Jonathan Goodish wrote:

HOWEVER, I've now discovered another potential problem. In my quest to
inspect ground connections, I pulled the cover on the rear bulkhead to
look at the glideslope receiver, strobe power pack, and battery. The
master relay is next to the battery, and I nearly burned myself on it
after a 30 minute flight--it was that hot. I let it cool down, and ran
the airplane for another 5 minutes and it was almost too hot to touch.

Since the starter cranking current goes through the master relay, then
if the contacts in the master relay develop high resistance, because of
the voltage drop, you would not be able to start the engine. The coil
in the master relay should pull less than 2A, which would mean that the
coil in the relay is dissipating less than 20W. 20W of dissipation will
make the case feel too hot to touch...

Not all solenoid relays used in aircraft are interchangable. For
example, the one that is used as the master in my Cessna is rated for
continuous duty, and its coil pulls about an
Amp. The starter solenoid, however, is rated only for intermittent
duty, and has a much
higher coil current (about 3A), probably so it holds in stronger... I
have seem people mistakenly replace a continuous duty solenoid with one
that is intended to be used as a starting solenoid, and then they
wondered why it got so hot....