The tach in my '79 runs about 300 RPM higher then I get with my Actron CP7529 digital readout is there anyway to calibrate it?

 

I have a 74 which is mech. tach so I don't know if it would apply to yours but I noticed the tach needle is on a coil like a thermostat. You can adjust the tension on the coil by twisting the surrounding bracket therefore changing the tach readout. You'd have to take the panel out to reach it but that's all I can think of to suggest.

Brad

 

Well, at one time it was calibrated. And, I suppose that calibration could change over time and with use/wear & tear.

But, the first thing to check out would be to replace/refurbish/repair the 'tachometer filter' that resides on the intake manifold and is [electrically] between the tach and the instrument circuit board (which then feeds the tach). That filter is there to eliminate any electrical spikes that the tach could misread as rpm 'pulses'. So, if the filter is bad, the tach might read higher than it should.

At least, that would be a lot easier and less expensive to try...before you start pulling your dash apart.

 

I will check that out. I take it that it gets the pulse from the distributor. If that don't fix it I will live with it. It is not that big of a deal that I want to pull my dash apart

 

Rick-

Actually removal of the tachometer is very easy on your car. It’s the 1975-1977 guys that have a horrible time. Your tach comes out from the front of the cluster housing.

Here is a page on how to do it.
Click Here:
Corvette Tachometer Testing, Removal, Board Installation, Video Link 1978-1982

Originally the tach board was calibrated by having a scribe placed in the fixed resistor or what is called a DIP (Dual In-line Package). As the gauges age different things can cause them to become out of calibration. The analog movement contains resistance oil inside and as the gauges age this can leak out. You can usually tell when this is the problem by tilting the tach forward and backward in your hand. Good condition tachometers will usually hold the needle position while older wore out ones the needle will drift. The capacitor that holds the calibration can also fail...

Your best bet is to purchase a US made tach board and install it. We make the tach boards and yes we calibrate them to be accurate based on our own formulated scale. This does not mean the boards are 100 percent accurate on every tachometer basically because of the same condition mentioned above! Movement condition!

So what is the solution, for a correct calibrated tachometer it is best to send it in and allow the board to be match calibrated to the movement! Our new boards have an adjustable 15 turn potentiometer on it that allows the board resistance to be tweaked. The better the "pot" the more precise the reading.

Can this be done at home with a new board? I'd like to say yes.. But no, you really can't....

Below is a picture of where the tach filter is located on the intake. I don’t think this is your problem, it could be but it is probably just the old dog lying down.

There is a ton of info about tachometer's on our web site.. How to remove the tach, how to do a zero set, and so on. There are some diagnostic tips there as well.