Alternator Charging, Testing, and Output

by Morris Arthur and David Schwarze

Morris Aruthur writes:

I spoke with my local alternator rebuilder today who just happens to be familiar with buses. (He does the engine work on a friend's Bus.)


An alternator should only run at full capacity for no longer than a minute or so...Normally it should run at about half-capacity.

David Schwarze responds:

I've heard this too, although it annoys me that an alternator is apparently not designed to run at its rated output for any length of time.

Charging and voltage regulators:

The green wire from the voltage regulator delivers a variable voltage (from 0-12V) to the alternator based on the voltage of the battery. The alternator then delivers more or less current (Amps) out of the fat red wire based on this voltage.

To check the alternator without voltage regulator:

With the fat red wire hooked up to the battery (or starter bolt), unplug the wiring harness from the voltage regulator and jump the green wire to 12V and start the engine. This tells the alternator to deliver full capacity current to the battery. You should see ~13.8V at the battery. If this works and you are not getting the 13.8V when the harness is hooked to the voltage regulator as normal, then point fingers at the regulator. If it doesn't work, then you know your alternator is bad.

Thank you, I've been wondering how that worked. I guess you could also hook up an ammeter between the fat red wire and the battery to measure the maximum output of the alternator, right? I also guess that you should not run it in this "test mode" for more than 60 seconds based on your first statement, or risk overheating the alternator.

High output alternator alternatives:

My rebuilder says it's easy to put a Delco 200A alternator (able to put out 130A continuous) into a Bus. It has it's own fan so you don't need to worry about hooking up boots to the fan housing. He's put one into his friend's Bus and said he would set me up with one for $150-200 ...

I asked my rebuilder about this recently, and he said that he wouldn't recommend putting any non-bus alternator in because of the close proximity of the alternator to the exhaust and other hot engine components. He said that models without the cooling duct would overheat because of inadequate air circulation, even if they have their own fan. I think he's right, especially in the case of a high-capacity alternator such as the Delco 200 amp.

He says the 55A model I have won't last very long charging multiple batteries...

I disagree. My 55 amp has been charging not two but three batteries for many years. The reason it's not a problem is because batteries charge in a non-linear fashion. They start out drawing a large amount of current, typically 20-30 amps if you just started the car, 50-100 amps if you wore it down overnight with the headlights, and then taper off very quickly (like within 60 seconds) to a much smaller charge rate of 10-15 amps, which will remain more-or-less steady until they are fully charged. When they are fully charged, they will draw less than 1 amp from the alternator (older, worn batteries may draw 3 amps continuously).

The thing that will kill an alternator is if it is overloaded by current-draining accessories like A/C, fog lights, refrigerators, etc. If you have a 55 amp alternator and are using 50 amps worth of accessories while you drive, you risk killing your alternator because the accessories don't "charge up", they will just keep sucking down the power and never give your alternator a chance to cool down.

If you're still worried about dual batteries and your 55 amp alternator, you can make life a little easier for it by not turning on headlights, wipers, etc. for a minute or two when starting up with a low auxiliary battery. This gives the alternator time to overcome the initial spike of power demanded by the low battery before additional demands are placed upon it.

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