Starter solenoid 'earth wire'?

[Joshua Blackh4t]

Yesterday I almost get stranded on another airport when my M20E wouldn't turn over.

I assumed the starter motor was worn out but after some checking it turned out to be the starter solenoid and a couple of well placed firm taps made it work enough to get me home. Yay for good old mechanical things.

However, when trouble shooting it, I came across a wire which seems to go from the solenoid exciter to earth. Which doesn't make sense. Can anyone explain this? Photos attached

[N201MKTurbo]

You may find a diode under the heat shrink. If so it is what is called a flyback diode. When you open the circuit to a big inductor like the solenoid coil, the magnetic field collapses and tries like hell to make infinite voltage back through the circuit. The diode gives this charge a discharge path and saves the other electronics in the circuit from the abuse.

[33UM20C]

I couldn't find a proper wiring diagram of the specific solenoid you have. But its probably P/N: 001464 if you want to keep looking or need a new one.

Here's the service manual that has wiring diagrams for your M20E. Use the S/N to narrow your search if you need them

1966 Mooney-Service-Manual.pdf

[Z W]

That looks a lot like the flyback diodes sold at Aircraft Spruce:

https://www.aircraftspruce.com/catalog/elpages/solenoidassy.php

It acts as a one-way path for electricity and keeps the charge in your solenoid's coil from traveling back up the switch wire and causing a spark in your starter switch.

If you have trouble getting your exact part number of replacement solenoid, you can get a Lamar FAA/PMA replacement, very nice units:

https://www.aircraftspruce.com/catalog/eppages/sts-12s24s.php?clickkey=8060

If your master solenoid looks like that one, now might be a good time to replace it too, or at least throw a new one in your spare parts bag so you aren't left stranded on the ground somewhere.  And if the rest of your battery-to-starter wiring looks as corroded as the wire ends in the photo, just cleaning the connections with a wire brush and adding some dielectric grease might be a good idea, and might be all it takes to get everything working right again.

[Joshua Blackh4t]

Thanks everyone,

My mechanic cleaned the contacts and thinks that was all that was wrong with her.

I'll have a look at the rest of the wiring when i pick her up.

My mechanic seems to think the flyback diode was incorrectly installed. Not sure why. In any case, apparently its all working now, yay. 

Thanks for the link, looks like the part.