Alternators 101

[Piwoslaw]

There are a few threads with less or more information on alternators, but none with full information on how an alternator works and what is the best way to add an OFF switch. Since the alternator is an inportant and dangerous engine accessory, I decided it would be a good idea to understand alternators before messing with them. I'm no expert, so I ask (pleasepleaseplease) anyone who can pitch in to do so. Anything educated that I might add would only be cut&pasted from a randomly googled site, so I'd like someone to expain, in their own words, how an alternator works, what kind of alternators can be found in cars, how many wires do they have and what are they for, what is a field wire, supply wire, what is the best/safest way to disconnect the alternator, etc. Pictures and diagrams will be helpful, as will be links to any posts with good information.

To start somewhere, I'll use my car's alt as an example.



You can see two wires coming out, one thin with a connector and one thick with a bolt. When under the car I checked (with the help of a mirror and uncomfortable position inflicting great neck pain) that there seem to be no other wires connected to the alt. (The other wires on the picture go to the A/C compressor immitiately under the alt.) I checked voltage between the two wires and got 12.4V with the engine off, the thick one being '+'. Checking the voltage at the battery I got 12.4V and 13.8V with the engine on. After unplugging the thin wire the voltage reading stayed the same. Unplugging the thick wire noticeably changed the sound of the engine, and dropped the voltage to 12.5V. I'm guessing that the thick wire is the '+' supply, and that there is no '-' supply, it just goes straight into the chassis as mass. But what is the thin wire for? Unplugging it doesn't even light up anything on the dash.

So my alternator seems to have a thin wire (what for?), but I've read that some alternators can have even more extra wires, 2, 3 or 4. I would think that an alternator only needs two big cables ('+' and '-').

[nemesis]

Most alternators have thin wires going to the voltage regulator and big wire that goes to the battery. Think wire is the charge wire, thin wire tells the regulator on what the voltage is at, so it knows when to charge and when not to charge, the main job is to keep the voltage between 13.5-14.2 volts, no more no less.

[some_other_dave]

Older alternators, yes. But newer ones have the regulator inside them.

Usually one thinner wire is there to run the "battery" light.

Grounding is done through the chassis of the alternator, into the alternator bracket, to the engine block.

-soD

[dwtaylorpdx]

Most newer Chrysler products have the regulator integrated to the computer, so the alternator just has the diode pack. Jeeps, Trucks, not sure of the cars.

Research the specif applications functions and trouble shooting guides carefully.

Dave