I did a 3G swap about 6 months ago and used a brand new alternator that I got from Autozone. It was for a 95 V6 Mustang application, 130 amp model. I rewired the whole cab and charging harness around the same time, it's all new 4ga from the alternator to the charging solenoid. Connections are all good, nothing is loose or hacked.

Recently, the voltage gauge started getting very erratic. All the lights (headlights, dash etc) dim accordingly when the voltage drops, so it's definitely an overall voltage problem not just a loose connection at the gauge or something. Happens when idling, revved, driving, ALL accessories on or off etc. Seems to be no rhyme or reason to it, as it doesn't kick on at a magic RPM or something. It'll drop down to 11-12v driving down the road, then kick up to 14. Sometimes it'll even get close to 15v; it spends most of its time around 13.5v it would seem.

I just added a second 4ga ground today from the block to the frame, previously I just had the 4ga ground from battery to block. The symptoms started a month or so ago, and that didn't make it better.

Voltage regulator? Bad alt? Am I missing something obvious?

 

Make sure the yellow wire is looped back to the charge post.

Josh

 

While I'm not sure it will fix your problem, add a ground strap from engine to firewall. I have seen lack of this cause some very strange things.

 

2x... 'experienced that during the first-time fire up with efi. That one bonding strap cleared some very weird symptoms.

 

I've got an 8ga wire from engine block to firewall...not big enough?

I added a Ground Lug to the frame, this is where the battery ground and block-frame ground both go. All 4ga. I'll try adding a better cab to block ground and see what happens...

 

You should be okay then. I added a few extra grounds from the core, cab, and bed to frame just for insurance.


I'd say the problem is with the alternator itself or an integral component thereto.

 

alt tested OK on the tester in the parts store...tried 3 times and passed every time.

brought it home and reinstalled, still doing the same thing. i threw a voltage meter on the battery while it was running and its matching the alt output pretty closely. so, which part should i throw at it first?

 

The regulators are really easy to change out on a 3G. I'd snag one at the wrecking yard and give that a shot.

Also as a shot in the dark check all of your battery cables for corrosion. I've had cables corrode internally and cause issues.

 

Couple questions. What is this charging solenoid mentioned in your original post and will it do this sitting in driveway? Also attached normal 3G diagram just to double check.

 

Sorry for the confusion, I meant "starter relay" when i said charging solenoid. The wiring is as shown on the diagram.

I'm taking the opportunity to relocate the alternator higher on the block, I noticed I was getting some higher RPM squeal from the power steering belt only wrapping about 15% of the water pump pulley. Moving the alt forward and up so that I can wrap that around the crank and water pump pulley; it currently only grabs the rear most crank pulley.

I'll check back once I get it rolling again, just need to order a few spacers and parts to get it all lined up right. I did not like how low the alternator sat, it is very vulnerable to mud and water down there.

 

Belt slipping is a possibility with a v-belt and 130A alternator. My 90A 3G upgrade slips but usually on startup. I really need to go to a double "V" setup. Let us know what happens.

 

put a new voltage regulator in and new battery (the one in the truck was robbed from the T-bird which is going up for sale soon). problem seems better, but seems to still persist just not as bad.

 

finally sorted this out. after doing everything including throwing a NEW alternator, new coil, LED bulbs everywhere, 4ga ground from frame to block and block to cab...turns out it was a bad crimp and solder on a battery cable.

DOH