High Level Tacho - Low Level Drive (peer review please)

[Craig Jennings]

Howdy,
Seeing as there is weird info online and not much is clear in regards to driving a high level tacho with a low level signal I figure I'd take mine apart and apply Ohm's Law as I figure it's got to be simple enough surely!
97 Mazda B2200, link atom g4.

So I dragged out my tacho

You need to see what is + 12V, what is Ground and what is 'switched ground' or whatever that is, signal I suppose, from coil.

Now that you've found your signal, check to see what size the resistor is (hopefully there is one and I've not wasted your time).
Mine is a through hole resistor which is nice and simple, they may exist in thin film too but those are easy also, don't be discouraged. You'll want a fine tip on your soldering iron though.

So mine is a 42k resistor. Now using Ohms law 200V / 42000 =  ~0.005 amps. (guessed at 200V, might be 400 but with such a large denominator it doesn't really matter)
Because we have only 12V to play with we go backwards to find the resistance needed for that amperage
12 / 0.005 = 2400, so we need a 2k4 resistor.

So I took out the 42k resistor, replaced with 2k4 resistor and put it back in.

Video of tacho working:

http://i801.photobucket.com/albums/yy295/Bangbug_bucket/video-2014-06-30-21-48-13.mp4

RPM sweep works nicely too, thanks link!

[Craig Jennings]

And for those with fine taste in Kiwi metal will be able to identify the backing track

[Adamw]

You can also usually make these high level tacho's work using a relay coil like this:

http://www.haltech.com/wp-content/uploads/2013/07/Relay-Wiring-for-Tacho-Output1.pdf

[Craig Jennings]

I thought about that one, would be handy if you didn't want to delve into the dash.
Nice addition Adam, now we have it covered hopefully!