Need help diagnosing speedometer issue

[Zoro]

A few months back the speedometer in my '84 Eagle wagon quit working, the needle always bounced a little bit when it did work so I assumed the cable seized up. A few weeks ago I jump in the car and for some odd reason the speedometer is working again as if nothing ever happened, in the back of my mind I thought "I knew it would fix itself one of these days!" Unfortunately the next day the speedometer died again. It started off working somewhat but it seemed to be showing a slower speed than I was really traveling. In about a mile's worth of driving it was back to not working again.

Consulted the AMC FSM for 1984 and it seems that the only thing NOT covered in great detail is the speedometer cable system. There is an extensive chapter on the "Cruise Command System" however.

Here's what I know:
When the speedometer is not working the cruise also doesn't work
When the speedometer decided to randomly work the other day the cruise worked until the speedometer stopped working again
Underneath the car everything appears to be connected as it should be

Here's what I'm trying to find out:
The location of "speed sensor" for the "Cruise Command"
Is it under the hood or somewhere behind or up under the dash?
Where does the cable come thru the firewall?

Anyone else have any ideas as to what might be the issue with my speedometer?

[Psomion]

The speed control sensor is in the engine compartment if memory serves me right. I remember it being located against the fire wall in the engine compartment in front of the driver, but I could be wrong. It's fairly obvious to find if you follow the cable. If the steel wire is frayed it will sometimes work as you've seen. Could also been stripped gears I'd think... easy to check by pulling out gears from transmission. You're right about not bothering with the upper speedo cable, sounds like it's working fine

[BenM]

If you have a loose cable or exhaust leak it will melt the cable and cause havoc as well. I believe it comes through the firewall fairly high up, almost directly behind the speedometer, near the brake booster. I've replaced mine at least three times; make sure you clip it back down away from the exhaust.