I have a 79 L48 and the speedometer does not work. It worked about 6 months ago. I have checked the back of the speedometer and the cable is all the way in. Not sure how to check the cables or the thing they plug into in the engine compartment. Not sure what to check next.

 

The thing on the fender is the cruise control transducer. You could pull the cable leading from it to the tranny and check if it will turn. It shouldn't. Pull the one from the transducer to the speedo and turn it with a drill motor or something and see what the speedo does. At this point you should know where the problem is. I actually installed the cable going into the tranny on my '79 with everything in place,it's a little tricky and you have to hold your mouth just right. If that don't work,you could put a jack under the tranny,pull the crossmember and lower the tranny enough to get the cable in and out. Seems like nothing is easy on the vettes! When you but big things in small packages,things get crowded! If the vette challenges you too much,work on school buses and vans for awhile! Trust me,vettes aren't that bad!
Ron

 

I will try it later this week or maybe I will get a chance tomorrow. I have to get the speedometer working so I can get the state inspection done on it so I can drive it to work on the base. Do you know where the wires come from go to that are on the cruise control transducer? The cruise control has never worked since I have owned it.

 

If you do the drill thing, make sure you are spinning in the right direction or the needle w/ probably just stay put. Usually you can just twirl it w/ your fingers but that requires someone to watch speedo for movement.

 

According to my "full color wiring diagram" from www,ClassicCarWirinG.com the 2 wires from the cruise "regulator" go to the control switch on the steering column.
Ron

 

That'll be a good excuse to buya special tool: a steering wheelhub compressor (so you can remove the lock ring). While you can do it w/o it, you won't have to use as many four letter words in doing so.