So I know this is a beech forum but I am flying with a friend right now and we are trying to head home in his cessna 172. When he turned on the master switch the starter engaged. I am guessing this is a bad starter solenoid???

I have no knowledge of the wiring of a C172, so all of this is entirely at your risk.

It might be worth removing the cowls and tracking the wire/s out of the starter motor. If there is a thin wire and a fat wire into the starter motor, see if the thin wire goes to a relay that's separate from the starter - it may be that that relay is stuck in. If there's just a fat wire, see if that fat wire goes to a relay that's separate from the starter - it may be that that relay is stuck in.

If it were me and it was an emergency, I'd give any relay that's connected to a wire that goes to the starter motor a good tap with a calibrated tapometer. If there is a thin wire and a fat wire going to the starter motor, I'd give the starter motor a good tap as well - on the bit to which the thin wire is connected. I'd guess the solenoid was in that area.

CAUTION: If I did this, I would triple check that the master/battery switch/es are OFF, and make sure everyone's well clear of the prop when I tested the success of my tapping by turning the master/battery switch/es on while at the controls and on the brakes.

CAUTION: If my tapping was successful and the starter did not engage when I turned the master/battery switch/es on, I would watch the ammeter and other electrical system indications VERY closely after start, to confirm that the starter motor disengaged after start. If the starter motor did not disengage after start, I'd shut down immediately and apply AMUs to the problem.

I reiterate: I have no knowledge of the wiring of a C172, so all of this is entirely at your risk.

There is nothing that would tell me that after it started the same condition might not exist? If it stayed stuck after start:

You would not hear it.
You may or may not have a high current draw on the ammeter
You will, eventually, fry the starter, alternator, battery... in no particular order

I'd not fly it.

Just sayin...

My experience is with a C 185, I suspect they are the same. The key switch (when engaged) powers a single conductor that goes to the solenoid. When this wire is powered the starter solenoid on the fire wall is engaged. The solenoid is about $150-200. If you disconnect the wire at the solenoid and its powered (with the key off) then the problem is with the key switch it the wire, if it's not powered then the problem is the solenoid.

Andy

What Mike said. Do not take chances on this. To many things can go south in a hurry.
Now, if I were stranded in an unfriendly place and the natives were doing their best to spoil my party, I would disconnect the leads at the starter and secure them with duct tape......(everybody has duct tape), and then hand prop the little Lycoming and be on my way to the nearest safe haven.

Not sure how it's possible to fry a whole bunch of electrical components without any prior abnormal indications in the cockpit after start, but as I say I don't know anything about the circuitry on a C172.

In any event, Del's suggestion seems to me to be a potentially safer solution in an emergency, provided that:

- the people involved know how to do a safe hand start, and

- it's certain the starter motor is not permanently mechanically engaged.

Sounds like a stuck starter solenoid/contactor. Easy to troubleshoot, solenoid should be forward of the firewall. Battery might be as well depending on year. A generic solenoid could be picked up from auto store to make it home. Could be the ignition switch as well...

You are right Clinton. I should have placed more emphasis on safety. Thanks for your post.
This condition can be very dangerous for the OP and for bystanders. Please be mindful and careful. If you are not mechanically inclined, please seek out someone who is.

No - thanks for your post. It highlighted an option that I had not considered for this problem but would now consider in an emergency. (I have hand started an A36 with a 520! The scary bit - which should have been unsurprising - was the ease with which the engine sprang to life. )

i hope it was a 2 blade prop

Nope. 3 blades.

It is the start solenoid or the key switch. If its the key switch disconnecting the small wire from the solenoid will stop it from engaging the starter. if its the solenoid they are rather inexpensive item. Just make sure when you buy a solenoid it is a starter solenoid and not a master solenoid. In a lot of planes they look interchangeable. Also if the engine is equipped with the shower of sparks system just pulling the prop thru will most likely not start it. I think you may get it energized if you have the key on start when pulling the prop thru. Good luck and be careful whatever you do.

Have C172s ever had a shower of sparks system?

Doesn't matter if its a 172 or a Bonanza. A starter has high current draw. There is a warning to not have some starters engaged for more than 10 seconds, some as long as 30, before they could get hot enough to damage them. If the starter is engaged while the alternator is working, the current the starter consumes can potentially over heat the alternator from excessive output. If you have a load meter for monitoring alternator current, you will note the high draw, but typical ammeter installation won't necessarily show it. Battery will be drained because the alternator can't keep up with the current draw.

About a week ago, had the solenoid stick on an S35 I was doing a dynamic balance on... everything shut off in the cabin, and the starter keeps on a spinning. One needs to act fast in disconnecting the battery. Another bright idea from Wichita - not run starter current through the master switch...!