Starter relay/ignition switch misbehavior

[Michael]

Have any of you ever had this happen?

Yesterday I flew through some rain between Palm Springs and French Valley. After grabbing a bite to eat at the field restaurant we climbed back in the plane to do some touch an go's. I put a friend in the left seat for his first experience flying a 260SE. As I talked him through a hot start I watched him take his hand off the ignition switch (key) and yet the engine kept cranking before starting up and running normally. Since I wasn't in the left seat working the controls I really thought I might have imagined it. Then while talking him through some of the avionics etc. I noticed the low voltage light was on and then some of the avionics were cycling off and on as I revved the RPM's some to see if I could get the light to turn off. I then smelled an electrical burn smell and pulled the mixture and flipped the master off. Upon turning the master switch back on the engine cranked by itself.

A mechanic talked us through (on the phone) removing the cowling and tapping the starter relay. This worked and the plane behaved normally. My understanding is that the relay got stuck (maybe because it got wet) and the light was actually indicating over voltage, something about the starter acting as a generator? I'm taking it to a mechanic on Monday to have this looked at a little closer.

Michael
97AV

[Todd Peterson]

Yes I have seen this twice. Unfortunately one of the times the airplane was in my shop when it happened.

For some reason the starter relay will stick which then shoots the full line voltage right to the starter when the master is turned on. In the Cessna handbook it states that you never want to turn on the master without clearing the prop first. A warning I never much thought about until it happend to me.

There is a small wire on the starter relay that you can unhook which removes the starter switch from the circuit. If the prop turns over when the master is turned on then the relay is bad. If the starter doesn't turn over then the starter switch is most likely bad.

The other problem here is that if the starter sticks on when the engine is started the starter will not disengage as it normally does after the engine is running. Unfortunately there just really isn't any way you can spot this problem. If this happens it will quickly overheat and destroy the starter.

While this is not a common problem it points of the need to always have the prop clear when you turn on the master switch.