New to site, need help. Just bought a 1990 F250, 58000 miles. Truck is in great shape. Mechanic replaced battery, belt and alternator yesterday. Picked truck up today and ran great, until I stopped at a red light. The volt meter dropped to the red. I shut off all accessories and it came back up. Played around with different combinations and found that it was affected most by the heater fan. Some needle movement with lights, blinkers and radio but minimal. Drastic needle movement with the fan and only when the vehicle is stopped. Just looking for some input before I bring it back to the mechanic. Thanks

 

It sounds like the belt is slipping a little or the alternator is bad. If you have a test meter, see what the actual voltage is doing. The fan draws a lot of current. Welcome to FTE.

 

Thanks, for the response. The belt, alternator and battery are new. I spent some time on the site today and my problem seems to be fairly common.

 

Tom237,
I recently bought a 91 Explorer with a battery drain. First truck with a voltmeter instead of amp gauge. So being new to voltmeters i didn't know what to make of the needle dropping with the lights and the blower running. Running at idle it would hug the number 18 side. With a demand it would point straight down, be in the center of the gauge. I didnt' know to test the volts with demand being placed on the system. That would have been my clue the regulator was bad not the Alt (hence where the short would likely be). It was only putting out 12.3 volts with a demand. With the replacement it holds 14.8, load demand or not.

Ok so i fixed that, thought all was well. Made four very short trips, won't start. that problem was the Universal aftermarket battery clamps. They'll look clean on the outside and be all full of green and rust on the inside. uh, where the cables bolt to the clamps, Previous owner didn't have them very tight either.

I don't know if others have had similar experiences or if it's just my luck but i'm finding out that "new" parts sometimes doesn't mean much. A bad "new" starter just about ate the flywheel teeth up on my 82 before i got it through my head that that new sound was not the way a 351 starter sounds. I was used to old big blocks. the front bushing in the starter was just sloopy as all heck, so now i check em.

 

What bugs me is a voltmeter face with high, low, and a picture of a battery in the middle. Like it would really cost a lot to put numbers from 10 to 16 on the scale so the reading would mean something. This reminds me that I need to make some progress on putting real guages in my Ranger.

 

Don't you just love those "idiot" gages?

Seriously, check it with a meter. The alternator may not turn fast enuf at idle speed to keep up with all of the devices you have on. This is a normal "problem" with any alternator setup, in other words it is not a problem.

 

Thanks for the responses. Been driving it a couple of days and it is still doing it but it does not seem to be a problem, just an annoyance. As long as it keeps starting, I'm not going to worry about it or throw anymore money at it.

Would a 100amp alternator help the problem?

 

I would save your money.