Calibrate vintage automotive tachometer?

[LG63]

Looking for some sort of inexpensive signal generator that I can use to check the calibration on an old (12V) automotive tach. I'd be happy with just checking a single reading like say 3000 rpm. Since the tach signal comes from the negative post of the coil I understand there will be a frequency to rpm conversion but get lost when it comes to wave form shape.

I saw somewhere online that an 8 cylinder engine at 3000 rpm would produce a 200 Hz signal.

Electronics aren't my thing so I would need something that could work out of the box or with minor modification.

 

[Pontiac787]

If you need to test the easiest way would be to hook it up to a vehicle that already has a tach you can compare it to. Is there a reason why you think it would be out of wack?

 

[Sevenhills1952]

V8, 8 lobes on distributor turning half speed of motor at 3000rpm would be 200hz (pulses per second).
There are pictures on line of oscilloscope views of the primary circuit, where you would connect tach, but if you have or could borrow a known good tach, dwell meter I would use that with your tach connected to verify accuracy.
That's the best test because they're designed for that, otherwise you'd be trying to use a signal generator with a short duration positive pulse of over 100v.

 

[LG63]

use a signal generator with a short duration positive pulse of over 100v.

I guess this is what makes it more complicated than I thought. Probably not likely to find such a signal generator on Ebay for cheap.

 

[Mechanical Noise]

I don't see why a tach would be fussy about a particular waveshape. It's going to have internal circuitry to standardize the signal, anyway. Ages ago, I built the Heathkit dwell tach. It was calibrated against the 60 Hz power line frequency through a resistor. I first used the resistor setup then figured it was safer to use one of the filament transformers I had handy. When checked with 120V-resistor combination or 12.6V/6.3V transformer, the readings were the same and made sense.

 

[Sevenhills1952]

You may be right, and I still have all the Heath equipment I started building as a teen until the late 1980s. The reference you mentioned was to calibrate it using a 60Hz sinewave, but the ignition pulse is not that.
Like pontiac787 said just try it on a car with a working tach.
I have lots of test equipment but I would rather simply put an automotive test meter on a running motor with the tach to make sure it works and is accurate.

 

[LG63]

Like pontiac787 said just try it on a car with a working tach.

True, this is a simply solution if you have car around with a distributor, working tach, and coil neg available. Unfortunately it's been several decades since I've had anything like that in my fleet

 

[Sevenhills1952]

Sorry. I thought your original post was "Looking for some sort of inexpensive signal generator that I can use to check the calibration on an old (12V) automotive tach."
So...if you have an old 12v tach you want to test...why not temporarily connect it to a car/truck WITH a working tach to see if YOUR tach works?
So simple.
Let me know if you don't have a test vehicle.
I assume it's an old tach you want to sell...ebay maybe? If so let me know. I have NOS tachs, test equipment, etc. I'll be glad to experiment, design a simple circuit so you can test yours.

 

[johninct]

I did the same thing that you are asking to do.....very , very easy. I bought a cheap digital non-contact tach off of Ebay and it worked great!!! You could then check high and low RPM's and calibrate as necessary. I actually like my cheap tach better!!