Tach will not work with Petronix

[Doctor510]

I have a 72 Datsun 240Z with an Ignitor. I installed it as per the instructions. It runs great, but the tach will not work, any suggestions? Works great with points. I understand that the tach is a "current sensor" type, unlike most aftermarket units which operate off a signal from the negative side of the coil.

I put the old coil back in (current coil is a Petronix), started the motor, tach went to 1,000 and then it dropped back to "0".

[tamo3]

Easiest way is to swap with 280z Tach or after market Tach.

http://www.classiczcars.com/forums/showthread.php?t=42405&page=2&p=365658&viewfull=1#post365658

How to Swap. Blue's artcile.

http://www.classiczcars.com/forums/thread47191.html

[SteveJ]

Did you bypass the ballast resistor?

[Doctor510]

I did not, I have others that the tach worked with it.

[Randalla]

SteveJ, I also have a 1972 with Pertronix. My tach works fine and I have not wired around the ballast resistor. When I wired around the BR the coil got too hot to touch because the current was not being stepped down. Wondering if the issue may be the coil on the car. Pertronix has several coils, each with their own internal resistance values. I'm certainly no expert but wondering if someone else can either confirm or dispel my theory.

Edited November 7, 2012 by Randalla

[beermanpete]

If the wiring is correct the tach has failed. If you remove the tach and car runs the wiring is wrong. If you remove the tach and the car does not run the tach is bad.

There are 2 black/white wires in the wire loom going to the coil and ballast. If they get mixed and installed incorrectly the car will run but the tach won't work.

One of the black/white wires should have power when the key is on. This goes to one side of the ballast resistor.

The green/white goes from the ballast to the tach.

The other black/white wire comes from the tach to the positive side of the coil.

Basically, the power goes from the ignition switch, to the ballast, to the tach, and then to the coil.

Connect the red (Pertronix) wire to the + terminal on the coil.

Connect the black (Pertronix) wire to the - terminal on the coil.