04 buick LeSabre 3800 intermittent high voltage and voltage drop when shifting

04 buick LeSabre 3800 engine. Main concern is high voltage at highway speeds and at start up being at 15v. Second concern is voltage dropping to battery voltage 12.9 (12.9 bc its overcharging) then with a little rev of the engine it will go back to 15v. Put it in drive and another voltage drop. Going from park to reverse is more pronounced voltage drop.. new iac valve, new front transmission mount, cleaned throttle body and egr, new spark plugs and wires. Tested coil pack with the danner method and is ok at idle but couldnt test coil pack under load.

Where and how are you getting these voltage readings?

I first noticed on the dash gauge the voltage drop and then hooked a multimeter to confirm and surprising enough the dash was very accurate and read the same on the multimeter. Did a voltage drop on all charging wires expect starting wire and didnt have a voltage drop on the wires. The battery cable from underhood fuse box to battery under the seat. That cable actually has a crimp in the wire like someone put the seat on it and flattened it a little bit but its saying no voltage drop on the wire reading was .09. i forgot to mention in the post i have a power steering pulley that doesnt spin as freely as it should. I just got the car not too long ago and trying to get it all together. My main concern was the high voltage especially at highway speed where the alternator should be working only at 10% but the voltage is 15v. The battery is under my back seat where I have children so i dont like the high voltage and im assuming the voltage drop may be a culprit. Also checked the alternator harness wires. I have good ground, 4.8v on my reference and 3.6v at the signal wire when at idle. I didnt do a voltage test on the signal wire bc i have nothing to compare a good voltage to. Thanx again to all u guys

Looks like a bad voltage regulator in the alternator. Put red lead of volt meter to the B+ post of alternator and black lead to ground. Switch meter setting to AC volts. If reading of .25 to .50 alternator is bad.

Sorry i starter the explanation on youtube and mentioned it but forgot to mention it here that i put a new alternator in and still had the same issue bc my first thought was the regulator. Also put a new battery in bc this one is older but still same readings so i returned the battery and alternator.

Man, I was wrong on this system (we talked on YouTube for other reading this). You were right about what you read in you Haynes manual! I really hate those manuals and let that dictate my thoughts lol

From looking at the diagram the Dash Integration Module is the one that is the brains in the system BUT it is NOT the one controlling the alternator. The ECM does this on the gray wire (pin 29) "Gen Field Duty Cycle Sig" and then it also sends a turn on or wake up on the red wire (pin 61) Gen Turn On Sig. 
The Dash Integration Module talks to the ECM via a class 2 network and tell is what to do based on the two inputs it sees on (pin A12 and B12), these are simply battery voltage and battery ground inputs. This is where we need to focus in my opinion based on your symptoms. 
So we need to take direct measurements from these two locations (if possible) and compare to what you read directly at the battery. 
Do you have the ability to see data on the Dash Integrated Module? If you do, then we can maybe skip this step (direct measurement), at least initially.  
 
We have to be careful sharing service info here. If you need additional help with this as far as component locations and other service info. Here is a link to the DIY version of this program that I am using.  eautorepair.adtrk.biz/?a=10817&c=24&p=r&s1=

I think you may have a poor signal getting to the Dash Integrated Module (low voltage on the battery positive sense wire or poor ground on the ground sense wire) we need to measure these to confirm. Also do the measurement I am calling out in this pic, this will answer the "low voltage here" (at the fuse) I have stated in the pic because it is shared with the alternator on that pin and the DIM. 
Also as an easy thing to do, that battery ground cable, right at the battery, is a split cable and right where they connect together, we want to take that apart and clean those areas really good.

I do not happen to have a scan tool that can read that data. I only have a 20$ obd2 that i can use with an app but it doesnt communicate with modules. I can read the voltage at the sources. I have the same exact manual u have actually, now i realize its not a hanes manual. But anyway i can get a reading. So ground and power at the dim and ground and power at the ecm ( on those pins of course) and compare to battery voltage correct?      Side note after watching hours apon hours of your videos for the last year and a half and having no mechanical experience i am proud of myself that this is the direction i was going in which is a hard diagnosis bc most cars its just the regulator or ecm controlling it and also so grateful for the experience in diagnosis ive gained by watching your videos. So much more to learn.  Not trying to kiss ass btw just honesty.

I'm just glad you are here and glad to help! And I didn't even know that part of your story. I never get tired of hearing how much this helps others. Looking forward to the results of your tests.
Thank you!

Something else that is VERY important. ALL of your voltage measurements should be compared directly to battery ground, right at the battery, NOT the frame or body especially because of the remote location of the battery. And when you do your ground tests, it will be ground to ground voltage drop tests NOT resistance okay?

the Dash Integration Module is the one that is the brains in the system BUT it is NOT the one controlling the alternator. The ECM does this on the gray wire (pin 29) "Gen Field Duty Cycle Sig"

Hello ScannerDanner. I have a question. I could be totally off and maybe this doesn't apply to this particular alternators operation but I have in my notes that the F terminal just tells the PCM about alternator load.

During the late ’90s, we started seeing the L terminal on the CS series four-wire regulators connected to the PCM, as well as the F terminal. This led many techs to assume that the PCM controlled the field current, as it does with Chrysler. It does not.

www.motor.com/magazine-summary/are-you-s...art-charging-system/

Dave Hobbs explains as such @ 2 hours 19 minutes in this Youtube video:

 

Ok guys. Sorry been busy with work. So far i have measured voltage at connectors a12 and b12 of the DIM module. Orange wire (a12) to ground on battery, same as battery voltage. I was reading 14.67.. ground (b12) to ground on battery was weird to me bc it read only 0.00. check my connection and still 0.00 on ground.

    Today i was testing voltage on these wire (pic inserted) i also just happened to test the daytime running lamp relay control. Everything mentioned on pic was the same voltage 14.7.. but the drl relay control wire was 15.23.. the control output wire (b8) was 14.23.... Led dimming signal was 10.4 key on engine off. Everything else was measured engine on. i think i might be onto something. Jus not sure what now lol. 

So i have a .5 voltage drop on my ground wire from engine to frame... orange wire checks out ok (at rear fuse box) ground is still a little weird to me at the dim.. but going to change my ground wire that is clearly corroded as well...Field duty cycle wire voltage is 3.6 at idle and drops to close to 0v when given gas.. so now i have to check that wire at the PCM to see if im getting 5v at PCM and only reading 3.6 at alternator. Theoretically it should be closer to 5 at idle im guessing? But thats where i need ur input.. I had to go through more scanner danner schooling to learn more electrical lol.. thanx for the parking light short to ground video on ur neighbors charger and all urs and ur brothers help. Much respect 

Have an 2002 Buick Lesabre with exactly the same symptoms, what turned out to be the problem if you don’t mind me asking, thanks?

Still have the same issue, havent fixed it yet. But also still need to take out the fuse box and test the orange wire danner was talking about. Seems to be a common issue without any definitive explanation.

Thanks for the reply, will keep you updated if I figure anything out. By the way, probably unrelated but I also randomly get the “ security” light to stay on solid after starting the vehicle but seems to run fine however if you shut off the vehicle and go to restart it cranks but won’t start unless you turn off pull the key for a few seconds and try again then it busts right off. Anyway just curious if you had similar symptom with the voltage drop issue? Take care.