Tach Reads 1k Too High

[kupcar]

Hi All, I'm having an issue with my tach. It reads 1k too high throughout the rev range. When I turn the ignition key on the tach will go to 1k with the motor not running. Once the motor is started the tach reads around 1950. I did replace the ground strap, the reading stayed the same. The car is a 76 930. Voltage regulator? The tach does not bounce, it just reads high. Crazy thing is every once in a while the tach will drop to the correct reading. Anyone have any ideas? Thanks, Jeff

[Bob Ashlock]

This really sounds like a tachometer-gone-bad situation. The way to confirm this is to (a) make sure the battery voltage at the tach is 12-14VDC, (b) the ground of the tach (relative to the chassis) really is 0V, and finally (c) the tach input signal should be 0 volts when the engine is NOT running. If you have an simple digital voltmeter, confirm that you are measuring 0 Volts DC and then switch the meter to AC and confirm you are still getting 0 Volts. If the meter shows voltage on the AC scale, that would be an indication pulses are arriving that should not be there, and that would mean the problem is coming from elsewhere. (speed control unit, or ?) My suspicion is that the tach is just bad.

[kupcar]

Bob, Thanks for the reply. I guess I'll get the multimeter out. I was hoping it was a common problem. What seems strange is when the ignition switch is turned to the right in I believe the accessory position, the tach will read 1k and my other gauge needles will move a bit. I hate electrical problems. Thanks Again, Jeff.