TOPIC: Tachometer for a Bigtwin

I would like to have a working Airguide tachometer in my dash. I understand that there is a transmitter required for this to be possible. I also believe they are rare. Is there a source for a schematic for something like this? Thanks.
Don

Don,

The Airguide (and most all other makers) originally had the pulse module (many people call them a transmitter) seperate from the actual gauge. The later models married the two by putting them in one case. So depending on what model gauge you find, it may need the module, or it may already be inside the case. You'll have to do your homework on which models will work. And yes, the modules can be very difficult to find.

The old tachometer gauges were nothing more than a very sensitive volt meter inside a fancy chrome case. The pulse module converted the high voltage ignition pulses into a constant DC signal of very low voltage (far less than even 1/1000th volts) As the speed of the engine increased, the pulses to the module would be converted to more DC voltage, making the needle rise or fall, depending on motor speed. On a magneto equipped engine like yours, the tach is driven off of the coil primaries - the same circuit serves to feed the "kill" switch wires.

I've never seen a schematic for one anywhere. The circuit essentially uses a smaill transformer to step down the ignition impulse voltage, then a diode connected to a resistance - capacitance newtork. It isn't complicated, but haveing the correct values of parts is essential. Since I'm an electronic technician, I do have all of the parts in my shop - with the exception of the pulse transformer. I should dig out some of the old tachs that I have stashed away & measure the values of the parts. I might be able to make modules for people that way. Unfortunately, I'm busy with more pressing projects at this time, so it would be a "back burner" project.

Bruce

Flush mounted in dash had transmitter built in. Surface mount had separate transmitter.

Maybe this will help. www.classicboatwork.com/classicparts/gauges/airguide/airguideinstructions/Airguide%20653%20Instructions%20Tachometers.pdf