What would make my voltage drop from 14 to 11?

check your grounds on left side of engine block and ground for ECM. the ECM tells the alternator to charge or not. also, check the pink wires on the body relay box under left hand side of dash. as well as ignition 1 fuse. one other thing is to check the battery junction box for loose or cut wires near engine on left hand side.

Checked all grounds i could find , fuse, and battery junction box is okay .
started up and went back to 14.7, but battery light in message center still coming on.

make sure your battery terminals are clean, if you have the side post type which you do the bolt gets corrosion behind it and does not make good contact. Other than that recheck the battery because they can have a bad one even if new. Otherwise you will need a tech 2 put on it and normally dealers only have that. Of course your gauge could be wrong as well.

I know most people on here do not come back to tell your out come, and how will we ever know what works ! I kept checking grounds and everything else and all was fine so i decided to try a new alternator and belt from AutoZone even though the others checked out okay. So i put them on today and so far so good the battery light in message center went out and stayed out. I will keep fingers crossed and hope for the best. Thank you hmac300 for all of your ideas and Happy Holidays!

Your problem was intermittent from the start so any test would have been inconclusive.

Okay, so my problem may be a little more involved, but I hope I can get some help. My voltage started dropping from 14 to 10 at times, causing the engine to cutout. At first I noticed mostly when maneuvering in the parking garage - braking and turning. Initially I thought maybe my battery (bought a new one), then the alternator (had it checked several times), then possibly a vacuum leak off the brake power booster. However, even when I just slow down and start turning the voltage will drop, then come back up to 14. If I am stopping at a red light, the voltage will drop (but not enough to cause the engine to cutout, yet!) and then come back up to normal. Any suggestions guys? The truck is a 2005 Silverado with a 5.3L engine (4WD) with about 75,000 miles.

This is most likely a bad alternator, but would require some in depth testing while the problem is occurring to eliminate all other possibilities.

the only thing i can think of here is to check the ground for the PCM as the PCM tells it to charge or not. it may be losing ground like in wetter weather and causing this. this is about the only thing i can think of unless the diode bridge in the alternator is messing up.

Hey, i am back to offer a suggestion. My truck is still doing fine and i replaced alternator and belt as a chance it was the cause and both were original at nine years old. And i had mine checked out at two different places and they said it was okay. I have 39,000 miles on it. Seems to me it could very well be a slipping belt and or the alternator in your case . Belt is a lot cheaper to try first. Let me know what you find out.

It uses a spring loaded tension-er so if there are no frozen components or a bad tension-er, slipping is unlikely but would make a very loud noise if it was slipping.

Update!! (but still no solution). So after going over the whole thing myself, took it in for new brakes and tires, etc. The service guys were checking everything they could think of pertaining to the voltage drop, that actually was happening while they had the truck. We rechecked all the grounds, all good. we rechecked the battery and terminals, all clean and good, rechecked the alternator and voltage regulator, all checked fine. cleaned the mass air flow sensor, all good. but the problem still persisted. The readout kept indicating just "low rpm" so no panel warning light and no specific code. we pulled off the TAC module and put a new one on, but no difference in reading (so we put the old one back on and saved $250.00). The fleet service guys have not ran into the problem so they could not offer any help.
This problem has only started over the past six months, but it seems the tachometer has been acting up for about a year and half (GM dealership will not do the cluster recall fix for some reason) so we can only get an accurate rpm reading with the machine.

Any new suggestions? Thanks guys.

if this vehicle and is under a recall and the dealer refuses to do this, then contact GM and see what they say. they may send you to another dealer or not. but if it is under recall it should be fixed no charge. that can be your whole problem anyhow.

So after checking so many things, I came across this suggestion on another blog.

"There is a service bulletin for that problem on your vehicle. It involves two things. A software update for the engine controller and a cleaning of the throttle body."

Bulletin No.: 04-06-04-040
Date: June 14, 2004
Subject:
Idle Instability, Low or Rough Idle with Electronic Throttle Control (Clean Throttle Body)

So the cleaning is no big deal and the guys at the first shop cleaned the mass flow air detector (since I have a K&N air filter they worried about the oil on those filters). The software update required a Tech2 so the dealership checked my current version and then updated the software for $120.00. So far no problems with the voltage or oil pressure dipping when breaking or making low speed hard turns in the parking garage or on the mountain switchbacks. I will keep an eye on it and if anything else develops I will be sure to let you all know. However, in this day and age of internet and email, it does not seem that it would be that hard to contact specific owners about recalls, software updates, or other safety type issues pertaining to their vehicles. Thanks for all the help guys!

Most companies if there is a software update will only do current year and not contact for anything further. Unless it is a specific recall.