for your consideration concerning tach troubleshooting and when, if and with what to replace, calibrate fix..Jim
1. The tach circuit board is the most likely problem why the tach is not working, or working correctly, but not always. On 1975-76 Corvette tach the printed circuit board usually show a burned spot, actually melts the thin copper wire and the problem is obvious.
2. Checks to do to see if the tachometer is working for 1975 to 1982 Corvettes, or if 12 volts or ground are missing, or the signal from the distributor is missing. The tach needle usually stays about 1000 to 1500 RPMs ,when the vehicle is off, this is normal .Turn the key to the on position engine not running, the needle should go to zero. If the needle goes to zero this indicates the tach is working and 12 volts and ground are present. If the needle does not move,either the circuit board is bad or 12 volts or ground are missing to the tach. Remove the tach from the dash and apply 12 volts and ground, tach should go to zero if working. Now , start the engine, if the tach does not move off zero, the signal from the distributor is missing, the likely problem is the tach filter, follow the wire from the distributor cap, can't miss it, about the size of your thumb. Use a wire and bypass the tach filter all together.( Disconnect both ends completely.) If the tach works now ,that your problem.The Tach filter also, if not working correctly makes the tach work erratically, jumps around, etc. bypass if you think it's bad. On a lot of 1978 to 82 Corvette the problem is the tach is reading to high, 1000 to 2000 or more to high, Circuit board is bad, need a new board. Also on 1978-82, sometimes the needle pegs past 7000 RPMs, it stay there because it can't get back to zero because it's bottoming out .You need to move it back to zero by hand, take the lens off. If it pegs to the bottom out again eitheron the zero side or 7000 side, then the board is bad.
3.All these instructions assume we are dealing with a factory tach and wiring etc, that has not been messed with or replaced, etc. Note, if the needle is removed for any reason, you just can't put it back on at any place and expect it to read right.(IT's not like Speedometers, also Speedo and Tach needles are different ). To rezero the needle 12 volts and ground must be applied to the tach, then the needle must be set on so it reads zero. Also re zeroing is not calibration, also switching circuit boards, 1978-79 only, 1980-82 only. 1980 to 82 tachs have a hole for the choke light, 1978-79 don't have this hole.1978-79 tachs Calibrate differently from 1980 to 82. Another problem in warmer climates, the needle has a dampening effect, there is oil inside the meter head, if this dry's up, the needle swings freely and causes it not to settle down .fluctuate a lot .A new circuit board will not fix this problem.
4. Purchasing a new circuit board, *** Note, the original G.M., Delco circuit board sold over the counter were not calibrated, so buying a new old stock board will not make your tach work.
FIRST, ask if the circuit board is calibrated to read correctly for your year corvette, if not don't buy it, you need special equipment to calibrate it. .
TWO, Does it come with instructions, how to re zero the needle ,(a must for 1975-1976 tachs),possible problems you may encounter, etc..
THREE, are there three insulating washer, if yes, stay away,these type have problems. I believe there made in China.
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