Voltage spike problems

[DickTrickson]

So my 91' 7.3 idi has been doing some weird stuff lately. Ive always had little electrical quirks. As of late my batteries are dying over night. Theyll charge fine once i get the truck running but if it sits a few hours its a hard start if it starts at all. I've also had to replace my windshield wiper switch once and my headlight switch twice. Well after replacing my light switch this last time, my voltage started going through the roof. Driving to work my ac compressor started making a God awful noise, pulled over popped the hood and the red power wire was melted and the compressor smoking. I grabbed some channel locks pulled the wire and the pulled the belt because the compressor was seized. I jumped back in and noticed that voltage was maxed when I went to take off. When I accelerated it spiked, at an idle it seemed fine. Limped it home home in 5th gear at 600rpms because if I went faster the voltage spiked. Any thoughts on what it could be? Voltage regulator? I think I have one that's kind of by my starter relay but have read there's one also in the cab? Any advice would be appreciated. Electrical stuff is not my strong point and feeling a little lost.

 

[IDIBRONCO]

Voltage regulator? I think I have one that's kind of by my starter relay

That's where I would check first. Yes you probably do have a regulator there. When Ford switched to the serpentine belt setup, the alternator was switched to the 3G style which have a regulator that's mounted on the alternator itself. The one in the cab is probably the one for the gauges and may have not been used after 1986.

 

[Black dawg]

Pretty common failure with the old external regulator. I bet if you go out after the truck has cooled off good, that the regulator will be warm to the touch.

Read up on doing a 3g alternator conversion. I would not waste any money on a new regulator. The aftermarket ones are probably the most failure prone part of these trucks. I always try to save old original regulators from stuff that gets scrapped because they will last longer than an aftermarket one.

3g alternator is probably the best mod you can do to one of these truck.

 

[franklin2]

A bad alternator will cause a drain also if one of the diodes inside it is shorted.

 

[Rocknit4x4]

I had a similar problem, did the 3g conversion then started having an issue with the voltage fluctuating between 12-15 Volts at idle but steady at 14 when the rpm’s were above idle. I found a ground strap in bad shape and replaced it then replaced the voltage regulator on the 3g - it was good for about a year and now the issue is starting to come back. I’m guessing I need to go look at all the ground straps again, but thought the experience might help you if the 3g conversion doesn’t fix all your problems - check the ground straps.