How to fry an Alternator

Looking for informed facts here, not just more opinions!! Is it possible to fry an Alternator on a Diesel engine by turning off the Ignition BEFORE pulling the Kill switch??

And if you believe Yes please explain why.

Anything is possible. Make, Model, Year, Miles?

It’s on a boat, Yanmar engine. But specifically looking for General info. about ALL Diesel engines with Kill switch. I believe this idea must have come from some archaic and/or random engine. I just don’t believe most… 99.9% would be a problem. Are you familiar with the workings of a combiner? Kind of a Fancy isolator.

I don’t see the connection with the kill switch and the alternator. The only thing the kill switch should be doing is to make sure the engine stops when you shut down. Otherwise it can keep running because it does not need electricity to run.

I’ll bet you have a 20 year plus+ car. All modern diesel cars that I know of don’t have a user kill switch.

UPDate.  OK it's a boat engine.  Sorry I don't have the knowledge to offer suggestions.

Enlighten us. When and where do you need a kill switch? Is this a lanyard type? The kill switch is a fail safe mechanism if the driver falls overboard the engine will stop, and should operate independently of the ignition. If I understand the operation of the kill switch you do not need to activate it after turning off the ignition. So you need to understand how the kill switch performs it’s duties. I am sure it was not designed to fry the alternator, but if cutting off power from the alternator to kill the engine is the process there is a potential for damage.

Modern alternators are able to protect themselves from almost ANY form of abuse…They are overload protected and temperature protected…About the only way you can damage one is connecting a battery backwards or reversing the cables during a jump-start attempt…That will blow the voltage regulator and the rectifier trio…

I see what you are saying…You mean disconnecting the alternator while the engine is still running…That is not a good thing to do as the regulator loses it’s voltage reference (it can’t see the battery any more) and voltage can soar to the point that the regulator and or rectifier are damaged…Some alternator designs can protect themselves from this and some can’t…

When you say “kill switch,” are you referring to a fuel shut-off device?

I used to drive trucks with a manual fuel shut-off to stop the diesel engines. It never made any difference whether I pulled the fuel shut-off and stopped the engine before turning off the key, or turned the key first and then pulled the fuel shut-off. The alternators never got fried, regardless of the shut-down technique.

On modern cars, if you disconnect the battery while the engine is running it can wreak havoc on the alternator. I don’t know if that applies to your situation.

I suggest you read Caddyman’s post and use that as your guide.

Yes Fuel shut off, Exactly what I wanted someone else to say! I have a boat owner trying to blame me for frying the alternator because I turn off the switch and immediately pull the fuel cut off. There seems to be lots of chatter about this in the boating world and I’m curious why. I’ve been around trucks, tractors, and Generators a long time and don’t understand why it would EVER be an issue.
Anyone know where such an idea may have come from?

What’s the relationship between you and the boat owner?

I don’t believe it’s possible to damage the alternator using your description so my answer is no.

I amend my previous statement. You did nothing wrong.

Turning off the ‘ignition’ switch (Diesels have no electrical ignition system) cuts the fuel to the engine and turns off the electricals. Flipping the cut-off switch also cuts off the fuel, but not the electricals. The electricals will behave like a stalled engine if the ignition switch is on, but the cut-out is off. Turning off the ignition should not damage anything electrical, by design.

Depending on how the alternator, ignition, and battery are wired I’d say yes, it’s quite possible you could generate a load dump that could fry the alternator.

The windings in the alternator have a lot of inductance associated with them. If the alternator is supplying a large amount of current and that load is cut off the alternator will try keep putting out current. This can result in large voltages (well over 100V) being generated, which can be enough to cause damage.

This may be possible if the wiring were cobbled together and not consistent with the manufacturer’s design. If that were the case, and there was a special procedure to shut down the engine to prevent damage, the boat owner was at fault for not letting this guy know.

I would like to work this from the other end, do you have any info on what parts of the alternator are damaged, and the specifics about the alternator and the circuit it is connected to?

THANK YOU!! Exactly my point and there IS evidence that there are other issues on the boat. Just one of which is that there were several loose battery connections, one was stripped, which I tightened and repaired perhaps too late for the alternator.

No I don’t know what was damaged. Wish I did. BUT strongly suspect damage was due to loose battery connections and that would damage diodes which wouldn’t be conclusive either way.

My thinking is it should be the same as an automobile engine. When the ignition switch is turned off then the voltage to the alternators exciter is turned off. Without the exciter field the alternator action is dead. I can understand what Seraph stated but the same cutoff action happens in a gasoline engine also, just not as long perhaps. If I had to speculate something as a possible cause of the damage I would have to say the loose battery connections. If there was some sort of surge caused due to the action then having proper battery connections may have limited the surge to a lower and safer level.

Have this alternator tested and see just what the problem is. Having more knowledge in regards to this situation will help make the best decision.

We see this type of thing a lot in regards to SRS and other systems. The cars have a diagnostic system attached to the SRS system but people don’t want to make use of it in regards to getting their problem fixed. In your case there are at least two chain parts stores (that I can think of) that will run a diagnosis on your alternator, but you have not used their services. For my knowledge what is the idea behind loose battery cables damaging alternator diodes, do they go short or open? the diodes used in alternators are some pretty robust components, they deal with loose battery cables daily and handle the situation well.

Thanks for your input. More info. is always good but I had rented the boat and no longer have access to the alternator. (I’m a HUGE fan of fully diagnosing, So often I see symptoms “repaired” without addressing the cause. Don’t have to look far to find MANY examples of that! Gov’t spending… financial crisis… Health Care…)

My Understanding is that loose connections or disconnecting the battery can cause current to “backup” and damage diodes diminishing or terminating Alternator output. Undersized output cable can do the same thing. Thanks Again!!