77 Corvette Speedo, Odometer, Tach Problems

[David S.]

Hi Guys,

The speedometer and odometer on a 77 Corvette that I've working on do not work. The tach works on and off. Starting with the basics of where the cables or connections go, can someone walk me through how to trouble shoot and fix this issue.

Thanks,
Dave

[Lynn H.]

Re: 77 Corvette Speedo, Odometer, Tach Problems

David,
Your speedo is mechanical (cable driven), so you might check the cable first (easiest fix). After that you may have to remove the speedo itself to have it checked. If those prove out, the next thing to look at would be the drive gear in the trans, these are plastic and can wear out. Might actually be easier to check this before removing the speedometer. The tach is electronic, and if you are getting it to work intermittently, I would first check the connection at the distributor to make sure it is making good contact.
Lynn

[Edward J.]

Re: 77 Corvette Speedo, Odometer, Tach Problems

David, Lynn has you on the right track for the speedo- just add one thing,If the car has cruise control its a two piece cable -just go under hood and un-hook the cable front the cruise transducer and have a helper watch the speedo while you spin the cable by hand, it should move with the rotation of cable, if no cruise control unhook at transmisson and spin the cable and check for movement of speedo. if theres movement then start looking the gears as Lynn says. cables seem to the most likely causes with speedometers.

The tach. may have a filter attached to the dist. which if that year has it may be something else to look at.If so you can by- pass it and hook to dist. just to try.

[David S.]

Re: 77 Corvette Speedo, Odometer, Tach Problems

Thanks Guys. Would there be any effect on the speedo if the cruise control is busted and does not?

-Dave

[Edward J.]

Re: 77 Corvette Speedo, Odometer, Tach Problems

Dave, I suppose it would depend on what the problem is with the cruise, is it a mechanical issue, internal gearing , electrical, I say first step is to check the cables - upper and lower. they should be a easy check. the lower cable I would un-hook both ends to check, you may need a extra set of eyes to look to see if theres movement when you spin.

[David S.]

Re: 77 Corvette Speedo, Odometer, Tach Problems

The cruise control servo is broken, and the vac hoses are not connected. So to back track, I'm going to disconnect both the upper and lower cables from the transducer. Then spin the inside line of the cable, while having someone sit in the car and see if the speedo moves? Do I disconnect the cable from the transmission too?

Thanks,
Dave

[Edward J.]

Re: 77 Corvette Speedo, Odometer, Tach Problems

yes, to make sure the lower cable is good, un-hook both ends and spin the cable at the transmisson area and watch tha upper end to check for movement.