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jlking2's Avatar
Join Date: Mar 2006
Location: Fernandina Beach
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964 Battery Drain

Hope someone with a 964 can point me in the right direction and solution...have a '93 RS America that has developed a drain on the battery. After a full charge on the battery approximately 2 1/2 days of not driving I get a ticking noise under the passenger seat and everything is dead! After recharging battery and drive it things are fine again. After another 2 1/2 days of not being driven I get the ticking and everything is dead. I've checked what I think are the basics i.e. radio, blower switch for fan, window switches etc. all are off or function normally. Any suggestions???

Thanks Jerry K
Old 09-02-2010, 05:32 PM
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If it were me, I would put a volt meter inline between the positive pole and the battery cable. It should show the drain on the battery. Then systematically pull each relay and fuse, one at the time, watching the volt meter as you pull each relay/fuse. Eventually, you'll find the circuit with the fault.

Good luck.
Old 09-02-2010, 06:10 PM
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Wow.. where to begin? There are so many things to point out here, please bear with me.

First, make sure you have a DMM (digtal multimeter) with both VDC and VAC capability.

First, check the battery. Engine off, set DMM to VDC and check voltage between the terminals. You're looking for 12.6-13.2 VDC. If less/more, your battery may have issues. The 6 cells in a 12 volt car battery each produce 2.1-2.2 VDC. Less, and you may have damaged or sulphated plates in the battery. More, and there could be an internal short in the battery. But, first, try charging it on a low amperage (1.5 A Max.) maintainer, then check again for the values I stated.

Now, start engine and DMM set to VDC, again measure current between battery terminals - this is your alternator voltage. The value for a Bosch alt. s/b 13.5-14.5 VDC. If not, bad alt. or diode pac (aka voltage regulator).

Next, set DMM to VAC, engine running, again measure current between the battery terminals. If you get any value other than zero, you have one or more sets of diodes going bad and allowing reverse flow which will drain the battery.

Next, set DME to mA and disconnect one of the battery terminals (engine/ign off - key out). Attach one DMM lead to the battery cable and the other to the naked battery post. IIRC, for a completely stock 964, you should have a key off current draw of no more than 20-30 mA from radio pre-sets, alarm, etc. If more, you have an open circuit somewhere. This can be isolated by removing each fuse one-by-one and seeing if the reading changes. If so, that circuit or component is the issue. But, be sure to check all fuses such as one in the radio - 1 Boxster guy had an 850 mA key off draw but pulling all the fuses and relays didn't affect the measurement, turned out to be the pcb in the radio and he hadn't pulled it's internal fuse (need to remove it from dash for that), Becker repaired the radio and issue solved. Also, another possibility, esp. as our cars age, is a stuck relay which stays closed and allows current draw, so pull each one-by-one and note any change in the DMM value. If measurement alters, try swapping in a like relay to see if that fixes the issue. If so, replace the original relay.

Next check all the door switches, 'Frunk' light, engine light, cigarette lighter and any obvious exposed wiring for chafing, such as that going to the Frunk/engine lights, door lights, engine bay, etc.

You'll likely find the answer in one of these procedures. If not, unless you are well versed in automotive electronics, it's time to call in the pros. Good Luck!

Old 09-02-2010, 07:41 PM
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If this ticking noise occurs while you are driving, its the door switches.
Old 09-03-2010, 05:50 PM
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The drain on my 92 C2 came from the very small fan in the back of the HVAC control.

Good luck,
Hugh Lindberg
1972T....Now looking for 72-73 911 w/ 2.7 MFI....
1978 SC - 74 RS Inspired w/3.6 - sold
1992 C2 - sold
1983 SC - sold
Old 09-04-2010, 04:48 AM
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If your alternator is operating as it should, then I would suspect the battery.

Test the battery by performing a load and capacity test on a fully charged battery.
All the best,

1995 Carrera 2 - Speed Yellow - Alps Touring
Old 09-04-2010, 05:40 PM
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I am having a problem with battery drain on my 92 C2 which started with a residual drain of .47 amp. This draw was traced back to fuse 1. The ECU was pulled and a faulty transistor replaced (Thanks Tore B!). The draw was reduced to .05 amps, about normal I think with a aftermarket radio. A new battery was just installed. The alternator was pulled and tested ok but had the voltage regulator replaced anyway. The drain is still .05 amps and the battery is STILL discharging over a couple of days. How can this be? Anybody with any ideas would be greatly appreciated!
Old 09-19-2017, 09:16 AM
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