How to wire a vintage Airguide Tachometer for Magneto Motor?

[Steven Lichty]

I am trying to wire up my vintage Airguide model 690 Seaspeed Tachometer.  I have a 1960 Evinrude Lark II 40. The materials I have been able to find for this tach indicate that I am to leave the jumper wires disconnected on the back of the tach, as it is a two stroke two cylinder with magneto.  There are two wires coming out the back; a black and a yellow. I believe the black should go to a good ground (I could take it right to my negative bus panel on the fuse box).  The yellow I do not know where to connect. I assume it needs to come from one of the magneto primary wires. Would it be ok to wire it up to the back of the ignition switch to one of the black connections that are each marked ‘M’? As I believe these are the magneto connections.  Thanks.

[Gary H.]

Hi Steven- It looks like you can wire it to either M terminal on the ignition switch. They go to each ignition point set. Wiring a tachometer to old cars with points in the day, the trigger wire would attach to the negative side of the coil. The factory wire that attaches to the negative side of the coil goes to the ignition points. As far as which wire, black or yellow going to the M terminal, you may want to see if you can get a wiring diagram. I do not know if you reverse the polarity if it will hurt the tachometer.

Gary

[frankr]

Not familiar with this exact tach but yes that would be the normal hookup. It counts the pulses that occur each time the points open to that M cylinder (once every revolution).

[frankr]

It will work connected to either M cylinder but to avoid troubleshooting confusion in the future connect it to the one that does Not lead to the vacuum cutout switch.

[Steven Lichty]

good tips, thanks you guys. Finding out which black magneto lead is the one that is not connected to the cutout switch will take some sleuthing… the wires disappear into the shrink wrapped bundle that goes from  the back of the switch all the way back to the junction box and onto the fuse panel, and then the other side of it is another shrink wrapped bundle that goes up to the motor.  Will need to put a test light on some long leads….

[frankr]

Personally, i would connect to either one.  In fhe first place, the cut-out switch does nothing 99% of time. the other 1% is when the motor is in an overspeed condition due to over-revving in a no-load condition, such as revving it up too high in neutral or busting a shear pin.  IF and when hat happens, the tach might stop working temporarily until the motor speed is under control and the cut-out switch returns to its normal function, which is doing nothing.  My suggestion was simply to eliminate confusion when trouble shooting.

IF you see such strange tach behavior wh3n the cut-out switch is doing its thing, simply switch the tach lead to the other M wire.  Problem fixed.

Now figure out why the motor is over-revving.

[phil]

Found this in my stash of airguide info

http://www.omc-boats.org
http://www.aerocraft-boats.org

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