I have recently purchased a 79 350 manual vette. The Tach doesn't work at all and the speedo reads very high. I don't know if a previous owner has changed anything as I am still sorting through the numbers book. The speedo is very frustrating as we have a lot of speed cameras where I live so knowing the actual speed would be helpfull. Anyone have any ideas as to where to start. Thanks.

The tach and speedo don't have anything to do with each other. I guess that's the bad news.
The speedo is gear driven from the transmission, through a cable, to the speedo head in the dash. It may go to the cruise control then the speedo head. None of these will make it read too fast. The gear in the transmission may make it read too fast if it's the wrong gear. This can be changed or checked by taking out one bolt that holds gear holder into the trans, where the cable connects. Just pull it out and count teeth or check the color of the gear. Color is not reliable after several years. Now check the tire size and gear ratio of the diff. Go the the parts book charts at Chevy and see if it's the right gear. If it is, then the problem would be in the speed head itself. It can be removed and serviced. You may be able to clean it and be okay. Don't get anything on the face of the speedo or the odometer numbers. A light penetrant like WD40 will work fine. Then gently blow off the excess. If this doesn't work, it will need to go to a specialist. Electrosound on Brookpark Rd in Cleveland OH is one place. Hopkins and Kipps in Akron OH used to do this, I don't know if they still do. There are others.

The tach is fed from the tach lead (brown wire) that comes out of the distributor, at the side of the cap. It goes to a tach filter, the goes to the tach in the dash. The tach is fed with ignition power and ground, and the tach signal. It could be disconnected, or the tach filter could be bad, or an open circuit to the tach. If the signal is okay and you have power and ground, the tach need repaired. Same places. There is probably one close to you somewhere.

Ken Styer

kstyer >> you mentioned the tach filter,,, do you know the "proper" mounting place for it? Mine is just laying loose next to the distributor. Thanx

Ken, Thanks for the information. I will start with the gear ratio first as taking the dash out looks a bit more fiddly. With regards the tach, what does the filter look like and where would I find it? I have noticed that the brown wire from the distributor has be cut and re-connected at sometime in the past.

The tach filter is in line in the brown tach wire. It is usually near the distributor or mounted on the firewall. I have found one under the dash.

Ken Styer

If the filter IS mounted on the firewall it must be grounded somehow. It does precisely jack if it's not grounded. The proper place for it is bolted down to the intake manifold, just beneath the distributor. then it's grounded thru its mounting tab..

Ken/Theo, Thanks for your help. I haven't started on the speedo yet but there definitely is no filter on the tach so I need to get one before I can say there is anything else wrong, hopefully it was removed by a previous owner who didn't know what they were doing and fitting one will resolve the problem. Again thanks for your efforts.

There is an older instruction from Corvette Fever on this. It doesn't say too much about the Speedo, but does give a detailed description about rebuilding your tach.

http://www.3will.net/~jawillis/vette/tech/corvette_fever/jul97/eN.htm

Hope it helps.
Glenn