Try to check the the voltage on the left post under the hood (or even closer to the alternator).
- If the value will be the same (ca. 11v)l then unfortunately it looks like smth bad with alternator.
- If the value will be higher (better) - then I would check the grounds, the cable back to the trunk.

 

So there are 3 lugs in the junction box under the windshield cowel on my car. I suppose one is for starter, one for alternator, and one for house battery.
Which of these would be connected to the alternator? I think I should be able to test alternator output through that.

 

somewhere in the middle of this thread you got a photo with information what is what

 

Hello @virtual_law ,
You are on the wrong side of the cowl, the actual cable connections for the alternator and the starter are on the engine side, so no reason to lift the cover at the windshield, unless you wanted to clean all 5 connections, I gave all this information in post number 11 of this thread, but I will post pictures again here to make it easier.

As to the starter battery having 14 volts at idle, that is fine, and not to come off as rude in any way to @sazam , Sazam, please list the part number and location of installation of this so called "DC/DC converter", as I have never seen nor heard of this in our first generation GT'S or FS's, nor that any such converter would pump the voltage up to 14 volts for the starter battery, let alone just the starter battery, this is not a system like the Bentayga that converts 12 volts to 48 volts..

14 volts at the starter battery is fine, just as 13.4 to 14.5 +/- out of the alternator is fine, of course depending on the load placed on the alternator at idle, as to the house battery having 11.65 volts at idle, a couple of things may be a play here, one, the house battery itself, you could charge it and have 13- 14 volts sitting on the bench right after charging, and as you said, you have 13 volts after a full charge with the CTEK, in the vehicle I assume, question is, how long does it take to reach the full charge, and most importantly, have you had a load test performed on said battery, as I see you did a capacity test in the other thread, but how about the "start load" test as shown on the tester I posted the picture of, because, eventhough the battery shows good voltage, it may have lost capacity and life by being discharged one too many times, therefore drops in voltage, and or causing a problem with receiving proper voltage from the alternator, like the battery may be shorted inside, the other issue may be with the battery control module that is located behind the house battery, as from my understanding, it switches to charge the starter battery, BUT, as looking at the wiring diagrams, the alternator cable comes up from the alternator to the left connection point (green) at the junction box as shown, two of the three points under the cowl cover (yellow & blue) take the alternator's current and feeds it directly to the house battery's 300 amp mega fuse (green on the house battery as pictured) and the main E-Box harness (blue), the two red points are strictly the positive from the starter battery to the starter solenoid/motor.

So, I am thinking two things, one, verify that the 300 amp fuse at the house battery bus bar is good, better yet, check all of them, four mega fuses on the face, and one on top, second, verify the battery is healthy with a load test, because in my mind, I don't see how the starter battery can have 14 volts, but the house be 11.65, as I said, and to be clear, the cable from the alternator/junction point at the cowl is a home run to the house battery positive bus bar 300 amp fuse, therefore, if the alternator is outputting 14 volts +/-, then you should see it at the house battery itself, unless the mega fuse is blown, therefore, the alternator is still feeding power to certain items which may include the battery control module, therefore allowing the charging (14 volts) to the starter battery.

Sorry for the long explanation...

Johnny

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@Johnny Hotspur GT ,
DC/DC - here is the diagram from service manuals:

 

Priceless info as always sir. Your posts read like a J.K novel!
Thank you for resharing, I was actually skimming the thread on @sazam 's advice when your post came in.

Went straight to the car today, started it, and read voltages at Alternator connection (green) on the engine side junction box, getting 11.6v. On the starter battery connection (14 and above).

Alternator Mega fuse: I had all mega fuses checked for continuity earlier on the house battery (a lesson learned during some hard days with my 599), and they all reported fine, including the alternator fuse as you've shown. However, this was checked in situ. Should I remove and recheck fuse separately?

House battery: I haven't done a load test yet, will do so shortly. I will use my TopDon tester to report its findings.

Corrosion: I don't see any particular buildup near the engine side junction box nor the cowel side so have put off cleaning for now, especially since I'm seeing the same volts at alternator junction (green) as at the battery terminals at the back of the car.

Parasitic draw: I currently have all possible culprits removed by way of fusebox under steering. I have the fuses for alarm, infotainment, telephone, TV tuner and Navigation removed. I see a P01308 "Roof Electronics Control Module No Signal/Communication" but I do not suspect it to be causing a draw.

Given that starter battery is seeing 14v and above, the Battery Control Module and the Alternator itself can be ruled out? My Battery Control Module was replaced by a 2004 Phaeton ebay purchase in March '24 to cure a charging issue with (yet again) the house battery. There are no errors reported on the scanner under "Battery Charger" either.

 

These are my battery results.

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so you have the same voltage under the hood as on the battery....
i would go now closer to the alternator then (directly on alternator pods) and check its voltage - to eliminate the problem with alternator or rather confirm the problem is with alternator..
- if again it will be 11.6 when running the car, you have problem with alternator itself
- if surprisingly it will be higher voltage then smth wrong in the cables or connectors (grounds, corrosion, defects) all on the way to the pods under the hood and further to the back of the car

btw: have you scanned address 71? maybe here is smth interesting too.

 

Sorry for the delay, was fighting with multiple cars, including the Bentley.

I inspected all possible junctions, the ones at the firewall, both battery posts and their relevant grounds. Everywhere else was clean, but I have found massive corrosion on the left battery's chassis connection. Seems like water had got in at some point and not been evacuated in time. I missed this. It felt like a possible culprit, so I cleaned it, removed all the rust. Refitted all and connected right side battery negative then left side (this is incorrect but it skipped my mind).

Here's the peculiar readings:
Battery voltage before reconnecting everything was 12.81v
When negative was connected and boot lights powered up, voltage dropped to 12.6v.
I started the car and voltage at idle dropped to 11.95v. Right side battery charging normally at 14.3v.
Waited about 10 minutes, then turned the car off. Checked left side battery and it had jumped to 12.4v. What does this say?

Seems like nothing really changed after the corrosion cleaning.

I did scan address 71 on the computer, it has no errors. It allows me to trigger Parallel solenoid which I did, and I can hear the clunk in the boot, so seems all right.

When you say alternator pod, do you mean the junction bolt that the alternator connects to on the engine side?

@Johnny Hotspur GT I think you mentioned that a bad alternator could actually be drawing power from the left side battery than supplying to it. Is there a way to check this?

 

you simulated this with vcds on addr 71 or did it thru the key left/right?

 

I simulated key left once, but that did not result in a clunk. However, through my scanner, (Thinkdiag 2), I can trigger the solenoid on address 71 "Battery Charger" and could hear the clunk

 

ok, so you want to make sure the bridging just works? you said you were able to trigger this by scanner option, but you are not hearing it triggers using regular manual procedure...

- check voltage on your left & right battery posts before triggering the solenoid to fire - record value
- do the trigger
- check voltage on your left & righr battery posts after triggering the solenoid to fire - record value

compare if it logically means they were bridged

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repeat the same using key-based manual procedure - you will get the answer


btw - long time no news here, did you solved the main issue related to charging or not?

 

Ahh where do I even begin. Im in a new world of pain.

2 weeks ago, I spotted a lot of corrosion on the left side battery negative terminal's connection to the chassis, so I disconnected right side battery, then removed left side negative terminal, then removed the chassis connection bolt and removed the cable, cleaned all the corrosion, put it back together, reconnected batteries, trickle charged left side battery to full, put the key in the ignition, and switched ignition and saw my heart sinkkk, instrument panel/cluster was completely dark even though ignition was on. The car started up though and I did a 20 min drive. Parked the car in the garage, took key out, then put key back in anddd nothing. Ignition isnt turning on anymore. Zero response. Though lock, unlock buttons on remote are working fine.

I had to leave for a week long racing event 2 days later, so I figured Id disconnect both batteries and let her sit for this week. When I got back and reconnected, the cluster came back on showing me the 'boot-open' 'door-open' sign, accessory lights in boot powering on, but this time, putting key in to the ignition, I get "Immobil Active" on the driver instrument panel.

I figured maybe I messed up left side battery chassis ground connection that I just cleaned, so I went back there, loosened the nut (it was actually fine), retightened it, connected both batteries back up, boot lights power up, I insert key into ignition, and now again, driver instrument panel is completely unresponsive.

Ive checked all fuses under steering wheel and fuses and relays in trunk, and nothing seems blown. Mega fuses on left battery are fine. Couple of posts on the forum led me to Ignition Relay no. 35 in the engine compartment fusebox, which I removed and thoroughly tested with 12v (it clicks, shows good resistance, perfectly fine). There's an Ignition relay fuse 40A in there, no. 26, also fine.

So now im gonna ctek both batteries to full, and see if that changes anything.

Meanwhile, I came across this:
This video replicates the problem I was having BEFORE the instrument panel decided not to respond at all. However, I hate how the guy in the vid has rolled the carpets back, is there a safer way to remove carpet partially to access thermofuses and Kessy? Mine's a RHD car mind you, so we're working with passenger side.

Its a sad week. But thank you for all the help!

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sooo... you have more electrical issues that it was before...

1. you had the charging issue (alternator issue or cabling issue, other may be the battery control module issue)
2. then you found out the parallel-solenoid is not ticking when you want to bridge via key (using scanner it is)
3. now you have multiple electrical goblins eating your car

ad1. did you checked if alternator itself do the right charging level (using voltmeter check the connectors closest to the alternator / on alternator)

ad2. suggest to check as I described earlier

ad3. is your car rebuilt after flood damage, or car accident? now, or somewhere in the past? and/or there was any water inflow to the driver side carpet area? corrosion, wiring cuts etc.

to get to the kessy and to do it in elegant way, you need to take out the seat, then all the covers and get the whole carpet out... otherwise you may end up with destroying some foams etc.