Pulses on GT500 Ignition

[CJF]

I have an electronic Polaris snowmobile tach that I would like to use on my road race project bike. From what I have read, I should just make sure the number of pulses from the GT500 ignition matches the number of pulses the tach was made to read. Can anyone confirm the number of pulses on the GT500 engine?

[rbond]

If I understand, you want to run a electronic tach off of the ignition of a GT500? If this is correct, the are two magnets on the outside of the rotor that signal when to fire the plugs. there is one coil on the outside that senses these magnets. It may be possible to position a similar coil 180 degrees opposite the factory to get a signal to your tach. This should give you two pulses for each revolution of the crank. It may be that you will have to stick with the OEM tach. I have both a speedo and tach from a '77 GT500. You can have them for the cost of shipping......

[tz375]

On the Polaris tacho I have here, it's marked as 6 pulse. I thought that related to the three raised portions which trigger the pulser coil, but in fact, the way it's wired is straight into the AC from the alternator.

Most of the older Polaris sleds did not use DC and had no rectifier, so they used the AC pulses.

If that one is form a later rectified sled, the circuit may be different.

[CJF]

If I understand, you want to run a electronic tach off of the ignition of a GT500? If this is correct, the are two magnets on the outside of the rotor that signal when to fire the plugs. there is one coil on the outside that senses these magnets. It may be possible to position a similar coil 180 degrees opposite the factory to get a signal to your tach. This should give you two pulses for each revolution of the crank. It may be that you will have to stick with the OEM tach. I have both a speedo and tach from a '77 GT500. You can have them for the cost of shipping......

Rbond...thanks for the generous offer, but i actually already have the stock tach for the bike. I am trying to run an electronic tach to eliminate the need for running with a tach cable. After reading your post, I looked at my rotor and did see the two magnets. I am thinking the fact that there are 2 magnets would mean that there are 2 pulses per revolution of the engine. The magnets are 180 180 degrees apart on the rotor. So, the magnet would trigger a pulse against the pickup 2 times per revolution of the engine. Make sense or am I missing something?

[CJF]

On the Polaris tacho I have here, it's marked as 6 pulse. I thought that related to the three raised portions which trigger the pulser coil, but in fact, the way it's wired is straight into the AC from the alternator.

Most of the older Polaris sleds did not use DC and had no rectifier, so they used the AC pulses.

If that one is form a later rectified sled, the circuit may be different.

I know it is a 2 pulse because of the stamp and that it fits the Polaris Indy 500 model from the mid to late 80's....not sure of years. I tried looking over a few snowmobile forums to see how these are wired up on a snowmobile, but no luck finding the info yet. There is a yellow, black and red wire coming out of tach and the red wire is attached to a light bulb which I won't be using. So, this leaves the yellow and black wires. To your point, if the tach is made to run of ac, where do you think I would attach the yellow an black wires in order for this to work? My guess would be the black goes to the frame as a ground and the yellow is attached to one of the wires coming from the stator plate. I'm just not sure which one.

[tz375]

Correct Chris. Black is motor ground on a Polaris and yellow is power. usually that means un-rectified but regulated power. They use a one wire zener diode on s small square of aluminum for a heatsink.

On yours, if it has the alternator intact, tap into any one of the power leads out of the alternator on its way to the rectifier.

I would expect that stealing juice from the pulse coil cause problems, but only one way to find out right? try it and see.

[CJF]

Sounds good...I'll try it out and see what happens. Thanks