear all the way forware and back. Probably more than an inch movement. Is that the problem?
Click to expand...


There is a snap right on the driveshaft to limit the movement of the gear. Should be about 1/4" movement.
 

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