Stock Subaru Tach with Lem G3

[Matt Alexander]

so, I'm helping work on a turbo swap into a '98 Legacy 2.5GT (you'll see a couple questions here from myself and Grayguy, the owner, about it). we've got it wired in, and running. still need to do some tuning. but we're not having any luck with the tach. of course, I didn't actually read the section about the tach in the link manual, so I didn't realize it would be a problem until we got the car started and it didn't work. the stock subaru tach needs a pulse to ground to work. and the LEM G3 (obviously) pulses 12v+. I pulled the cluster out, and re-wired it so that the normal signal wire get's grounded all the time, and the signal wire from the Link is attached to the + terminal on the tach. it was a long shot, but worth a try......well......that didn't work either. I was thinking use a small relay of somesort. wouldn't have to carry any load (just an amp, maybe 2, I suspect). but would have to be very quick, and resistant to wear. but that would be triggered by the 12v+ coming from the Link, to ground the signal wire for the tach. good idea? other suggestions?

[ashesman]

To be blunt, the problem will be a wiring issue, setup problem or tacho fault.  All our plug in ECU's and adapter based plugins, as well as all wire-ins to subaru's that I have done wire the tacho signal directly to an auxiliary output.  Wire the factory ECU's tacho signal directly to an auxiliary output.  Configure the auxiliary output as tacho.  If it is Aux 1, 2, 3 or 4 then make sure you set the multiplier value to 1 (remember some RPM x 0 = 0 RPM) and duty cycle to about 25%. 

Just for your info, all LEM Aux, Fuel and Ign outputs switch to ground (not 12v).  The tacho signal is about 150 Hz (150 pulses per second) at 4500 RPM, so a relay would not work as it could not switch fast enough.  In that case you would use a transister to invert the signal...

To test your install, set the auxiliary output to "Test PWM", set the frequency to about 100 Hz and make sure the tacho goes up and down as you vary the frequency...

Let me know what the problem was...