1989 Ford F-250 voltmeter/alternator/windsheild wipers

CS7519

MEMBER
1989 FORD F-250

Electrical problem
1989 Ford F250 Two Wheel Drive Automatic 96000 miles

I have an 1989 diesel f250 that I just purchased. It has two batteries both new, and the previous owner just changed all the belts. It has not been driven long distances so it is possible the batteries have not been fully charged.

My problem is the first few days I drove it the voltmeter was only reading around 10 or 11. It now reads slightly above 13 when running and with engine off around 10, but that is with glow plugs heating up.

My Problem
I don't know what it should read but when I turn on the windshield wipers while running the voltmeter slowly but steadily drops, I noticed while driving in rain that when the lights are on and the winsheild wipers are on high the wipers eventually slow down.

I am not sure if it is a wiper motor issue or my alternator. Things appear to stay normal when lights and stereo are on but wipers cause a major difference

1 Reply

MERLIN2021

MECHANIC
17,250 POSTS

Two different charging systems are used on these, does yours have an internal regulator on the alternator? Or external? Internal reg= no more than battery volts + 2 volts at 2000 rpm, external reg it's 2.5 above.

Test wiper motor for amperage draw like this.
Bronco & "F" Series 1. Disconnect linkage and electrical lead from motor. Connect negative lead of ammeter to battery positive post and positive lead of meter to low speed connection at electrical plug. Measure and record current draw. 2. Move positive lead of meter to high speed connection at electrical plug. See Fig. 1 & 2. Maximum allowable current draw for either speed is 3.5 amps.