I recently had to replace the master solenoid on my T-18. I know I
have replaced it at least one time and maybe twice in the previous 20 years of operating
this airplane. So I got wondering what was happening and why I was having to replace them.
I cut one apart and found the contacts sort of pitted and rough, but
not really burned looking. Then I got to thinking about how it was mounted. I found some
articles in Van's newsletter that addressed that subject.
The Master solenoid should be mounted upright
(domed housing up) so that the coil is pulled down when activated. Thus gravity helps to
keep it engaged and bouncing over rough ground would not cause it to chatter or
momentarily break contact. Also hi-"g" maneuvers in flight would help to keep it
engaged.
Conversely, the starter solenoid should be
mounted with the domed head down so that the coil draws the plunger upward to engage. High
"g" maneuvers or jolts could possibly activate the starter solenoid if mounted
the other way around. If this occurs, the starter engages the ring gear (in Lycoming
engines) with the engine running and generally has expensive results.
Solenoids are not designed to be mounted sideways. Internal parts can
gall & bind. The work best when mounted with the plunger vertical.
Also, a bit of deterioration occurs every time a switch controlling a
solenoid is turned off. The starter and master relays are heavy duty electromagnets. A
coil of wire wound around the plunger forms a magnet when voltage is applied through the
switch contacts. When voltage is applied, the current ramps up slowly in the coil causing
very little damage to the contacts. However, when the switch is turn off and the magnetic
field collapses, there is a large induce voltage across the switch contacts. (Think of
the coil as a large flywheel that is suddenly braked.. It can tear things up) It is
this surge of voltage and current that pits and burns contacts. (Just like a small spot
weld)
There is an easy way to prevent this damage. Radio Shack has diodes
that will do the trick. A 100 volt, 3 Amp. Diode will work.
The starter relay diode should be installed between the small terminal,
where the wire from the starter switch is connected, and the mounting lug (ground). Important;
The end of the diode with the red band should be connected to the small control terminal.
The battery relay diode is place between the relay control terminal and the large terminal
on the side of the solenoid connected to the battery. Again, on
the master relay, the end of the diode with the red band is connected to the battery lug.
Van also has these diodes in his catalog for a couple of bucks each.
They have the proper size terminals to install directly on the solenoids. They are; ES
DIODE STARTER, and ES DIODE MASTER.