Alternator new, still low voltage

MURDOCK1

MEMBER
FORD F-150

Ford
F-150
1991
Mileage: 120,000
4.9 Liter
Automatic Transmission

Recently I was having electrical problems. I had on-going issues for a couple months. Mainly sudden decrease in power, heater fan working in low speed, lights would dim, or cigerette lighter go dead for a short time.

Five days ago I went out to start up the pickup and it was dead. No power at all. I jumped it off another rig thinking maybe I left the ignition on, as it tends to stick slightly. It started up and ran perfectly. After shutting it off, I came back two hours later to find a dead pickup. I tried this one last time with the same result. After testing the battery and alternator we discovered the Alternator was toast, and the battery seemed to have a dead cell. Replacing both, it seemed fine for 10 seconds, untill the volt meter droped down to barely below normal.

I have pulled fuses and looked for shorts, but come up with nothing except a headache and a sore back. Wires have been check to battery and terminals replaced. Nothing seems to work. The alternator pumps out a little juice, and then the battery starts draining slowly.

I let the pickup sit dead for a day and came back. I started it up and the volt gauge went to normal, then dropped back down to barely below after I let it run for 10 seconds. I am extremely confused as it seems to get the power from the start, then its almost like something is telling it to slow down. Is there some sort of reset that needs to be done?

Help would be extremely appreciated as I am about to pull my hair out.

2 Replies

SERVICE WRITER

MECHANIC
9,123 POSTS

Wouldn't be surprized if the replacement alternater is bad right from the get-go. I it see more than I care to.

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Saturday, February 17th, 2007 AT 6:54 AM

LEO 57

MEMBER
3 POSTS

My problem was not identical, but maybe it will help. My charging light came on. A test on the alternator in the truck (1998 F150 4.2L 160k) indicated it was bad. I bought a new one (not reman) at Auto Zone. When I installed it, the light was still on. Bench test showed OK, but a test on the truck said check wiring. After much flailing about, it turned out that there was a blown fuse in one of the small wires that attach to the plug-in harness on the alternator. The wire runs from the alternator to or near the battery fuse block. The wire goes in and out of a connector (about 1" square) which houses a 20A fuse. Hope this helps.