Speedometer

If the lower cable is badly twisted on one end, suspect a locked up servo.

Yup. It's what the speedometer cable goes into. The upper and lower speedometer cables are attached to either end of the green shaft. As the shaft spins, the orange weights fly out further due to centrifugal force. As they fly out more, the levers pull the yellow sleeve to the left. The black lever, (anchored on top), is at rest with both blue valves closed. It is not attached to the yellow sleeve.

When you press the "Set" button, the black lever is magnetically attached to the yellow sleeve at its current location. As long as vehicle speed remains steady, the sleeve and the lever do not move. When the car slows down, springs pull the weights back in toward the shaft. That pushes the sleeve to the right and it takes the lever with it. As the lever moves to the right it opens the blue vacuum port. Engine vacuum enters the chamber behind a rubber diaphragm. That increased vacuum causes the diaphragm to pull harder on the throttle cable and that brings the vehicle back up to the set speed.

If speed increases too much, the weights fly out further, the sleeve moves to the left, and pulls on the lever to expose the vent port. That releases some vacuum and lets the diaphragm and throttle cable relax.

What can happen is one of the four links holding a weight can break and let that weight fly out so far that it catches on the housing. The green shaft won't be able to spin. Since it is locked up, moving the car causes the speedometer cable to turn on the transmission end which twists and shreds it.

It is also possible that only the plastic drive gear for the speedometer cable is worn or broken although that is not real common. The clue will be found by disconnecting the lower cable from the servo and pulling out the center cable. If it looks fine with a nice clean square on each end, suspect the drive gear in the transmission. If the cable is mangled on one end or is impossible to pull out, suspect the servo. When the cruise control still works and the speedometer does not, suspect the upper cable or speedometer itself.

I should mention too that there were some AMCs that used a version of a common aftermarket electronic cruise control. The aftermarket version was very nice but it got its speed signal from magnets attached to the drive shaft. On those models, they would still work fine if the speedometer cable broke. I don't know this for a fact, but I would expect there is some type of speed signal generator inline with the speedometer cable for an original equipment system. That means the speedometer cable would not go to the servo, but the system still would not work if the cable broke.

AMC built their cars with everyone else's parts, Chrysler transmissions, Ford generators, GM brakes, etc, so it stands to reason they used someone else's servos too. Any chance you can post a photo of it?

My guess would be any servo that will connect to the speedometer cables will work. Logic dictates that they aren't going to make different instrument clusters with or without cruise control so you can assume the upper and lower cables will spin at the same rate. The servo doesn't really care how fast the speedometer cable is spinning; it is just going to try to keep it spinning at the same rate as when you pressed the "set" button.

The biggest difference, besides the speedometer cable threads and physical mounting would be the electrical connector and the throttle cable connector. If I had the service manual for your car and the car the new servo came from, I might be able to figure out which wire goes where, but to make life a lot easier, look for the right part.

I get a lot of parts from "Pull-A-Part". That is a chain of very clean and well-organized yards where you pay your buck, then throw your tool box in one of their wheel barrows, and you can spend all day there. This chain has 23 yards between Indianapolis, Ohio, and southern Georgia and Alabama. Parts are very inexpensive. I did an internet search and found no Eagles but the same servo might have been used on other models. There's an AMX in Atlanta, a Concord in Baton Rouge, two Eagle 30s in Canton, OH and Charlotte, NC, a Spirit in Indianapolis, and a Concord in Knoxville, TN.

There are similar yards all over the country but many of them are very proud of their parts and charge accordingly.

Oh. That's not the servo; that's just an electronic speed sensor. Instead of the mechanical flyweights in my sad drawing, you must have an electronic controller that pulses the vent and vacuum solenoids on and off in response to the speed signal.

It seems hard to believe that this sensor could lock up but I would have to assume that's what happened. Can you spin the center where the speedometer cable went in? If it spins freely, try spinning the cable that goes up to the speedometer. I've seen some speedometers come apart and lock up. Sometimes when they do, turning the cable very slowly will make the pointer go all the way up to its highest reading, then lock up.

If the sensor is locked up, I would be looking for some way to take it apart to peek inside, hopefully without destroying it. If something plastic broke and let the spinning part move off-center and hit something, epoxy and silicone gasket sealers work well to fix stuff like that.

Where is that place? I'm in Wisconsin too, sorry to say, but have never heard of them.

Dandy. Happy to hear you can buzz off, ... Ah, ... I mean SAIL off into the sunset.