Pertronix tachometer correction

Hi Jim. The resistor is fitted inline with the wire connecting the Ignitor to the Negative pole of the coil. The resistor was just a regular small resistor that I purchased from Radio Shack that contained approximately 100 or more resistors that are color coded. Dont recall if they were rated by Watt capacity.

Regards, Larry

Sounds like they were likely 0.5 watt. Thanks for the feedback, Larry — I’ll give this a try.

Jim

This still doesn’t seem to make sense to me. Going from memory here, on my 6cyl engine with Pertronix, the switched +12V ignition source comes from a fuse block, goes through the current sensor inside the Tach on a white wire and connects to the plus terminal of the coil. That puts the switched ignition +12V on the coil. The Pertronix plus (red) wire is also connected to the plus terminal of the coil to supply power to the electronics in the Pertronix. The negative (black) lead of the Pertronix is connected to the negative terminal of the coil and the Pertronix switches the coil negative terminal to chassis ground every time a spark is required. How does it do that with a 10K ohm resistor in line with the negative lead?

That’s the way I originally understood the fix to be. I used a 10 ohm, 5 watt resistor in-line with the wire that coils through the sensor on the back of the tach. However when wired that way, the engine would not start. Remove the resistor and it would start.

Jim

Hi Tony. Given my dim recollection of OHM’s Law, the resister just changes the amount of current to the Ignitor by approximately 50% which changes the RPM indicated by the same amount. For a better explanation, call Terry’s Jaguar which states that a 10K resistor is required in order for the electronic tach to work properly.

Regards, Larry

Larry, in applying the Pertronix to the V12 ignition system it must be wired differently from my 6cyl and Jim’s 6cyl engine, or have a different Tach that I’m not familiar with. Because a resistor in the negative lead to the coil on my car and Jim’s will not work. The engine won’t start as Jim pointed out… Back to Jim’s problem, my Tach was jumping around from 2 to 5K RPM and I managed to settle it down with a 1.0 uF 600v Mylar capacitor across the + terminal of the coil to chassis ground.
Anthony

Either you were wearing welding goggles when you did that job. Larry, or your mechanic needs glasses or truth serum. :slight_smile: . Pertronix don’t make a V12,points replacement system because no production V12 ever had points - they all had electronic ignition from new.

That fact pieces it together

Hi Peter. I guess to be technical, the ignition system in the V12 is part of the OPUS system that uses an amplifier to supply current to the distributor. I referred to the OPUS rotor and connection part in the distributor as “points”. Sorry if my terminology confused you or anyone else.

Regards

I wouldn’t have bothered remarking on it (mustn’t say ‘pointing’ it out!) if :slight_smile:

  1. it wasn’t open to confusion with the far more common 6-cl system,
  2. if wasn’t somewhat related to the core problem and
    3 if topics didn’t get dug up decades later by noobs.