I have a 2005 MCS, I installed the Scangauge 2 into the obd 2 port and my voltage while the car is running AT IDLE is at around 12.2 with the a/c on. If I turn all the loads off than I get to about 13.3-13.5 volts. If I turn on ALL the loads than it will get as low as 10.3 volts. When I rev it up with all the loads placed than it will go up to about 12.4-12.6. I don't see any warning lights or any noticeable dimming of the lights on the car. I also put a multi-meter on the battery itself and noticed the same issue.

I have replaced the cooling fan because the low side did not work and the high side was making a weird noise.

But most importantly, I replaced the Alternator with a new Denso unit to replace the Denso unit I had in it.

What's going on?

 

Maybe the bearings of a component off the belt is going bad resulting in "drag" on the accessory belt when at idle or water pump going bad? I had similar issue originally diagnosed as a possible issue with original alternator, turned out to be the water pump.

When the water pump was off/inspected, the internals were almost jammed/gummed up, and I do regular flushes and it was second water pump too. There was no indication of the pump causing cooling issues either. My Scangauge II always indicated 177-181*f too.....

 

At that point than ill wait until I service the supercharger.

I also replaced the belt because the old one looked like poop. No dice.

I also removed the 100Amp fuse to see if the power steering pump was causing the voltage drop because when I tuned the wheel I saw a significant voltage drop.
I still had the issue.

The weird thing is that when I first start the car after letting it sit a while, the voltage for a minute or so after initial start up is around. 13.5.

Why the hell is there a light on the dash that doesn't light up when something like this is going on?

 

Wouldn't that also cause the whole engine to run like crap at idle due to the extra drag? The car runs great and the battery has never had any starting issues yet. its just the voltage is too low at idle.