If the battery is 100% good, you might still want to check for wetness and corrosion in the harness running under the driver's carpet since you are seeing all those electrical funny things happen.

 

+1

The wet wiring bundle or low voltage on the battery are the most common causes of random unrelated electrical issues.

BTW, a power hatch stalling partially up is the first sign of a weak battery, even though it will still turn the starter motor over, which is weird.

 

Can you detect wetness on top or do you have to pull up the carpet?
It is really baffling that a weak battery will start 8 cylinders but it won't actuate a rear latch?????

 

My comment applied to not the latch,but the power lifttgate, which I think was only added on 957 generation Cayennes and wouldn't be on your 2004. The liftgate stalling somewhere on the way up is a weak battery indicator, but when that happens, the vehicle will still usually start. Sorry if I confused you but thought others reading this might find the information useful.

 

Yeah, I actually caught that. I meant to say that the power latch should take even less power that your lift, yet it fails to open with a weak battery. Then you go get in the car and start it so you can open the back. Either way, it is odd

 

I would guess it has to do with voltage drop - the higher the load on the battery, the lower the voltage will go. That's basic electricity. The starter and it's electronics were probably well designed to run at lower voltages, likely down to 10v if not even 9v, as that's how far the battery can drop under that kind of a load. The control module running the tailgate may be dropping out below 11v or something along those lines, so when the lifting motor just starts and has a spike in draw as motors do, the voltage drops and the module stops the whole show.
Just my theory.
But, in your case, after you described all of the other symptoms you're having, I agree with others that you must have a larger problem going on, not just tied to the volt meter. Do you have windows fogging up from inside (moisture evaporating)? Or have you had water ingress in the past?

 

Been really busy so haven't had time to check anything yet. No fogging inside and never had a water event that I'm aware of. Do i actually have to pull carpet to check?

I like your theory about the lift gate and latch. Makes sense

 

Jumped into my 04 today and voltage gauge shows 12, start it and it shows 14, then as soon as I indicated to leave the carpark the volts show 8 and I have no indicators!!!
got home, restarted and same condition - 8v (0) on gauge and no indicators, then waited 2 mins and restarted, all normal again...

No warning alerts

 

May sound dumb, but worth checking the battery terminals at the post. My DIY guy (me) is a baffoon, and those were loose, which triggered a cascade of odd things and a minor coronary event when I thought it was the alternator…

and check for water under the carpet. And check voltage at the jump posts.

Mercifully, alternators are pretty hardy in these things. More likely a battery or wiring issue, 9 out of 10.

 

So the car drives fine, starts fine, no problems at all other than the voltage gauge in the instrument panel, right?
If yes, I'd start thinking about a problem with that gauge.
A simple test, useful to have at all times, is to a digital voltage gauge that you can plug into the 12V power outlet (cigarette lighter). If that reads as it should, it will confirm things.
An alternative, if you don't have that, is to plug an OBDII reader in and monitor system voltage there ("live data"). If OBDII says you have 12-14V and the instrument panel voltage gauge is pegged to the minimum, that will be a strong clue.

 

You have some electrical wiring problem, not a battery/alternator problem. If your car was showing 8v for more than a split second, no chance in hell the car would start.
No indicators is another sign that you may are loosing power somewhere. Or ground, of course.

 

I had the EXACT same thing happen to me twice.

First time I was on my way to the grocery store, about 10 miles from home. I'm at a stoplight making a right turn and I notice the turn signal indicator arrow isn't flashing...could have swore I turned it on, hit the stalk and it is on. I can see the reflection of my turn signal off the truck in front of me, so I know they are working. Then I notice the volt gauge is bottomed out. At first I assume the car is shutting down accessories due to a dead alternator and immediately head back home.

As I'm driving home, the stereo and hvac blower is working. I'm thinking the stereo would be one of the first things to shut down followed by the A/C. Also there were no warnings on the display, no idiot lights and all other gauges were fine.
As soon I get home, I leave the car running, grab my VOM and check voltage at the jump post...14+ volts, its' charging fine.
Turn it off, start it back up and the gauge is now at 14 volts and signal arrows are working.
Several weeks later, I'm ready to leave the grocery store and it did it again. Turn off and back on, and problem is gone.

When I got home, I disconnected the ground for a few minutes and then reattached it hoping to reset things. So far it's been almost two years and it hasn't happened since.

 

See your problem there is using a Cayenne as a grocery getter. It is offended. I bet this doesn't happen at WOT on a twisty road...

(I kid, I kid....)

 

So your situation is (was) turn signal indicator failing to indicate (although turn signals actually working) and instrument panel voltage gauge pegged low (although apparently no voltage problems).
@bob_oz 's situation is instrument panel voltage gauge pegged low, and no other information.

Could be the same. As mentioned, definitely worth considering that nothing is wrong outside the instrument panel. And holding off on selling the car or replacing the alternator.

 

I assume indicating to leave and no indicators means his turn signal arrows were not flashing, exact same symptoms as mine. Hopefully some weird glitch.