Alternator 101

Here are all the sites I used during my research :

Other than ground, there are three basic wires on an alternator : BATT, FIELD, and SENSE. The trick is to find out which one is which, and of course, what it does. The above alternator is from a 90s Land Rover application. The big fat lug connects the big red wire which charges the battery (BATT), the two smaller ones are the FIELD and SENSE. The following diagram shows how I hookup up a GM alternator to my car.


BATT is the main output charging post on your alternator. If everything is hooked up correctly, it should be driving 13.8V DC constant voltage.

Alternator technology has improved over the years, but in the old days the current that BATT could sink depended on the speed with which it was spinning. The higher the spin, the closer to full amp capacity it could sink current. The older alternators even needed a kick-start to begin charging, so you need to get the RPMS above 1500 before it did anything. You'd notice a sudden increase in lights when it started, which was normal. Nowadays, alternators charge at 13.8V right away, with near 100% rated amperage.

The only critical detail of the BATT wire is that it needs safety protection. The worst case scenario is the regulator fails and the alternator overcharges and destroys all the electronics in the car. So the BATT wire, although is supposed to charge the battery, first passes through a fuse. In the old days, it was a fuseable link, nowadays, its just a quick-blow fuse. A fuseable link works like a fuse, except its a wire that can take short spikes of overcharging, but remains intact unless a prolonged overcharging burns it. A quick blow fuse, as is common today, blows with the slightest hint of over-current. They say you can replace a fuseable link with a quick-blow fuse, rated 20% higher than the alternator max output, and you should be fine. Im running an oven fuse, rated at 30Amps, and Ive never had a problem.


The FIELD wire has a couple functions. The full name is the "exciter field tap", has something to do with the actual science of the electronics, where there is a magnetic field which is excited, etc etc. Anyhow, its an input from the alternator, which activates and deactivates the charging. Its always hooked up to the ignition, so when you turn off the ignition, you also turn off the alternator from charging.

Another reason why its hooked up to the ignition switch, is that its a drain on the battery. In other words, the FIELD wire will draw a small amount of current, even when the car is off. If its hooked up to a physical switch, its cut off from draining the battery when the car is sitting.

Another interesting use of the field is that it can tell you that the alternator is actually working properly by being hooked to the alternator light on the dash. Also, saves the car companies from having to install a voltmeter, by instead having a low cost idiot light on your dash! So when you turn the ignition on, the ALT light should go on, then when the car is running, the ALT light should go off. Why does it do this?

If you hooked an oscilliscope to the FIELD wire you would see a square wave, as described above. Its a square voltage pattern, going between 0V and 13.8V. When the car is off, its stuck at 0V, which is why the ALT light goes on. When the car is running, it turns into a high frequency square wave, so the ALT light sees a square wave, which has an RMS value near 13.8V. IE, the ALT light isnt grounded, it truns off. Actually if you look really closely, the ALT light will have a tiny glow to it, because technically, its not 13.8V, its near there, so there is a voltage drop, and it will draw a bit of current and is actually on. Anyhow, if the alternator is malfunctioning, the FIELD will be 0V, and your ALT light will go on and give you the warning.


The SENSE line is a positive input to the alternator, and is meant to be feedback giving reference information on the load of electronics on your car. So its supposed to be hooked up the most electrically distant part of your car to let the alternator know if it should be upping the current to compensate. If its hooked up close to the BATT wire, which will be virtually the same voltage, then the alternator doesnt think it has to work that hard, and just does enough to keep the voltage steady at 13.8V. Not a bad thing if other systems in your car, like amplifiers, or fuel pumps dont mind getting a little shy of 13.8V.

Mopar alternator alternatives

If it fits, you can use it! It would be good if you had a friend who worked in the parts department who could pull some small alternators off the shelf and cross reference some numbers. Or, you could take a stroll through the local scrap yard and just see what would fit, thats what I did. As described above, Im running a GM alternator with little customization. Heres a pic :


1980 CHEVROLET CAPRICE CLASSIC, 5.0L 305 cubic inch V8 4BBL

Lowest rated amperage

DC-10SI 63 Amps


1985 to 1992 Toyota 4Runner and pickup trucks

Toyota part number 27060-35060 or 27060-35061

Bosch part number AL218X

For the Toyota alternator, you can get a custom bracket from here.

Also the Toyota wiring diagram is :

Alternator Dissassembly

Alternator Component Checking

  • Diodes can be checked at 20M setting, mine conduct in one way only, around 10M Ohms. Test from ground to diode, then test from diode to + lug.
  • Diode trio specs out similarly
  • Capacitor measures a measly 0.5uF, which is actually correct to spec. Probably so a bench test gives a nice line, but under real load this does nothing.