How to test a factory tach??

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The easiest way would be to "jerry rig" it up on a car with a tach you kinda trust and compare it in a few RPM settings. With the service manuals posted on here, you should be able to figure how to hook it up. Should pretty much just need power, ground, and a tach triger. If you have something like a stock ignition, coil neg. If you have MSD/ other special stuff, you'll have to go to their websites. I'm running a GM HEI module, and my little Autometer triggers fine on coil neg.
 
Actually you could if you have a way to generate a signal, but I don't want you blowing it up. Years and years ago, I put together a Knight kit (Allied Radio) tach, and it came with a large value resistor and a neon lamp. You wired them in series with 115V AC!!!! which provided an 1800 RPM calibration signal. Imagine doing that today.
 
OK, Steve, here's what I came up with, and IT WORKS

You need

1 A laptop or even any computer with a sound card and analog output. Mine is pretty low level from my laptop (phones, not speaker) and it worked. Obviously you need basic software, which most computers come with to play sound files through the system.

2 A transformer to step up the voltage, some clip leads, and a cable to "get" the sound out of your computer

3 Some sound files which I easily found and downloaded. I easily found some that worked fine right here:

http://www.ronelmm.com/tones/

I also looked around and found a 440hz tone on the www

The tones translate thus:

That is, 100hz x 60 to get hz per minute, and divide by 4 to convert to V8 tach reading (V8's fire 4 times every crank rev)

100hz is 1500RPM

150hz is 2250RPM

250hz is 3750RPM

440hz is 6600RPM

500hz is 7500RPM




4 A small amount of math to convert the sound file frequency to V8 tach readings



In the top photo, I started out buying a Radio Shack audio transformer, not ideal, and the only one they had in stock. Part no 273-1380.

I also experimented with a "wall wort" you want one that is AC output, NOT DC output, and better the LOWEST voltage output if you have a choice. This one, second and third photos, is a relic from the thrift stores

The third photo shows all that there is for the wall wort. Once again, it MUST have AC not DC output. You hook the analog output of the sound card to the low voltage leads of the soundcard, in this case the USB dongle

The 120V AC plug is then hooked one lead to ground on the car, the other lead to the tach trigger wire, which MUST be unhooked from the coil/ ignition

Photo 4 shows the hookup for the RadShack audio transformer

The black and yellow clips at top, hooked to transformer white and red, come from the sound card.

The black lead of the transformer is the center tap, unused

Ground one secondary lead, and hook the other to the tach trigger (For this transformer, green and blue)

Then, just power up the tach, fire up a sound file, and crank up the volume. Turns out my tach reads "a little low" up above 3750RPM

By the way, that's not an ammeter you are looking at, but a voltmeter!!
 

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I am thinking about a box with two leads to the car battery, and an output to the tach. It would have a push-button to select the RPM. A LED would blink the setting number. A table would be used to equate setting number to RPM for various cylinder engines 4, 6, 8.

For the DIY:
The parts list would include a 5V regulator, LED, MOSFET, inductor, resistors, capacitors, clip leads, prototype board, box, and uC (micro-controller).

If there is interest, perhaps I could source a pre-programmed uC. I would use one with an internal oscillator and calibrate when programming. I am not interested in making units, nor supplying other parts. I would provide a schematic.
 
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