Thanks frankr. Can you tell me how my motor keeps the battery charged? And whether there is something there that could be intercepted to provide a signal to the tach?

Since posting the question I have found a “Universal Tach Adapter” called “Widget Man” on eBay that employs the wire-wrapped-around-a-spark-plug-wire just like the TinyTach. However, the device resides inside the motor shroud (or attached to the transom) and provides a wire with a signal any ordinary tachometer can use. You can view it here: eBay posting here

Anybody know anything about this one? Here is what the seller says about it:

“Doing a plug-wire wrap is usually the best approach with mags of all sorts. The Schmitt input on the adaptor even makes it “tunable” by changing the number of turns — getting a reliable pulse is a simple matter of fiddling until you find a number that admits the initial spark pulse at all speeds, but excludes all the ring noise. If you have a lighting coil, the AC signal from one phase of the coil will also make a very clean trigger, and the BTA has ratios for nearly every configuration of alternator. For the output, depending on the vintage and make of the tacho, you may need to add a booster to the BTA output. Many marine tachos are back-EMF types that won’t trigger at 12V, even some recent ones. You can generally check by bouncy-bouncing the tach signal wire on battery+ to see if the needle twitches. If it doesn’t, then chances are you’ll also need a booster (see our listing# 142171581596). We do bundle shipping if you need both, just pay full shipping at checkout and we’ll refund the excess when the items ship. Thanks for your interest!”