I recentally purchased a 78 Silver Anniversary corvette, L82 4 speed, that has been stored for about
8 years. The car had 20620 actual miles, but when I put it on the road, the speedometer will only
register up to 52 mph. At first it would go to 35 mph, then 45 mph and then yesterday it went to 52
mph. As the car is moving the needle will slightly jerk. The mileage part works fine and I have
checked it against mileage markers on the interstate, and it is right on. Also the tack is stuck on
25 rpm's and won't move any. Has anyone had any sililar problems with there speedometer or tack,
or have any suggestions? I thought that the speedometer might break loose after running it for a
while and has done that to an extent. I've put about 250 miles on it sinse I got it. I also took the
cable loose, at the transmission and put some liquid wrench inside the cable housing. Thanks: John

John,
I haven't had that problem with my vette, but I have a friend that has had that similiar problem with inaccurate speedometer readings. I think in his case we had to end up changing the speedometer cable core. But, another option that you mentioned would be to lubricate your existing cable. But since your car has been sitting so long you will probably have to lubricate it from end to end. I think the correct GM lubricate is (AC ST 700) speedometer cable lubricate. Also, make sure that your cable casing bends have no less than 6 in. radius in them from end to end , this will cause inaccurate readings and a kinked cable.

As far as the tachometer I am not too familiar with how it corresponds with your speedometer, but check all your connections from your distributor. and your instrument fuses to make sure they are working properly. It also could be a printed circuit board problem. Maybe another member will have some input on this question. I will do some more research. Hope this helps.
-
- Doug

|UPDATED|7/5/2002 2:26:19 AM|/UPDATED|

JOhn:
Not sure if you've located or fixed the problem yet, but I just had to replace the circuit board in my 79 to get it working again. It was like $60.00 from CorvetteCentral. No problem. In and out. You may also want to investigate the same fix on your speedo.

Better late than never,
Mark Romanowski

|QUOTE|Pound said: I recentally purchased a 78 Silver Anniversary corvette, L82 4 speed, that has been stored for about
8 years. The car had 20620 actual miles, but when I put it on the road, the speedometer will only
register up to 52 mph. At first it would go to 35 mph, then 45 mph and then yesterday it went to 52
mph. As the car is moving the needle will slightly jerk. The mileage part works fine and I have
checked it against mileage markers on the interstate, and it is right on. Also the tack is stuck on
25 rpm's and won't move any. Has anyone had any sililar problems with there speedometer or tack,
or have any suggestions? I thought that the speedometer might break loose after running it for a
while and has done that to an extent. I've put about 250 miles on it sinse I got it. I also took the
cable loose, at the transmission and put some liquid wrench inside the cable housing. Thanks: John |/QUOTE|

One thing you might need to check, is if it has cruise, make sure the cable isn't too cramped at the turn into the main unit under the hood. It is a real sharp turn, and that could be causing some problems. When I got my vette, the cruise didn't work properly, so I disassembled it, and it had clearance problems inside. I rearranged things a little, put it back together, and it has worked perfect since. I also put on a new speedometer cable, which helped.

After I fixed my ignition switch, my tach didn't work initially, so I drove on for a couple miles, then all of a sudden, it started working. Don't know why, but it has worked ever since.

Gremlins, I guess!!

These old Vette's sure do funny things, don't they?

On the tach problem ,I would first check your tach filter . If you look at the wiring that goes to your distributor , there is a light cream or white colored wire the has a small unit the looks like a condenser that was used in old points type distributors . These units have a habit of failing and I am not sure they are available any more, I may have some info laying around to fab one up . If I recall correctly the components are available at radio shack and inexpensive .
On the speedo it sounds like a lube problem and there are lube tools available for just this problem . It would be prudent on your part to check it out as the speedo cables really do not tend to last long when they are dry. I will look around for the info on the tach filters and get back to you .

geo