Here, I whipped up a quick youtube video for you. This might not be the issue, but on the other hand there are speedometer repair places that can correct a weak spring and make other fixes if the head is worn out. Might be easier to find another speedo head if it comes to that.

https://www.youtube.com/watch?v=FyXBMOXMcYw

Chad

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Maybe, this could be the problem. How would you know?

 

If you exhaust other possibilities.

Describe the symptom a bit more - does it max out at anything above 1 MPH? Does it temporarily max out and then drop lower and sort of twitch back and forth or is it steady at a given speed?

Changing a gear one tooth up or down usually affects it about 2.5 MPH one way or the other and there are only a few gear choices over all. So, like I say, trying to correct a 100 MPH pegged problem with drive and driven gears and ratio changers can only do so much.

A person can disconnect the speedo cable at the head and pull it back out and reroute it into a side window so you can hold it in front of you while driving around the block. The inner spinning portion can be observed. It will try to creep its way out of the jacket but a finger tip on the end will usually hold it in place. A slow speed should equal slow revolutions of the inner core. Faster speeds should increase revolutions. If that all looks fine then the only thing the speedo cable hooks to - and it is the end of the line - is the head.

It's all very mechanical in operation - just the fine nature of that head makes it something that doesn't tolerate much abuse.

Chad

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When I step on the gas and just get going, it jumps to 55 then right up to max. It won't come down until I go in reverse. I thought I heard a different sound when I shut the engine off. Kinda like a spring unwinding. DOINK!

 

Here's a paragraph in the 1966 Shop Manual about how to test the head.



Chad

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Thanks for all the leg work and knowledge, Chad.

 

Chad, I have a feeling the resistance spring is worn out. Why else would the speedometer just peg out? If gears only make up a couple mph. What do you think?

 

Here, you can probably isolate it a little more using this technique.


Let me know if I can clarify anything. Don't use a super high speed drill/dremel or you will just max it out. And don't bend the core while you have it chucked up. You should be able to DX the cable at the transmission and use the drill motor to turn the core, thus turning the speedo, run the drill motor backwards (counterclockwise) to simulate forward and of course opposite (clockwise) to simulate reverse.

Let me know.

Chad

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Chad, I removed the cable from the transfer case and hooked up a drill. I ran it slowly counter clock wise. The needle jumps to 55 and then to 100 with hardly any rpms on the drill. The needle sticks at 100 and needs to run clockwise to get it to return to 0. The front of the gauge cluster is cracked and would need to be replaced. Do you have suggestions now?

 

See the cracks near the mounting holes on the face?

 

I'm going to go out on a limb here and suggest that it sounds like you have a defective speedometer head. I do see the cracks in your bezel. That has seen better days. It doesn't have the simulated engine turning either, like someone painted over or it has faded badly. If you can't salvage the bezel to reinstall it then they do make reproductions.

Instrument Cluster Bezel and Lens | Dennis Carpenter Ford Restoration Parts

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Chad, I would still need the gauges and speedometer? What is a simulated engine turning?Does anyone reproduce the speedometer head?

 

The term "speedometer" and "speedometer head" are really one and the same thing in this case. It is an assembly unto itself. Ford did not separate out anything that could be replaced in this part. See pics #1 and #2 in post #125 here.

https://www.ford-trucks.com/forums/1...l#post14616787

As far as I know nobody reproduces the original style speedometers. NOS or used would be the only choices as far as I know. I've never personally seen another NOS one other than the one in my picture.

Here's what looks like a decent used setup.

Ford Truck Speedometer Gauge Cluster Instrument Panel OEM 1965 1966 F100 F250

I'm not saying to buy that one, who knows if it works? But, you can see the complete setup there with the rear metal housing that holds the speedometer and other gauges in place.

Your Fuel and Temp gauges could be reused - the speedometer can be changed out independent of everything else.

The simulated engine turning is the swirl pattern on the face of the bezel around the gauges.

This is a Dennis Carpenter reproduction bezel.



And this is the NOS bezel. Very similar but it has overall a better reflection. You can hopefully see the swirl patterns in it. Also, notice the silver paint on the border around the inner opening. Ford did them like that and the reproductions don't have that paint on them. Easy enough to add that.





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Since FoMoCo made the speedometer assemblies, is it correct to think that any speedometer from 1966 from Ford or Chevy would be compatible?

 

Chevy would not be compatible, as it's a totally different speedometer, and...

Chevrolet is made by General Motors,