Voltage lowers when stopped
 

I have a 2001 grand Marquis, I got a new serpentine belt, battery, belt tensioner, and a Alternator that tested good at the parts store, yet the voltage lower to 12.8-13 volts when I’m at a stop sign in drive. When it idles in park it keeps a steady 14.5 volts with everything on, only when it’s drive and stopped does it have this problem. I’ve checked the battery ground, the wiring, and fuses just to be safe still can’t solve it. I would appreciate any suggestions.

Hello, Stitch, and welcome to the Mercury Forums -- such as they are.
You get a Voltage drop of 14.5 - to - 12.8-13 Volts at a Stop-sign while in Drive.
"When it idles in Park, it keeps a steady 14.5 volts with everything on."
"Only when it's in drive and stopped does it have this problem."
What's the difference !??
When the car is stopped in Drive, the engine drops below it's curb idle speed because of the drag on the transmission -- thus causing the Alternator to turn more slowly, and dropping the Voltage slightly.
Also, when the car is stopped in Drive, hopefully, someone has their foot on the Brake, which causes additional current demand, thus helping to contribute to that small Voltage drop.
I believe that what you describe is a Normal Condition with the Grand Marquis.
1995 Grand Marquis Restoration - Mercury Forum - Mercury Enthusiasts Forums (1)

Hello,
Thanks for the reply! I don’t believe it’s normal as both my Air Conditioning will slow down and my lights will will dim slightly at a stop. You definitely could be right it’s normal behavior, so I’ll probably stop playing around with it and just leave it be. Thanks again.

I also agree that what you see is normal operation of the charging system. Probably wouldn't hurt to atleast have your battery load tested to see what shape it is in.