Q: I recently installed a shift light tachometer into my 1969 Chevy C10 2WD pickup with a 250-cubic-inch 6-cylinder engine. The engine has an HEI distributor. Even though I followed the installation instructions carefully, I can’t seem to figure out this nagging problem. When driving, the tach registers anywhere from 5,000-7,000 rpm while cruising, even though the engine is really only turning about 2,200 rpm. I went back to the instructions and they said a bad ground often causes erratic readings. I replaced all the grounds on the vehicle, and even grounded the tach to the chassis, with no change. I exchanged the tach for a new one, but it does the same thing. The odd thing is even though the tach reads high, the shift light comes on at the right time. What should I do?
A: We’d start by checking to see if the tach’s selector switch is set for a 6-cylinder engine. Also, you may want to try the 4- and 8-cylinder settings to see if the tach was wired wrong from the factory. Most tachs come preset for 8-cylinder operation, but you should always double check the setting for use. Insufficient power or ground can also cause a tachometer to read abnormally high. As a precaution, check the voltage going to the HEI distributor. It should be between 12-13 volts with the engine running, not 9 like the original points system. Auto Meter has some good recommendations for checking voltage in the tachometer troubleshooting area of its website.
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I had a similar problem.. took forever to figure it out. Anyways it turned out to be a bad connection in the alternator. It was sending pulses all through the wiring. But only affected the tack.