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david (david)
Posted on Tuesday, October 21, 2003 - 12:09 am:   Edit Post Print Post

This subject still lacks a definitive answer.

How is the alternator damaged by turning the key off (diesel engine) or turning the battery switch through the "off" position while the engine is running?

This is the short answer: Usually turning the key off or on will have no immediate ill effect. The key may switch the generator's voltage regulator on and off, but that is not a problem except that the batteries may not charge with the key off. Also, the engine oil and temperature alarms will probably not function with the key off.

Disconnecting the generator from any battery by turning the battery switch off can cause a running generator's output voltage to soar and will often blow the generator diodes. It depends on whether the battery switch actually disconnects the generator from the battery or just disconnects the load from both.

I don't know whether the diodes are damaged by the voltage spike or by the subsequent current spike when the runaway generator is reconnected to the system. It really doesn't matter.

The most common symptom of diode failure will be that the batteries will not charge or will charge very slowly when the engine is running. Most generators have 9 diodes. At least one pair must be functional to get any output at all.

As I mentioned earlier, anyone who wires boats should think of a generator and a battery as a system. If the battery is always connected to the generator and only the load is switched on and off, damage will not occur. However, not everyone thinks this way.

The "off-1-both-2" switch is usually wired so that the generator can be disconnected from the battery in the "off" position. This is the principal case which produces the rule "don't turn the switch when the engine is running." Switching between batteries while running is only a problem if the switch is dirty or defective. You should get away with it, but better to just keep the switch on "both" when starting and running. An inexperienced person is more likely to accidentally turn the switch off and cause problems.

If a running generator is connected to the load and then both are disconnected from a battery, even momentarily, the resulting voltage spike can kill lots of equipment, including the voltage regulator on the generator. If you turn the switch off and nothing bad happens, maybe the boat's electrical system was well-designed.

The battery "buffers" the generator output in two ways. First, the battery absorbs most voltage spikes the generator may produce. The other way to look at it is the battery keeps a load on the generator and prevents the "no load" condition that produces voltage spikes. Various components can be connected across the generator output to absorb very fast voltage spikes which might get past the battery. I think it is most likely that they keep voltage spikes out of the generator (close-by lightning, for example) rather than the other way around when the battery and generator are always connected together.

Second, the battery supplies surge current that may be required when a load is first switched on. Therefore, the generator does not see much of the very fast current surge. If the surge current was drawn directly from the generator, it could blow diodes. Motors usually generate all the high initial surge currents on a boat. Incandescent lights also produce a current surge when turned on, but only a pretty large light will produce enough startup surge to worry about. So mostly they are not a factor on boats.

I suppose there are exceptions to the cases above where I think the action is safe, but I have not seen them.

david (david)
Posted on Tuesday, September 30, 2003 - 03:06 pm:   Edit Post Print Post

By the way, according to the Society of Automotive Engineers (SAE) list of terms, J1930, "generator" is mandated to be used in place of "alternator," hence the usage above.

The term "alternator" came into use as very early automotive models had external diodes to turn the AC output into DC. Diode rectification is now internal. Semiconductor diode rectification of AC power provides greater power, lower weight and higher speeds than the old "DC generators." As a result, useful power is produced at lower engine RPM without problems at normal speeds. The maximum RPM of today's generators is usually 10,000. Remember that the drive pulley ratio results in much higher generator speed compared to engine speed.

david (david)
Posted on Tuesday, September 30, 2003 - 11:24 am:   Edit Post Print Post

No, the alarms and system safety checks like low oil pressure and over-temperature will NOT work with the key off. This is to keep the alarm bells from ringing all the time, as there is obviously no oil pressure when the engine is not running. The over-temp alarm could be wired to work regardless of key position, but, as the two alarms are usually wired to the same audible alarm, it is usually not done on recreational vessels.

The key may do several things on a diesel, "ignition" obviously not one of them. This list may not be all-inclusive.

1)signal the generator voltage regulator that the engine is on. The regulator then provides power to the generator field, sometimes after a time delay or RPM check, and the generator provides power to the battery.

2) enable the alarms.

3) enable an electric fuel pump. if used.

4) open an electric fuel cut-off valve, if used.

It is important to think of the generator and battery as part of the same system. Catastrophic trouble from/with the generator is usually the result of somehow seperating the two, even for an instant.

Posted on Tuesday, September 30, 2003 - 08:54 am:   Edit Post Print Post

WHen you turn the key off from the 'on' mode will the alarms and system checks still come on?

hughston (hughston)
Posted on Monday, September 29, 2003 - 02:03 pm:   Edit Post Print Post

Okay, so we might not be able to recognize when we have a system that will get fried when you turn the key off or switch the battery through the off position, but it would be nice to know the root cause when it is a problem.

When I am explaining things to my Able 1 class with the Sea Scouts for example, I'd like to be able to give them a little more background. Thanks for the help.

david (david)
Posted on Monday, September 29, 2003 - 04:39 am:   Edit Post Print Post

As with many problems I rarely (if ever) see, I get a little fuzzy on the precise details after a while. So I would like to brush up on this just a little bit before I answer your question.

Your experience is not unusual, and you may have been a little lucky too. People have fried enough alternators and electronic equipment to warrant the caution. As I recall, there are only a few component configurations that will cause problems. Its just that you usually don't know when you have one.

hughston (hughston)
Posted on Sunday, September 28, 2003 - 10:37 pm:   Edit Post Print Post

We are all (mostly) taught that turning the key off while the diesel engine is running will fry the alternator, or cause certain harm to the electrical system.

Ever since I survived the first instance of finding the key turned to the off position while the diesel engine was running, I've wondered what the facts of the matter are.

Also, turning the battery switch through the "off" position while the engine is running is reported to cause harm. Survived that too. So, what's the deal?

I'm looking for a conceptual explanation, not the Ph.D disertation, but something better than the dogmatic "Just don't do it." Why does it (sometimes) not seem to matter? Under what conditions might it disable your charging system? Is it just on certain electrical configurations, or have the electrical gods just smiled on me a couple of times?

Need to know.


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