Mercury 525EFI won't shut off when key is switched off, when battery is switched off, or when kill switch is pulled. Only way to turn off engine was to pull ignition wire at back of motor. Replaced ignition switch and did not make a difference. This problem arose suddenly. Could it be the alternator? Any ideas? Thanks!

mike,i think it is a diode in the alternator.i have heard of it before.

Thanks Mike. Any suggestions on how to test that and/or fix it?

With the engine running, the ignition key off, pull the small white connector (2 small wires) out of the alternator. If the alternator is back feeding the electrical system, the engine will die.

Which would mean the alternator needs to come out, right?

If NOT your problem from above as mentioned,

Possible key switch and or harness problem

Possible missing a ground somewhere of even missing a connection to the battery.

Possible computer problem, I have had 3 that turned out to be a bad ECM / PCM 555's as well. So sometimes you can not rule anything out

2 where ECM 555's and 1 was PCM 555

i would disconnect all the alt wires,then start the engine,then shut it off.if it shuts off you have fount your problem,if it keeps on running john[BUP]can tell you what to do with the ecu.edit in,don,t let the heavy wire touch ground or anything else.i think this is safer than pulling wires while the engine is running.

have to jump back in I would not unhook any 12 v source wire(s) with the engine running. If by chance you get a voltage spike and it can be very possible - you run the risk of blowing relay(s) or even hurting the PCM. If I recall correctly all alternator wires unhook beforehand, I think the engine (ECM/PCM 555) will not start plus again the risk of a 12 volt spike.

I would take the alternator to a professional repair shop that can test it.

I want to know do you have a battery selector switch (perko switch) - have you ever turned it while the engine was running ? Next question is when it is off and your key switch OFF - No ignition key even in and you turn on your battery selector switch to any of the positions ( 1 or 2 or ALL) - does your fuel pump(s) come on ??? again just turn power on but no ignition key even in, can you hear your fuel pump(s) coming on ? If your fuel pump comes on then you have a bad PCM.

I'm gonna buy a kayak! Just kidding. Yes, I have a Perko. I did switch it to off the first time this problem arose hoping it would cut the engine off but it did not. I have not tried to turn it on without key in the ignition but will do so this morning and let you know. Thanks guys for the input.

I 've got to ask .What harm is there in unhooking just the exciter (switched hot) wire at alternator and starting then trying to stop engine . That is the only wire on alt.hooked directly to switched ignition hot. If engine shuts off it is alternator time