Autosport Labs

Trouble shooting my tach-hook up. I got my tach changed to electric from North Hollywood Speedo. I sent them a diagram of how the tach was run off the 2 coil wires and the schematic.

I hooked up with the double diode and single zener set-up.

Fired-up the engine. With the engine running at about 850 rpm the tach reads 2000.

How do I troubleshoot from here? Do I first remake the circuit and try again?

Does it make a difference that I am running the Acell coil rather than the stock ford one?

Do I suspect the tach from Hollywood Speedo has not been calibrated correctly, Perhaps it is configured as a regular Smith's Tach,

Any Thoughts?

I talked with NH Speedo and it appears they set the Tach up for a 2 cylinder engine thinking the wasted spark wouls function properly if only getting one lead from the coil, so it is reading approx 2 times what the input is.

Would it be possible to use a single leg from the coil and leave it set up as a 2 cylinder?

No harm in trying - report back...

Nice car BTW!

Sounds logical to me. I used only one coil trigger lead to hook up to the "tach sense" on a Subaru remote start. It works fine. You probably can hook it up directly, with no diode, but hook it up with the diode first so there is no possibility of feedback. What I mean to say is remove one of the diodes completely, and leave the other one in. Only remove the second one that is inline to your tach if the tach doesn't display the proper rpms.

First I hooked up the tach with the Diodes and the Zener and just one leg from the coil to see what I got. It wasn't quite right, so I yanked the diode and zener and hooked up just one wire from the coil to the tach. Bingo, 900 rpm.

So, I guess if your going to have an electric tachometer made into your old manual tach housing, have the shop set it up for a 2 cylinder engine. Then it's real easy to hook up.

Rod