NW911C4

Hi,

I have a 1991 911 Carrera 4 that I just replaced the battery. The previous battery died often and so I replaced the battery with a new one. However, the new battery is not keeping its charge when the car is sitting in the garage. I have the factory radio and no aftermarket alarm system. I measured the current usage when the engine is off and all lights are off. The current measurement is as follows:
- Total drain = 25mA
- Fuse 12 (Dianostics, Emergency flasher, rear window wiper) = 13mA
- Fuse 16 (ABS) = 1mA
- Fuse 35 (DME) = 2mA
- Fuse 38 (Radio/Booser) = 4mA

All other fuses recorded negligable current use (less than 1mA). Is Fuse 12 using too much current? The Emergency flasher is off and there is no rear window wiper, so the (on-board) diagnostics must be using 13mA when engine is off. Does anyone know how to diagnose the on-board diagnostics and know why it may be drawing too much current? The car runs fine.

Thanks,
LL
'91 911 Carrera 4

Kevin

Most battery drain problems result from the factory alarm system. Check to see if your radio alarm switch is attached and your glove box door is closing. Look through the previous posts, Adrian has answered these questions in the past many times. How long does it take to drain the battery?

NW911C4

It takes approximately two weeks or so to drain the battery. But at only 25mA, the power consumed is only 0.3 Watt (a small fraction of your standard light bulb). As I mentioned previously, the Radio/Alarm system only consumes 4mA, so I can rule out a faulty alarm system. The clock and interior light consumed next to nil amperage. I would like to know if the current consumed by the (on-board) diagnostics is reasonable or not?

Thanks,
LL
'91 911 Carrera 4

Adrian

Dear LL,
If you have a 72amp/hr battery it will take at 30ma current drain, 2160 hours to completely flatten your battery and around 1000 hours before you cannot start.
No problem, with 25ma. So why is your battery going flat. I suggest that you pop off the battery lead and connect the meter in series with the cable and the terminal and measure it again. Your posted measured current drain cannot be sending the battery flat. However your trunk is open. At the rear of the trunk lid is a light. Ensure this light is not on when you shut your trunk lid. From the current drain you mentioned you have not measured the whole circuit unless you have disconnected the trunk light. Another favourite is open your glove box and check nothing is fouling the glove box switch. I reckon one of your lights is on all the time. Trunk, glove box, engine.
Ciao,
Adrian
911C4

PS: you cannot turn the trunk or engine lid lights off, when opened unless you use the alarm switch or physically disconnect it. If you are not sure, all you have to do is remember where the light is and put an ice cube over the spot. or even water. The dry spot shows the light is on when it shouldn´t be.

jeff91C2T

Welcome to the club

I've been having the same problem but have yet to find it. Sometimes the car can sit for days...no problem. Other times it will kill the battery overnight. I've diconnected all the interior light but have yet to track down the problem. Clock and alarm have been checked. In my case, I think it's a relay thats getting stuck. But I can never seem to find it.

swellC2

I just had the same problem that Adrian was suggesting. Recently my battery was draining and I found that the trunk light was staying on. My quick fix has been to disconnect the bulb. I think the culprit is the that the push button switch stopped working. Anyone know if this is the first thing I should replace?

Thanks,
David

NW911C4

Adrian et al,

Thanks for the suggestions. I will try connecting my DMM again this weekend. Seems to remember that when I first connect the battery with the DMM in serial (10A mode), the current drain is 110mA for approximately 15 seconds, then it drops to 25mA. Additionally, when I measured the current, the hood light was turned off (I held in the switch). I have a Sears Die Hard International, its rated 700CCA, but I do not know the Amp-hours rating. I will let you know if I find anything....

Regards,
LL
'91 911 Carrera 4

Jacks911

To All with drained Batteries,

Two weeks seems the limit without recharge in my '91 C2. Everything seems to be working as it should and I paid one good mechanic to check it out ....So ....I jusy got the Porsche Battery maintainer and it works Great! At $50 it's less trouble than recharging or worrying about Battery condition. I would prefer to drive the car, but when I don't, I'll just plug it in.

Adrian

For those with lights problems. The switch is normally the cause. They are very mickey mouse things. It is quite possible that the switched earth wire has fallen off and is earthing out causing the light to come on.
110ma suggests a light on.
Apart from the items I have already mentioned and for those whose battery drain is somewhat intermittant, check out your carpet, sound proofing and under the passengers seat (LHD) for traces or evidence of water. The pax door in particular has a nasty habit of leaking water. Under the pax seat, right at the front is the radio booster, fuse 38. If this is wet it will drain your battery. I also recommend you check for inline fuses under the floor/carpet. When these are immersed in water they drain your battery.
The rear blower resistor if this shorts out will cause the rear blower to crank up and run. This will also flatten your battery.
Unfortunately it is a case of checking each circuit.
Mind you the most common cause of flat batteries is the switched lights.
Ciao,
Adrian
911C4

PS: I do not recommend using battery chargers to overcome current drains. Your battery should easily last four weeks of storage without any outside assistance.

Herbie

With standard alarm system, there is a simple check to do: after keylocking the door, and waiting 10 seconds, the LEDs within the secure ***** flash with a single blink if all controlled switch are OK (doors, trunk, glove box...) or with a double blink if something is still on, like a failed switch or a leaved open trunk.

Adrian

Dear Herbie,
This is I am afraid not accurate. This is what the book says, but not the way it is hooked up. There are two wires on the loom for each switch, not one. One wire is for the alarm the other wire is for the lamp. The earth is provided by the switch mount. It is very common for the lamp earth wire to fall out, they are both crimped into the end of the switch. Very mickey mouse as I said. If the lamp earth falls out, this has no effect on the alarm.
I can also state from personal experience and knowing how the system works that the monitoring cicuit and the flashing leds does not work as advertised. My trunk switch was missing in action. Completely disappeared yet the alarm never registered a fault and it never worked. Yet the Leds flashed correctly all the time. The PO had disconnected the trunk lamp to cover up this fault. Cost me a pair of damaged nuckles and 3 UK Pounds to fix the switch. Crimping the wires to the (after finding them) switch was the worst part.
You cannot rely on these statements in the owners manual I am afraid. The owners manuals are written by engineering on how the auto is supposed to work. Whilst a great reference book, you should have one in the auto all the time, I highly recommend that you treat system operations descriptions with some caution.
Ciao,
Adrian
911C4

Russ359

What about installing a battery cut off or disconnect switch on the neg battery lead? My 92 also drains after about a week. I was thinking of installing such a switch. Has anyone done this? Recommendations or concerns?

thanks

Russ