Misbehaving Tach

[Guest rlbleeker]

Have a '62 Skylark with factory tach, which works great, unless I have the headlights on. With the headlights on it reads about 500 rpm high (at cruise speed) and is somewhat erratic. The car has a rebuilt generator (last year) and a new regulator, and no other electrical problems. Really not sure where to start on this one.

[Reatta Man]

Your tach problem sounds like a classic grounding problem. The inacurate reading is likely because the light bulb in the tach is using part of the current flowing through the ground, and the tach can't get an adequate flow of current.

Look to see how and where the tach is grounded. If it is through metal-to-metal contact with the console, and then with the console grounded to the floor pan, make sure all screws are tight. If that doesn't clear it up, you might want to remove the screws, clean them with some OO steel wool (available at any hardware or home supply stores) and clean the area where the screws go into the floor pan, as well as the metal part of the tach housing that touches the console. Also check the connection for the wire(s) coming out of the back of the tach. Make sure they are clean and tight.

If you still get a bad reading, I would consider running an extra ground wire from the tach housing directly to a good ground, such as a major brace support inside the dash or another thick piece of metal.

There is also the possibility that the engine has a weak or bad ground. Check the negative cable from the battery to the engine block. Make sure there is no corrosion inside the end of the cable connected to the battery. If there is, remove all the cable that is corroded, or replace the entire cable. Make sure the cable is tightly mounted to the block.

Good luck.

Joe