Help please! Humming noise and automatic start

[CTsalt12]

Hey everyone,

I encountered a strange issue yesterday on my way back in from a good night fishing.  So I shut my engine off at the dock, and a humming noise persisted from the engine.  This noise definitely does not occur normally.  

The only way I can get the humming noise to stop is shutting off the battery, so that's what I do.  Then I turn the battery back on in a few minutes to
Check it, and the engine immediately begins starting without me
Turning turning the key.  It's almost like
The starter never shuts off.  Ive gotte get back ok the water ASAP!  Any help is much appreciated!

[Blue Agave]

May be the ignition switch.

[John Jones]

May be the ignition switch.

Just about has to be that or a short in the motor harness.

[fitz73222]

I'm in the ignition switch camp; stuck in start mode or the starter solenoid is stuck closed. Unplug the engine wire harness, under the cowl; its on the starboard side on your engine. Turn the battery switch back on and see if the starter engages with harness unplugged. If she tries to start the solenoid is bad or stuck closed. If it doesnt try to start the keyswitch is bad. This is a quick test to get you back and running and getting the right part you need.

Fitz

[CTsalt12]

Thanks fellas.  This is a job for my mechanic anyway, not me.  I'm pretty useless for now.  Appreciate the help though, you've gotta be right about the ignition switch, it seems old, maybe original to the boat making it an 89?  Is that possible or would it have had to be replaced when the engine was replaced?

[Blue Agave]

It's possible, but if it was working when the motor was replaced it would not be necessary.

[GoneFission]

This one is a little odd - Does the motor start OK at first with the switch?  If so, the starter drive is OK and the drive gear would move up if the switch was turning the starter motor on, and the starter would turn the motor over...  

What kind of motor is this?  Is it an HPDI motor - if so it could be the compressor running.  Any chance it's a stuck solenoid on the trim/tilt motor?  Did you try tilting the motor up or down when it was making the humming sound?

[fitz73222]

This one is a little odd - Does the motor start OK at first with the switch?  If so, the starter drive is OK and the drive gear would move up if the switch was turning the starter motor on, and the starter would turn the motor over...  

What kind of motor is this?  Is it an HPDI motor - if so it could be the compressor running.  Any chance it's a stuck solenoid on the trim/tilt motor?  Did you try tilting the motor up or down when it was making the humming sound?

He has an '89 I4 Merc 115, same as my '95's; carbed, these are the 2+2 engines that idle on the top two cylinders and then jump on all 4 when you get to 1800 rpm's. They're a little quirky and a factory service manual is a must to understand timing and synchronization. Despite the quirkyness, they do work and Mercury produced them from '89-'05. I really think this a key switch stuck in start mode internally.