jls wrote:

And what's it for? Voltage spikes maybe? It's just under the voltage
regulator on the firewall of a 1966 172, which, btw, is a wonderful flying
airplane, as I can attest from experience.

Whoever installed this device did a sloppy job because the wire was out of
the oversized terminal on the switch side. Anyway it's apparently a tiny
resistor wired to the starter switch on the solenoid and the other end of
the wire runs to the solenoid's hot terminal.

Likely a 1n4007 Silicon rectifier installed to comply with a
Cessna Service Bulletin.

It is there to suppress a voltage spike caused by the collapsing
magnetic field in the coil of the starter solenoid. This happens
just as you release the starter key switch.

There should also be one on the master (battery) solenoid. If your
aircraft has "jump start receptical", then there should be a third
one on that solenoid, too.

MikeM