Electrical tachometer on GT500, what pulses, where to read?

[Nicholas]

Hi all,

Hoping someone can share some wisdom on installing an electronic tahometer on a GT500A (1976 with CDI). I dont know much about this and would really appreciate a 101 on where the best place is to take the pulses and how they are calculated for the GT500? I believe the sparks fire upon each rotation and within a single cylinder at a time...excuse me if I am incorrect.

I am looking at the nice MMB electronic tachometers and not sure which one is right and what polarity and pulse rate to pick...
http://www.mmb-messtechnik.de/index.php ... 21&lang=en" onclick="window.open(this.href);return false;

The mechanical interface is 1:4, but I dont think this is related or can be translated...

Any advice or pointers would be greatly appreciated!

Nicholas

[H2RICK]

Nicholas, you seem to have a pretty thorough understanding of what the "question" is.

All the electronic tachs I've ever seen for early electronic ignition systems (aftermarket) take their input signal pulses from the +ve side of the 12V terminals on one of the high tension coils. Most of the post-1985 factory ignitions changed to take the tach signal from the ECU instead. It was probably that the ECU
designers forgot to put an output wire/terminal on the box itself. I've never looked into this closely enough to be sure why the change was made.
Some aftermarket electronic tachs have a dip switch that will allow you to select either 4 stroke or 2 stroke but I'm not sure if that would be the case in the tach you are looking at buying.
A SWAG says that if you used ONE coil's 12V +ve terminal as your input to the tach you should be okay because most modern tachs all seem to be made for 4 stroke engines with a wasted spark system with 1 pulse for every revolution of the engine......just like a two stroke.
I would contact the seller or manufacturer to confirm that they haven't got a divider chip inside their tach that "ignores" the second pulse on a 4 stroke motor. This would cause you to have a reading that's half what it should be on your 2 stroke engine.
Last bit of advice: do NOT buy any tach made to work on a Harley. There are so many different aftermarket
ignition systems and associated parts for "single fire", "dual fire", single output coil, dual output coil, etc etc that it can get VERY confusing.....for both you and your tach.

My .50 worth......
All clarifications/corrections are welcomed.

[tz375]

Does your ignition have two ignition coils - one per cylinder or one double ended coil? That will tell you which to purchase, plus they have the option on that form to tell them what bike it is for so that they can make sure you get the right one.

[Nicholas]

Hi, it is a single coil with two outputs. I guess it only needs to read from one of the outputs and it would be two pulses per 1 RPM. I have managed to get my hands on a configurable one, so I should be able to confirm the correct readings when I have it and the original connected. Thanks for your thoughts guys much appreciated! Its been a long journey, I should have my bike finished within the next 5 weeks. I should create a build diary as I have done everything!

Check out my dry run before I send it off to the powder coaters. Custom frame, rebuilt engine, custom pipes, tank and lots of thinking!