I have an old school VDO beetle tack form the 70/80s. The beetle engine is gone and I now have a obd2 Subaru engine in its place. I tried running the tach from the ECM signal and got nothing.
I also found this posted about LS's and old tachs,
" You're gonna need what's called a "Pull-UP" resistor. Just put a 1K ohm resistor from your tach power wire to the tach signal wire."
Can it be that easy ?
Thanks !
Don't know the answer , but my 73 Corolla Challenge car came with a Sun tach and I'm putting in an LS, so interested to see what needs to be done.
In reply to DeadSkunk (Warren) :
Gen III era PCMs (red/blue or green/blue connectors) are easy, they send out a 4 pulse per rev tach signal. This might even be changeable in config. GM engineered those early PCMs to drop right in to work with existing distributor/TBI gauges, which were engineered to drop right in to existing distributor/carb gauges.
Subaru... um... pass!
But the recommendation of a pull-up resistor suggests that the Subaru PCM supplies a ground signal (normal) but the VW tach needs to see the tach wire get pulsed between power and ground, so you'd need to supply it power so the Subaru PCM can pull that to ground. Easy way to do this is to put 12v on the line through a 1000 ohm resistor. I did this to get my tach working after installing an MSD in such a way that I could not use the MSD's tach signal output.
I do not know if this may hurt the Subaru computer's tach driver. It *shouldn't*, 1000 ohms isn't going to allow much current through.
An "old" Beetle tach will need a high voltage pulse, and most ecm can't provide that. Diy sells a nice, inexpensive tach adapter for this when the cyl count matches. If not one of the prgrammable dakota digital boxes is the way to go (useful for 4 to 5cyl swaps).
You also may be able to jumper the input resistor for the tach but that only works w the same cyl count.
http://www.msextra.com/doc/ms2extra/MS2-Extra_Hardware.htm#tachoout
Sorry if a duplicate post, the first attempt is not appearing on my phone.
Looks like this has a solution for all tachs.
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