HERE ARE SOME HELPFULL INSTRUCTIONS ON HOW TO CHECK AND FIX YOUR TACH PROBLEMS
Be aware that there is an aftermarket tachometer circuit board made Overseas for 1975-82 Corvette.These product do not work, They short out when installed, so now it comes with washer to cure the problem, they are not calibrated, so they don’t read correctly not even close,and no instructions or 1 year warranty. Bottom line is they don’t work. They don’t tell you that their circuit boards are not calibrated in their ads, but people think they are, do it yourself and save. The poor person who purchases these boards think he must have other problems with the tach, not knowing he could not possibly make these boards work correctly, if at all.
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1. The tachometer 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. 1977 to 1982 circuit boards don’t , and you can’t tell by looking at it.
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 needle should go to zero if working. On 1978 to 1982 Tachs, the Speedo and Tach housing’s main wire connection causes problems and should be checked. 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, you can’t miss it, about the size of your thumb. Use a wire and bypass the tach filter all together.(Remove , unclip 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 it 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 RPM’s or more to high, Circuit board is bad, you need a new board. Also on 1978-82, sometimes the needle pegs past 7000 RPMs and stays there. 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 and move it back. If the needle pegs to the bottom again either on 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 can NOT just put it back on at any place and expect it to read right.You must rezero the tach needle. 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 REZEROING 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 , it fluctuate a lot .A new circuit board will not fix this problem.
4. . *** Note, the original G.M., Delco circuit board sold over the counter were not calibrated, so buying a new old stock (N.O.S. )board will not make your tach work.