...................
How to test a factory tach??
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!!
