I have not seen anything like this before (1991 944 S2)
 

As above I have a 1991 944 S2 that has only given me fan resistor issues. It's a reliable car.

Last fall on a Saturday morning I came out and found the battery didn't have enough juice to turn the crank. I promptly jumped it and it ran perfectly. I know low RPM doesn't produce enough amps to charge the battery, but I let it run for a few hours just in case. It's my first Porsche so maybe it would. I came outside to turn it off and it was already dead.

Fast forward a few weeks and I found a catastrophic result. No start, no gauge movement at all. With or without jumper cables! My multimeter shows 1.87 volts without jumpers.

Wouldn't at least some lights show up with a 12 volt jump?

I have some the following:

replaced the DME with 3 different units
replaced the fuel pump relays
checked every fuse in the box

Dearest forum members - do you have any idea what's going on?

(I mean jumper cables and no response?)

Did you replace or have the battery checked? I'd start there.

Sent from my SM-N960U using Tapatalk

Second getting the battery checked. And a correction to your post - you said low rpm doesnt produce enough amps (volts) to charge the battery - even at idle you should have enough voltage to power the battery - it should read at least 13.5volts.

So if you can get the car started (you can even perhaps roll the car and pop the clutch while in 1st with the key turned to "on"). Check the voltage of the car while its at idle by putting a multimeter on the battery - again, it should read at least 13.5 V.

If it does not read 13.5V you have an issue with your alternator OR the ground/power connections to the chassis or to the battery (corrosion should all be cleaned on grounds and on battery terminals).

If the car doesn't start even with jumper cables, you likely have corrosion issues which is limiting the voltage.

Also, it should not take a few hours to charge the battery, more like 10-15 minutes around the block (scary to think you let the car sit and idle for hours but thats another story...)

To me it sounds like you need to investigate corrosion issues to the battery connections or ground/connection to alternator, or a bad battery. Secondarily it sounds like you may have a draw on the battery (which you can test by googling). Third, recommend buying a battery charger or booster (its useful for lots of situations).

I'm off to get a battery - will report back soon. - Derek

Thanks guys! It starts like a champ. That was the fix, but what was the root cause? Battery hit end of life? Alternator died? What do you think? - Derek

Too soon to tell, but if your alternator is dead the problem will present itself again after a few hours of driving

I buy a battery every two years (4000mi). Even on a trickle charger my S2 will eat them up

Take it to AutoZone they have a free tester which checks battery, alternator and starter all in one test machine. It's easy and free.

Sent from my SM-N960U using Tapatalk

What is it within the car that you think is doing/causing this? Parasitic draws, inferior voltage regulation?

Just interested,
John

Please take my suggestion and put a voltmeter on the battery while the car is idling. Record the voltage, report back. The voltage will reveal whether you have an issue with your alternator/wiring or not and help address the "underlying cause" as you so indicated you wanted to identify...

Not really sure, but it typically presents just before a track event (maybe someone is looking out for me), and the hood has been open for months while I prepare for the aforementioned. It has set for 3-4 weeks w/o trickle and it fires right up, so nothing to suspect a parasitic draw

It is also worthless. I am trying to get them to replace my battery from my Armada It goes from 12.7v to12.3 in 45 minutes - sitting in my living room so no chance of a draw from the car. They said it tested good (tested at 12.3 - had it fully charged when left house and arrived at store 45 minutes later). So I put the battery in the car and drove the car to AZ so it would be fully charged when they tested it. They came out and tested it and said it was good. Asked to be checked again and the battery voltage dropped from 12.7 to 12.5 (12.5 is only 75% charged - I lost 25% charge in less than 2 minutes) in the 2 minutes since the last test. The employee saw this and agreed the battery was bad but since the tester said it was good he could not replace it. Told me to go home and drain the battery down to nothing and bring it back. They do not bother tested completely dead batteries, they just replace them. NEVER trust the testers at these stores.
I had the same problem with the battery in the 944 3 months ago, but with Wallyworld. It took several times of bringing the battery in before it tested bad. It worked out though...I bought the battery in NJ and am now in AZ. After it tested bad the guy told me he would check to see if I owed them any money for a new one. I bit my tongue, as I was looking at the "free 3 year replacement" covering the entire side of the battery. He did all sorts of paperwork, went into the register and handed me $95. I told him I did not think I should be getting money back and tried to give it back to him. He explained that I paid more in NJ then they charge in AZ, and that along with the core charge - since I was returning a battery - came out to the $95. I was tired of trying to get it replaced I just took the money and new battery and left.