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.
Thursday, March 24th, 2011 AT 11:02 PM