Pertronix tach signal

[davepl]

I have a Pertronix III module in a stock distributor body. I have the tach hooked up to the negative side of the coil. The first tach I tried didn't work with it at all, but a second one of unknown origin (but still a repro - hope it wasn't NOS!) did work.

I still think it reads high, and my theory is that the Ignitor III does multi-spark at low RPM, which fools the tach that can only see the coil.

Normally I'd think if you had an external ignition box that did multispark you would have a tach wire output that was only triggered once per cyl, but there is no such tach wire on the little module, so...

Is there a solution? If its multispark throwing it off, no amount of tach filtering is going to fix that.

[Coley]

Interesting....so you only have one working tach to test with at this point?
do you have an extra generic tach you can check against?
I'm running a Pertronix unit in my stock distributor and so far so good but it is the 1181 Pertronix unit. (not sure what the difference is between that and the Pertronix III).
It starts great, super reliable...no problems or issues and seems to run the tach without an issue.
That said, I don't think these tachs are notoriously accurate but it seems to be pretty close.

All Good
Coley

[BrianG]

I'm pretty sure that I have read about this with the Petronix III. I seem to recall that other users have 'downgraded' to the Petronix II to resolve similar issues. I couldn't find the threads though. Maybe a tach filter would help, but who knows.

Edit: I see you were having a similar issue quite a while ago. Did you ever figure this one out Dave?
http://67-72chevytrucks.com/vboard/s...d.php?t=634958
I have a similar issue, but its with a new AC-Delco distributor. The tach reads 4,000 with the ignition on, but not running. Then reads up from there when the engine is running.

[davepl]

Unless the multiple sparks come so close together that a filter would eliminate them as noise, I'm not sure if it'd help.

That other one thread you mention was the first tach, which didn't work right with it at all (read 2000+ when key on, engine not running). All I could do was replace that tach.

[BrianG]

Originally Posted by davepl

Unless the multiple sparks come so close together that a filter would eliminate them as noise, I'm not sure if it'd help.

That other one thread you mention was the first tach, which didn't work right with it at all (read 2000+ when key on, engine not running). All I could do was replace that tach.

Bummer, that's what I thought you were going to say... looks like I'll be ordering a new Tach. The one I have is new and unused, but I bought it a year ago at least, so getting my money back would be a long shot at best.

So, is this the same vehicle, same distributor, and just coming around again? Has it been a continual problem, or a new one?

[davepl]

I've had this tach in it for a year or so, so I think I just did't follow up on the other thread. The "reading too high" would be a continual problem, it just was a minor nit compared to things like the trans not working, so haven't worried about it much until now.