Questions and Answers

Ford F-150 Has A Hot Alternator Q. Mr. Ciulla, I need your help. I'm having alternator problems. The following is info on my vehicle: 1987 Ford F-150 Pickup 174,000 miles 300 cid inline 6 cylinder Electronic Fuel Injection Manual transmission ABS brakes P/S, A/C no cruise control

To keep this short and to the point: Started truck one morning and noticed that according to the voltmeter gauge in the instrument panel, the alternator output was low (around 11 volts).

Came home and found that battery was dead. Connected jumper cables between my truck and my other vehicle and let the battery charge up for a few minutes. Started the truck and connected my DMM and found that indeed the output was low. Turning on things like the lights, radio, and fan only made the output drop without the immediate rise in output voltage that you would see with a healthy alternator.

I also noticed that I was getting a field current all the time; even with the key turned off. Normally a hack saw blade will stick to the alternator case when the key is turned on, engine off. I noticed a good magnetic field around the case regardless of whether the key was on or off.

I disconnected the negative battery post and connected a test light between the negative battery post and the clamp and the light came on, key turned off. It went out when I disconnected the electrical connector from the regulator mounted on the back of the alternator.

Some sort of internal short I thought. No wonder the battery was dead when I came home from work. BAD alternator. I'll just go buy a new one. I also noticed shortly after I shut the engine off, or more accurately smelled that the alternator was hot; according to my DMM, around 192°F. The thing would certainly burn your hand; I found out the hard way! I don't think it's normal for an alternator to run this hot, but I thought, "Hey, it's a bad alternator." I had no problem turning the pulley after I took it out. The bearings turned freely.

Installed a new one that behaved normally except it got hot too. Too hot to put my hand on it, about 175°F and that's after it had cooled down for about 5 minutes. I'm going to leave it down until I figure this thing out. If I burn up this alternator after only having had it for a day or two, the boys down at AutoZone won't take it back.

Why is my alternator running so hot? Is it normal for an alternator to run this hot? I never looked into what the proper operating temperature was for an alternator. What tests/checks should I perform?

Any light you can shed on this would be greatly appreciated.

Sincerely,
Mike
Ideal, Georgia

A. When an alternator gets hot like that, it is working too hard. What you need to do is find out how many amps it's putting out at idle. With a fully charged battery and no accessories on, it should only be putting out, about, 7 to 10 amps. If it's putting out more than that, there is something making big demands of the alternator. Usually a shorted battery will cause a big draw from the alternator.

It is normal for an alternator to get hot, how hot I don't know. I never measured the temperature. If the alternator is putting out low amps at idle, then I would say the system is okay. Have it thoroughly tested however, don't take my word for it.