Rev counter misbehaving with uprated electronic ignition

[David C]

Fitting a fancy new electronic ignition had the unintended consequence of causing the tachometer on my 410 to vastly over read. This is because the rev counters on early V8s use current sensing loops rather than voltage sensors on newer units.

I chose to update my tacho by replacing the 1960s electronics with a new circuit board made by a company called Spiyda trading on Ebay amongst other outlets. Cost of board about £45. Time to fit about an hour once the unit was out of the car added to about another hour running in a new wire from the coil negative to the tacho.

Instructions are pretty good with helpful videos on YouTube. A major advantage of fitting the new board is that the unit can fairly readily be calibrated during fitment. This is fairly straightforward but requires a mobile phone or laptop with a high level audio output. I struggled a bit as my laptop wouldn't produce enough level.

Nice to have accurate measurement and smoother needle movement.

[Green411]

This is interesting. I'm in the process of uprating my 411 S3's Pertronix Ignitor 1 to an Ignitor 2 which senses dwell and more importantly doesn't get upset or fry if I leave the ignition on. In other words it's got more electronics in the module.
I fitted the Ignitor 2, connected red and black leads to the coil, fired it up and re-timed it, and hey presto: a misbehaving rev-counter. The motor ran fine but the tacho read 1500-2000 rpm at a 750 rpm idle, ok and about right but with some flickering on a test drive, and shot up to 3-4000 rpm whenever I put the brakes on. Weird or what?
I note in the Ignitor 2 instructions that the red doesn't have to be connected to the coil +ve, it can also go to a switched 'live when cranking' feed from the fusebox, which should get rid of the problem. Haven't tried it yet as I put the Ignitor 1 back on in a huff, but will update when the Ignitor 2 is fitted again.
BTW the Ignitor 1 was a big improvement over the points, I had quick, helpful responses when I emailed Pertronix Europe, initially about how long I could leave the ignition on without starting before Ignitor 1 damage occurred - answer was only 15-20 seconds, which I hadn't realised when I first bought it.
I must do my research thoroughly, I must do my research thoroughly, I must do my research thoroughly.....