I'm trying to recalibrate my tach VIA the grannies speed site. They state I should use an old car battery charger for a signal. I don't have this and have no access to one. Is there another source that can be used that will output a 120hz dc wave?


https://grannys.tripod.com/rx7tachrecal.html

 

For an ad hoc signal generator, I've used a cheap USB sound card in the past. Drive it with a signal from the software program called Audacity. It's free and will allow you to generate tones of various frequencies, not sure about square waves but definitely sine waves. The signal coming out of the sound card will be in the hundreds of millivolts range possibly up to 2 volts, so you might have to drive it through a small audio amplifier to get closer to 12 volts.

Edit:

Something simpler might be to buy a rectifier and a 12VAC wall wart...

https://www.amazon.com/dp/B08DFKTRVF

https://www.amazon.com/Reliapro-ACU120100D0531-Adapter-Transformer-Straight/dp/B00B8861T2

 

I tried one of the 12v transformers I had. used a battery to power tach. Then used the same ground on teh transformer. When I touched it to the signal it would just make the needle bounce a little. I assumed it wasn't putting out a "pulsing" 120hz. Same if I just use another 12v battery and ran a jumper to the signal input. It just jumps a bit off 0 rpm. Then falls.

I'm wondering if the unit I have is damaged. I've never seen it work.

 

Making things harder than they are. I have a stand alone ECU. I just opened teh laptop and adjusted the pot. to match RPM on the ecu. works like a charm. For anyone else doing this, a standard 12v ac/dc converter does not work.