Very Odd '96 993 Battery Issue

[daepp]

I've been driving a 72 911 Targa for a quarter century, but around Christmas I picked up a 28,000 mile Midnight Blue/Cashmere 6 speed Cabrio. With such low mileage I would never have guessed I would experience such an odd problem.

Bottom Line Up Front: When I turn the key off, not everything turns off - intermittently.

One day I went to start the car and the battery was DEAD. I mean nothing, not even a dim dome light.

I charged the battery and drove to work. Around 1 pm one of my employees came in from the parking lot and asked me why my backup lights were on, to which I replied, "That's impossible, the key is in my pocket." No such luck, he was correct. When I had parked I had backed in, and just like on my '72 I felt no concern leaving it in reverse. But the lights were on, and when I moved the gear shift to neutral they went off. And back to reverse, they were on again.

So I was thinking, well, maybe there have been some changes in the decades between my two P cars, so I called the dealer I got it from and had him check any other 993's he had - and as you might have guessed, none of them behaved like mine.

As I said it's an intermittent problem - it occurs maybe every third time I use the car. And so now every time I park the car I put it in reverse and go back and check the backup lights to make sure they're off. IF the backup lights are on, I cycle the key switch to the second position - one or two times, and then they go out. Weird, right?

I believe this is a LARGE drain on the battery - more than I would guess two little lamps could do. Before I had any real clues I did put an amp meter on it while everything was off (including the backup lights) and I only recorded a .35 milliamp draw.

Of note:
- When the big drain is happening I haven't found any other clues than those two lights being on - like, no dash indicators lights are on, no fuel pump noise, nothing.
- I was thinking of replacing the electrical half of the key switch, but once when I was parked in reverse - and had checked that the lights were off - I returned to the car an hour later (for something I forgot in the car) and the backup lights were on.

Anyways, I know that's a long writeup, but if anyone can point me in a particular direction, I would sure appreciate it! Of if you need any add'l info, please just ask!

Thanks so much!


PS A couple of pics for fun!



The day I first saw it in person. Delivery from Connecticut might have been the longest wait of my life



Next to my old, tried and true '72

[KNS]

I have a 1995 993 - this probably isn't related to your problem but it just might be because it was so strange.

For a while I had an intermittent problem where many of the caution/waring lights would glow dimly - on then off. no rhyme or reason. On a bright sunny day you might even miss it but at night you'd definitely catch it. I couldn't make sense of it. I believe I replaced the battery and still no improvement. This went on for months (no drivability issues).

It came time to replace brake pads and I noticed that one of the pad wear sensors, the part that makes contact with the pad, was broken. Still together in the little slot in the pad but broken. I had a feeling that broken sensor was intermittently closing a circuit. I eliminated the pad sensors at all four corners and the problem was solved.

That one little broken sensor was making the electrical system respond in strange ways. Even if that's not your problem I suggest eliminating your pad wear sensors.

[daepp]

Wow - great catch on your part! Thanks for posting.

I have a fundamental problem with not knowing just how much of a current draw I am experiencing when the battery goes dead, and whether just those two backup lamps can produce enough to kill the battery stone cold dead.

[76FJ55]

Sounds like something is keeping your X bus hot with the ignition turned off. it could be an ignition switch problem, or there could be something else back feeding power to it. I'll try digging around the wiring diagrams and see if I can come up with any specific items to check.

[daepp]

76FJ55: thank you, I would def appreciate it!

[Dpmulvan]

Get a meter and check the parasitic draw.

[Schulisco]

Originally Posted by Dpmulvan

Get a meter and check the parasitic draw.

^^this^^

and pull the fuses one by one to idenify those circuit(s) where the parasitic draw happens...
Also check the ground points on all over the car, make them clean and use a real contact spray, not the damn WD40 which evaporates quickly...

Thomas

[76FJ55]

here are a couple of my thoughts on troubleshooting.

the R33 relay is the relay that supplies power to the X bus. next time the issue presents itself, go and pull the R33 relay. with this relay pulled see if the reverse lights have extinguished.
If so, there are basically two options.
1. The relay is sticking in the closed position. replace the relay with a new relay (Part No. 191-937-503).
2. The other option would be that the signal to close is remaining hot. Relay pin 31 should have constant ground and pin 86 should be supplied power from the ignition switch. Check for power at the 86 pin socket. if you have power there with the key off you could either have a bad ignition switch or a short to power on the wire between the switch and the relay.

If the reverse lights didn't go out, then the problem lies outside the normal R33 power supply to the XX bas and there is something else other than the Relay supply the X-bus with power.

let me know what you find and we will continue from there.

[daepp]

Originally Posted by Dpmulvan

Get a meter and check the parasitic draw.

I measured .35 milliamp parasitic draw, but that is when the car is behaving properly. I have not measured the draw when (whatever is malfunctioning intermittently) is producing a draw.

[daepp]

Originally Posted by 76FJ55

here are a couple of my thoughts on troubleshooting.

the R33 relay is the relay that supplies power to the X bus. next time the issue presents itself, go and pull the R33 relay. with this relay pulled see if the reverse lights have extinguished.
If so, there are basically two options.
1. The relay is sticking in the closed position. replace the relay with a new relay (Part No. 191-937-503).
2. The other option would be that the signal to close is remaining hot. Relay pin 31 should have constant ground and pin 86 should be supplied power from the ignition switch. Check for power at the 86 pin socket. if you have power there with the key off you could either have a bad ignition switch or a short to power on the wire between the switch and the relay.

If the reverse lights didn't go out, then the problem lies outside the normal R33 power supply to the XX bas and there is something else other than the Relay supply the X-bus with power.

let me know what you find and we will continue from there.

That gives me a LOT to work with! Thank you for spending the time to communicate all that. And I will def get back to you.

Another couple of questions if I may: If the X bus is indeed remaining hot, do you know what circuits would be hot? The reason I ask is that - so far - the only sign I have of the malfunction is the backup lights.

Also, when you wrote, "...then the problem lies outside the normal R33 power supply to the XX bas" did you mean "X bus"?

Thanks again - I really appreciate it!

[76FJ55]

Originally Posted by daepp

Also, when you wrote, "...then the problem lies outside the normal R33 power supply to the XX bas" did you mean "X bus"?

Yes, X-bus. sorry, I'm not very good at proofing my own writing.

there are a number of items on the X-bus. the X-bus is the ignition switched bower that is hot with the key in the run position, but drops out when the key is turned to the start position to minimize the load during cranking.

examples of items controlled by the X bus.
windshield wipers, front and rear fog light relays, seat heating, AC compressor clutch...

a quick primer on Euro electrical systems.
Bus No. / function
30 / always hot 12V
15 / key switched hot in run and crank 12V
X / key switched hot only in run, off during cranking 12V
31 / Ground

see "Understanding European DIN Wiring" for more details.

[Schulisco]

I've never seen a wiring diagram of the 993 yet, but it would make me wonder when exterior lights like reversing lights are supplied by the X bus as they're security relevant...the X-bus is for disconnecting unnecessary things like heater and heated back window etc. while cranking the engine.

[daepp]

Originally Posted by 76FJ55

Yes, X-bus. sorry, I'm not very good at proofing my own writing.

there are a number of items on the X-bus. the X-bus is the ignition switched bower that is hot with the key in the run position, but drops out when the key is turned to the start position to minimize the load during cranking.

examples of items controlled by the X bus.
windshield wipers, front and rear fog light relays, seat heating, AC compressor clutch...

a quick primer on Euro electrical systems.
Bus No. / function
30 / always hot 12V
15 / key switched hot in run and crank 12V
X / key switched hot only in run, off during cranking 12V
31 / Ground

see "Understanding European DIN Wiring" for more details.

Man, I am again completely blown away by your detailed response - and again, thank you!

And of course I have a follow up: if the X bus is hot, does it mean everything on it would be hot? The reason I ask is that, so far, I haven't been able to detect any other "appliance" (for lack of a better word) that is on when the car is malfunctioning. No head lights, no blinkers, no blower, no dash lights, and no fuel pump. However, I am aware that may be a dumb question...

[76FJ55]

Originally Posted by daepp

Man, I am again completely blown away by your detailed response - and again, thank you!

And of course I have a follow up: if the X bus is hot, does it mean everything on it would be hot? The reason I ask is that, so far, I haven't been able to detect any other "appliance" (for lack of a better word) that is on when the car is malfunctioning. No head lights, no blinkers, no blower, no dash lights, and no fuel pump. However, I am aware that may be a dumb question...

Not necessarily. the X bus would typically be tied to the triger coil of the relay if a relay is used to control the device. for instance the AC compressor magnetic clutch is controlled by relay. the HVAC control unit is actually powered by the 15 bus. on even with X powered the control unit will not signal the relay to close and therefore the compressor clutch won't be energized.

Or the primary power could come from the 15 and the control come from the X.

basically the only time you would see a live component would be if the power came directly from the X-bus, or the X-bus directly controlled the trigger coil of the relay and power came from the 30 bus.

[daepp]

Originally Posted by 76FJ55

Sounds like something is keeping your X bus hot with the ignition turned off. it could be an ignition switch problem, or there could be something else back feeding power to it. I'll try digging around the wiring diagrams and see if I can come up with any specific items to check.

Update, solution, and credit where credit is due!

And, well, 76FJ55, was absolutely right about the X Bus.

We pulled all the fuses, but something was still remaining hot.
I told my local independent Porsche shop that I suspected it was the ignition switch, and that I was ok with skipping the usual diagnostics and any wasted $$ related thereto, I just had my suspicions.

Regardless they pulled all the fuses anyways, and found that indeed - something was still hot. And they knew that the X bus was an unfused circuit. So they then attacked the electrical portion of the ignition switch, and as soon as they had it in their hands they knew they'd found the "parasite" - I mean the problem.

In the 5 second video below you can clearly see the problemo: