Ask the Mechanic

Steve Thompson, Contributing Columnist

READER: I'm dealing with alternators on my tractors. Some have one wire on them, and others can have three wires or more. The alternator with three wires always runs the battery down if I don't disconnect the battery cable. The other tractor with one wire will also run down the battery if I don't disconnect the battery. Can you help me figure out the problems with these two alternators? They completely discharge my batteries when I forget to disconnect the battery cables, and the battery life on both tractors is shortened.

STEVE: Alternators usually work silently to keep batteries charged under extreme load conditions. They can charge more than a generator at low engine rpm, which allows your battery to stay fully charged during slow engine speed. It produces around 14 volts, which keeps your 12-volt battery ready for surge moments, such as several loads happening at once on the tractor (lights, cooling fan, AC compressor, brake lights). It can even charge turning in either direction, which allows it to be installed facing backward on a vehicle. However, unlike the generator, it must be excited with voltage, either externally or internally, to "wake it up" and make it charge.

Here is where you will find the problem with these two alternators running down your batteries. With an alternator of the three-wire type, the large terminal furnishes the amps from the alternator, and the pigtail's big red wire can go back to the big terminal, which is hot all the time from the battery voltage. The little white wire will more than likely be your drain problem. It must be hot to excite the alternator, but it must lose voltage when you turn off the key, otherwise it will run down your battery. The way you have your tractor wired does not allow this wire to lose voltage with the key off. The other type of alternator with only one wire is a very common choice because it eliminates all associated wiring. It excites itself from internal voltage when the engine starts.

In rare cases, after the alternator stops spinning, the electronic regulator is not disconnecting itself, allowing the battery to drain. This is rare, but it can happen. If the self-exciting diode is not working, the alternator will usually have a warm feeling to it after it has had time to cool down. This alternator will need to be replaced or repaired. Remember, the big post on the alternator is hot all the time.

TECH TIP

If you have alternators like the popular ones shown in the photo accompanying this article, with the engine running, the rear center of the alternator will become a strong magnet. If the alternator will pull the end of a screwdriver strongly to it, the alternator is charging. If the amp gauge does not show charge, you probably need to replace the amp gauge. Try this out on your alternator -- now that's neat.

**

-- Email Steve Thompson care of askthemechaniccolumn@gmail.com, and be sure to include your contact information and phone number.

[PF_0825]

(c) Copyright 2025 DTN, LLC. All rights reserved.