The other night when leaving work I noticed the red battery shaped light on the console was light-up. Also, the battery gauge was lower than normal; I'm thinking that after driving for a few minutes the alternator should charge the battery back to normal, it didn't. After about 5 or 6 miles the needle on the battery charge gauge dropped noticably. By the time I got home the headlamps were noticably dim, and the gauge was buried at 'low'. Alternator, what else?

Truck specs:

1999 F-150
4.2 liter engine-? (large 6 cyl)
6000GVW
2 whl drive

Question #1: How to check with multimeter?

Question #2: How to positively ID, for part replacement?

 

It's your alternator. If the battery goes bad, the vehicle will still run off the alternator once started. If the alternator goes bad, the vehicle will only run long enough to drain the battery. Best thing to do is take it to the parts store and have them test it.

 

If you have a voltmeter test the battery with the truck off and key in off position. It will read around 12.5 volts, although probably less in your case since your Alt. is most likely dead. Now if the truck will start, start it and while running test the battery posts again. If the alternator is charging it, it will read right around 13.7 volts, my guess is whatever reading you get off the battery the first time will be higher or the same to the reading you get with the truck running, which will mean the alt. is dead for sure.

 

Thanks guys, what I did after my first post was to start the truck, then take the positive lead off the battery, which killed the engine. Because of that, I'm pretty well convienced that the alternator is toast. I'm at work now, but in the morning I'll double-check with a voltmeter.

 

Try to buy the best alternator you can afford. Some real junky whiners out there...

Spotty

 

i put two rebuilt alt. on my friends truck both were no good, he bought a new one and it was ok. with a good guarante,if you have the money i would get a new one

 

That test is not valid on modern vehicles and it can damage electronics. Customers bring me cars all the time that they have done that test on and they rarely need an alternator. Some need ECM's, BCM's or some other module that they fried doing the "test".

 

dont ever pull the positive cable off a battery when running with an alternator. the alternator charges 13.7 volts because it has feedback from the battery. without the connection it will hopefully die. it is possible for it to jump to 20 volts and blow out the computer and all lights, etc.-- dont ever remove the cable from a running truck.

 

Yep, if the PCM lived, you're lucky. The battery acts as a capacitor that absorbs any voltage spikes. I have seen where someone did this on an older vehicle and blew all of the light bulbs out of the car because the alternator was overcharging.
You may be lucky because your alt. isn't charging.

 

Yikes, thanks for the head-up, guys.

I'll get a new alternator, just to save some grief, later on.

 

Good luck, hopefully it will be ok. Charge your battery before starting it with the new alternator. It's kinda hard on a alternator to charge a dead battery.

 

Thanks, I'm a little worried now though, when I get off work I may take the truck for a spin around the block, just to make sure everything's o.k.


On second thought, maybe I should wait until morning, no sense in messing up a good night's sleep, in case things aren't!

 

I also hava a '99, 4.2l v6. Last summer I had similar symptoms. I had the Batt tested AND the alt. tested both tested good when tested out of the truck at autozone and O'riellys but when tested in the truck the test said they were bad. The guys at the store were very confused. I ended up replacing both. The problem persisted, I took the alternator back and replaced it again. Still had the problem with engin light coming on and battery would not charge. I had alredy checked all the related fuses but i went back again double check them. this time I pulled each fuse and checked with a multimeter. Ah-HA! the fuse for the alternator field in the power distribution box was BROKEN, not blown but broken. I replaced the fuse and no more prblems. Dam 20 cent fused cost me hours of cussin. The power distribution box is built in sections, the fuse briged over two sections and vibration between the two sections I guess broke the fuse. Making intermittend contact.

 

Redjar31, that is another common problem. I had a Mitsubishi a few weeks ago that the owner took to Midas because his battery was going dead. The put a battery in it and he drove it for two days before it was dead again. He went back to them and they threw an alternator at it and charged the battery. Two days later he was back and they wanted to put a computer in it. This time he balked and brought it to us. The first thing I noticed was that the battery light didn't work when the key was on and the engine was off. It took two or three hours to find the broken wire in the harness behind the battery, but that $200 fixed what the previous $500 and the $800 for a computer would not have.

Beware the alternator test at AZ. A couple of years ago the alternator went out on my wife's 73 Camaro and I confirmed it by testing it myself. I pulled it off and took it to AZ since they sold it to me. The guy put it on the tester and eventually it asked him to input whether it was one of their parts or not, and it was. He ran the test and it passed. Since I knew it was bad I suggested we run the test again, but tell the machine that it was not an AZ part this time. He did it and it failed. His next comment blew me away: "Almost all of them that aren't ours fail and almost all of ours pass." Sounds like a scam to me.

 

ROFL That is hilarious there is no reason why the machine should need to know who made the part except for scamming people into claiming it was a good AZ part.