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2000 Porsche 996 - Battery Drains - The alarm Horn is a constant 3v an

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Hi all


So i'm getting the diagnostics done in a few hours time, so hopefully i will be able to repost tonight with some useful findings.


At the moment the fault is pointing towards the ABS unit or plug on it, it seems to be physically linked to the alternator regulator wire and is possibly not turning off or backfeeding current to the regulator. This unit is mounted neat the alarm horn, so this unit might also be affecting the small constant hum that I also have on the Alarms horn.


Depending on what happens tonight, i'll have to locate this 39 point on the car, i'll do some research but guess it might be a plastic connector with a few outputs - but at least i know the location.


I dont think the dash is powering the 39 plug in anyway as ive removed 2 fuses that relate to the dash power and the hum remains, so the ABS unit is the next item on the list.



"Right hand drive  - weldpoint 39 looks like it is behind instrument cluster" - this sounds hard to get to, i'll look at the precise location on a diagram and then see what I need to remove to get to it...



Cheers and have a good day,



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Hi all

Sorry to say that I still have this fault on the car, the battery is still being drained, i still have the hum on the alternator and the horn - around 3.7v on both. The drain is coming from the alternator, some 350ma from that wire staying positive 24 hours a day. It will switch to 10.v with the igintion on.



"Right hand drive  - weldpoint 39 looks like it is behind instrument cluster" - this sounds hard to get to, i'll look at the precise location on a diagram and then see what I need to remove to get to it...


I can't locate this point, does anyone know what it visually looks like or more precisely how to get to it? If i know its a small box or a black clip with 3 wires coming  out, i will further look.**



I have done some further work though:

1) I've unplugged the round X59 connector and that has 3.7v coming in to it and onto the regulator.

2) I also removed the rear door panel, carpet and side trim to locate X2/3 connectors, theres a white and a black one. The black one has the blue wire on it, and this measured 3.7v on it. I couldnt work out how to pull it apart, and theres not much room, so i didnt pull it in two to check, but it had 3.7v on it and it all looked visually as new.

3) I removed B1, B10 and one of the E fuses and the power gets cut from the Cluster, but the Hum is still there on the alternator.

4) I removed the Cluster and the Hum is still there. I checked the voltage on every pin on all 3 cluster plugs (black, white,Blue) and some have power 12v, 10v and one had 0.7v, but none had anywhere like 3.7v.

5) Ive unplugged the ABS plug on the unit by the spare wheel and the Hums still present. No thin blue wires there though.


So I've done quite a few checks, but still not resovled.


I also had the SnapOn diagnostic done at a fair cost, so not Durametric/PIWIS,  but it had 1 code P1602 but this seems to be a generic code that occurs if you unplug the battery.



So there isnt much more for me to check, except this Weldpoint 39----just need to locate this item! If anyone can help, let me know :)




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I do not know exactly where 39 is but you should be able to find the blue wire from the instrument cluster and trace it back to the connector .It is probably buried in the wiring harness which could require removing some pieces .Question-does the charge indicator lamp work ?Illuminated with key on and engine off and then not illuminated with engine running .

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I don't think connection point 39 is important at this juncture .The blue wire is being powered by something -abs-instrument cluster or psm .If all three are disconnected -visibly making sure the blue wire is in the connector - and disconnected from the alternator for measurement there should be 0 volts .The next test would be a test for shorting -ohm meter-blue wire at alternator and ground with blue wire disconnected at instrument cluster-abs and psm .

There is always a possibility that a previous owner modified something but unlikely .

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  • 2 weeks later...

Hi all


Finally the Fault is Fixed! but I had to do a skillful fix (bodge) as I couldnt locate what was actually causing the fault - so I've done the best thing and put in a Relay to sort the drain.


So basically I had a constant 3.7V going to the Alternators regulator at all times, then with the key Switched On, I was getting 10,8v on the regulators blue wire.


With the help of a lot of you guys on Renntech, I traced the wire back to "X2/3 Black" which is by the seatbelt mounting point on the left side of the car, to get to this you remove the rear left doort panel/speaker and carpet area. I determined that there was 3.7v coming into this X2/3 on the blue wire.  Using the wire diagram, we traced the blue wire into Weldpoint 39 which is behind the speedo, but not accessible without taking the dash apart, so I didn't do this. But this blue wire then splits into 3 other areas, the ABS, PSM  and ASR and then to the Speedo Cluster.


I took the speedo unit off (multiple times!) , the ABS unit and various other bits of trim etc, these blue linked wires all measured 0.7V with the ignition off, so basically the Extra 3v was coming from somewhere else, possibly Weldpoint 39 that I cant locate.


I did find a Tracker and checked every fuse, the CD player and removed every relay, but the X2/3 still showed 3.7v with everything removed or unplugged (except the battery of course).


I also had a SnapOn expert come and Diagnose the car, but nothing showed up of significance.


Then I brought my own Durametric PIWIS unit and Diagnosed with my laptop, I had a few codes but all unrelated.



So I couldn't locate the faulty 3.7v which was causing my drain.

To repair this fault, I removed the rear door panel/speaker then lifted the carpet and the plastic wire protector/cill thing, I then unclipped X2/3 by pulling the sliding locking leaver. You can see that pin5 (I think it was :) ) was blue and this is the alternator regulator wire causing me the problems of my battery drain. I  cut the wire an inch before it went Into this X2/3 block. I stripped and soldered on 12" of extra wire onto both parts. I Then stripped some of the sheathing off Pin 4 which is Black/Brown, this wire is 0v when Off and 12v with the key On. I soldered a 12" wire onto this wire, with a 5Amp fuse in the middle of this new wire.


I then insulated and cable tied these wires to make a tidy job.


After this I mounted a new quality 4 pin relay and wired it into these 3 wires, and added a 4th wire for the earth, this was secured under one of the bolts behind the door panel.


As I had 12" wire to use, the relay was tucked nicely behind the speaker, but very low down so that it wont be affected by the speakers Magnet. I can now easily access this relay in the future by removing the speaker only.


Relay wiring:


30 - Blue wire "3.7v" In , from speedo.

85 - Ground wire to earth point behind door panel.

86 - Thin wire to Pin 4 (Black/Brown) wire on X2/3.

87 - Remaining end of the Blue wire.


When you turn on the Ignition key, power is routed to Black/Brown Pin4 which powers the Relay. This then connects up  30 to 87 to connect the voltage to the regulators on the alternator.



Put all the carpet and door panel back in place and it all Works really well!

I've gone from around a 390ma draw which was draining the battery in about 6 days, to an amazing 30ma which is prefect and very low.


The alternator of course no longer  hums as its only powered with the ignition on.





So Thanks to Everyone who has replied, I cant single out people as I've had a lot of help. I've spent a lot of time on this, lots of evenings and around £150 to get to this stage, so hopefully this post might help other people if they also get a Humming alternator and battery getting drained.


Many thanks everyone!

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Glad to see finally you got it fixed! I think it should be fine but if it runs into any issues, you can always fix it by cutting (orphaning) the blue wire between the x2/3 plug and the cluster and ABS, then adding your own wires to connect the x2/3 connector, cluster, and ABS unit together. Will be more work to fish the wires through and dash and firewall though.

Edited by Ahsai
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  • 2 years later...

I'm aware this is an old thread and the OP has long resolved the issue...


For reference, the reason the horn hums is that the system is continuously applying a small amount of current and monitoring to see that it is present. If you arm your car, and then unplug the horn, you will see that the flashers will immediately come on.


The normal current draw, when all interior, exterior lights, as well as odometer illumination is off, and doors are closed, is ~50 mA (i.e., 0.05 A).

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