Ok so I have a 92 F150 5.0, when I have my headlights on, defrost on and I come to a stop sign my lights dim, fan slows down and the voltage gauge shows a drop. I have replaced alternators, batteries, add a 4 gauge ground from the engine to the body and body to frame. My next thought is to install a high amp alternator but Im not sure what to do. Any advise would be great.

 

Is the idle dropping when this happens? Or almost stalling.

 

If you've already replaced the alternator, sounds like you're not buying a large enough battery. I buy the best 65 series battery Wal Mart has and it carrys all of my vehicles at idle. Any issues with the battery and it's a Wal Mart problem. I also haven't paid for a battery since the 1990's as WalMart keeps replacing them- too cheap!

 

Yes the rpms drop a little too. The other issue i have with this truck is that when you start it, it has what i would consider a rough idle until it "catches" itself then idles normal

 

I get the best group 65 from carquest which has 880CCA. It's a good battery manufacture, East Penn, they also manufacture Deka.

 

It should go from the alternator case to the (-) clamp on the battery - nothing else will help this symptom. Read these captions:

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If you ultimately decide to upgrade the alternator (which shouldn't be necessary), just get a stock 130A 3G that fits your engine & wiring harness. Don't waste the time & money of an aftermarket alt that requires ANY modification of the truck.