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Discussion Starter #1 (Edited)
looking through my stuff tring to figure what tach i have as i rmber on the 4 gen tach the rpm red lines was different on the sixes vrs v-8 does anybody know what they are ?

my tach is yellow line at 4700 rpm and redline is at 5000 rpm as i recall after i put it in and started the car the rpm were way off too high (12years ago) so i just unplugged thinking it was for a six cylinder

can not run the car now is there anyway to check it out ?

at the time i had a msd modual in the gm hei dist runing a msd in cap coil , could these have been the problem ?
 

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Discussion Starter #2 (Edited)
still no thoughts on this ? only asking because tonight i may pull the dash

not sure what the tach cam out of , ebay

could be a inline 6 or a v8 or maybe even a buick v6 like in my omega
 

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If its a factory teach I don't believe its adjustable between 6 and 8 cylinder models. Have you looked into the aftermarket repro's... they may have an answer. Also a lot of factory tachs used a ballast ir resistor in the 70's... that may be why your readings were off.:yes:
 

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Discussion Starter #6
If its a factory teach I don't believe its adjustable between 6 and 8 cylinder models. Have you looked into the aftermarket repro's... they may have an answer. Also a lot of factory tachs used a ballast ir resistor in the 70's... that may be why your readings were off.:yes:
yes it a factory tach , not seeing any resistor in the wiring diagram
no there is no adjust between a 6-8 you can buy a inline convertor but before i do that i want to be sure that's the problem

here is the one i found

TACH-ADAPT

http://www.ashlocktech.com/TACH-ADAPT.html
 

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I've never looked at the internals of a 77-79 Nova tach to know for sure but all of the other late 70's (and 80's) GM tachs I've seen have a calibration resistor on the tach's internal circuit board. The resistor is a film on a ceramic chip and the factory fine tuned it's value by making a little cut in the film.

It is fairly common for those calibration resistors to increase in resistance as they age. I think it's due to moisture/condensation getting into that little calibration cut and degrading the film. Anyway, the increased resistance causes the tach to read higher than it should.

Here's a good writeup showing how to fix one in a 1979 Camaro ---> http://www.howtoalmanac.com/kevin/projects/automotive/tachfix.htm

I don't know for sure, but there is a good chance that the 77-79 Nova tachometers are using this same type of circuit & calibration resistor. The internal circuit board layout is likely different than the Camaro one shown on that site but the same basic repair procedure should still apply. Also note that once the calibration resistor has been replaced with a potentiometer as shown it doesn't matter if it started out as a 6 or 8cyl tach ... you can now adjust it to suit your needs.
 
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