[UPDATE ON PAGE 5 - Looks like this is SOLVED! Seems it was the Kessy unit]

I've had this parasitic drain for a few years now on my 2004 Cayenne S. Everyone has looked at it. All the usual checks done. I've done my own. The PCM unit is disconnected, the sat nav, all aftermarket crap that I can find is disconnected. But still, it drains - often overnight.

But - a recent realisation has made me wonder.

If I go to start the car and it's flat (say 9,10 or 11v) and won't start, I use a jump starter which works fine.

But I noticed if I turn the engine off after just a few minutes, it restarts again (noticed when I drove to the local bins and made the mistake of turning it off - but it started fine?!).

A mechanic helping me with a separate issue recently (who is one of many who has tried and failed to fix this before) didn't understand how it could do that. Obviously the few minutes drive couldn't recharge the battery (unless the jump start device is doing it but surely not?)

Today I did a test. I didn't check the voltage as my multimeter was in the house but it was so low even the doors wouldn't open without using the key.

I put the key in, and nothing at all. No lights. So lower than 10v I'd imagine.

I started it with the jump starter, disconnected the starter, got back in the car, turned it off and took the key out.

I left it 30 seconds or so, and then tried it again - and it started perfectly.

How?!

Then I turned it off and took the key out but this time the clock and something else that I can't remember stayed lit.

When I tried it again this time, nothing. Then the lights went out. And I had to use the pinhole trick to get the key out.

Tried it again, still nothing and the pin trick again.

Is it likely to be something to do with the ignition switch? I asked recently in another thread if anyone knows the part number (or non-Porsche equivalent) for the electrical part of the switch but sadly no one has replied. I did take it out last week to check the microswitches were working ok when I put the key in, turned it, took it out etc and they seemed ok.

I will forever be in the debt of anyone who can help solve this!

Thanks

 

Did the "usual checks" include an alternator output test and perhaps a new battery? Have you measured the parasitic drain? Amps, milliamps, microamps? After the computer goes to sleep (15 minutes after no electrical activity in the car..)

 

The drain has been measured by several people including myself. It's dropping around 0.7 amps. Test done properly - computer to sleep, doors open and latches jammed etc etc.

To be honest not sure about the alternator test, but others on here have said in previous discussions it won't be that because driving it charges iup (not just the few minutes I've talked about but a normal 'recharge' drive).

The battery has been changed about 5 times in the last few years. This one is just a few months old.

The fault sometimes goes away for a few weeks and I think it's solved, but always returns.

 

Dual battery?

 

No. I have a bose speaker and a spare tyre and i've been told before here that means no second battery. This has been on-going for a long time and there are a few threads about it in my name.

The particular bit I am keen to hear about is from someone who can say whether the fact it can start again so soon after being flat is normal. And what it suggests that the third time when I took the key out it went completely dead.

To me - for what that's worth - it suggests something other than the battery (new), or even a drain. The kessy system doesn't work either if that helps.

thanks

 

I cant recall exactly which combination of options rules out the possibility of a dual battery. Instead of merely assuming your dont have a second battery I would get to the bottom of that for sure. It sounds like you need to take it to a more qualified mechanic. Lots of guys can change the oil and do a brake job on these things but not many guys have the knowledge to handle a problem like what you have. You really need the factory scan tool and some in depth cayenne specific knowledge. Have you tried the dealer?

 

Are you measuring the battery voltage at the jump posts? If so, when the voltage appears low have you tried measuring from the positive post to a chassis ground? This would be a bare metal location on the engine.
Wondering if this could be a high resistance negative bond to the chassis, at the battery compartment.

 

Spare tire definitely rules out a second battery.

 

You can narrow down the circuit that has the draw by doing the following: Connect your multi-meter in series with the battery and perform a parasitic draw test (I'm assuming you know how to do this properly since you said that you do). Now you should be able to see the amperage draw on the meter, you said it's drawing about 0.7a. Now start pulling fuses, one at a time, until the draw goes away. Which ever fuse stopped the draw is the circuit you need to look at. I recommend replacing each one after it's pulled as not to misplace a fuse. Conversely you can do the same test by measuring the voltage drop across each fuse. I have a chart that shows what each value should be, but it's at work. You may also find general specs for this online. This looked promising after a quick glance: https://us.autologic.com/news/testin...e-voltage-drop
Hope that helps, let me know.

 

Thanks but that has been done several times by several people including myself. I thought I'd found it once (well, twice actually) - that's why the PCM is disconnected. The fuses for it were taken out by a previous mechanic who claimed that was the problem, but it was still causing a drain. I found other people mention this too. So I physically disconnected it as well. Why it should draw anything without the fuses in I'm not sure, but it seemed to make a difference. And - typically - it worked for about 3 weeks after that. Then started again.

It's something that the fuse box can't help with.

 

Yes, I am measuring from the jump posts but no, I've not tried it with a chassis ground. I'll do that today.

The one thing I've not been able to do personally is get to the battery as i don't have the right socket ratchet for the bolts holding the seat. As the battery has been changed so often (including by porsche themselves - although that was an expensive mistake) enough people have been down there that I decided it wasn't likely to be the source of the problem. But I guess I am at the stage where I need to do that. I'll get one today.

I'm not sure what you mean though. Could you explain the implications of your idea please? I presume if I measure using the chassis instead of the ground post, you are hoping I will see a normal reading and that could suggest a problem with something like the starter? Of course, worst case is it's a combination of several problems...

Considering the number of people who have failed to fix this I'm thinking it now must be something really glaringly obvious - so obvious everyone has missed it - or something really obscure. It's certainly not something a normal fuse check can find (and to answer another poster, the people who have looked at included electrical specialists, not just 'normal mechanics' - and Porsche did have a look at it a couple of years ago but here was too much aftermarket crap in at the time for them to do much but they checked some basics),

Thanks

 

Thanks for re-confirming.

 

Ok, I did that this morning. Car hadn't been used for 14 hours and was flat. Voltage on the posts was 9.10 - voltage on positive and some metal (it was hard to find anything grounded!) was more like 10.60, but jumped over 11 briefly too.

When I went back to the posts after that, I was getting the same 10.60 or so there. But then it went down again. That could be a dodgy multimeter (hopefully not but always possible). But otherwise does it fit in with any theories?

Thanks

 

No way a battery dead enough to not even light the interior lights on the first try will restart the car by itself on the second try after being jump started. Neither the alternator nor the 2nd battery can recharge a battery that quickly. From a discharged state as dead as you're describing, it would take hours to properly recharge that battery.

Here's another test. One of the first things that stops working on these when battery voltage is low is the rear power hatch. It will release and start to raise, then die in the middle. If starting the car with a jump always works, and then turning it off and restarting it soon thereafter always works, instead of restarting it the second time, press the hatch release instead and see what the door does. If it doesn't go up all the way, the battery is severely discharged, which could mean a bad battery or very discharged battery. If it does go up all way, then we need to look for something else - bad wiring, ignition switch, etc.

Another test - locking the car puts the computers to sleep. When you do that restart after a short drive or first jumping the car, had you locked the doors or left them unlocked. Try locking the doors, waiting for the lights to dim, then another 30 seconds, then unlock and try to restart it. Did it work? I'm thinking maybe the computers aren't waking up properly after shutting down.

 

You cannot troubleshoot effectively with a questionable meter. The voltage should be rock steady, even if its low.
If I was diagnosing this I would try bonding the battery negative post to another location on the frame. You could try this temporarily with jumper cables.