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1984 911

I cant seem to find out what is draining my battery. I can jump it and it will crank right up. I have tried to read through the manual, made sure all switches were off and just can't seem to find out what is draining it. I replaced the battery about a year and a half ago and would hate to think that might be it but I am just not sure. Can anybody help? Thanks for your time.

Old 11-03-2010, 08:08 AM
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If you don't own a multimeter, get one.
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Old 11-03-2010, 08:18 AM
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You need to determine either if your battery is bad, or something is draining it. How long does your battery take to go from fully charged to discharged? Is it being charged by your alternator? First make sure it is fully charged (use a battery charger if needed), then disconnect it, leave it alone overnight, reconnect the next morning and see if it can crank.
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Old 11-03-2010, 08:21 AM
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Do you have power windows and does your dome lights turn on when you open your door?

A bad door switch (that doesn't turn on the lights) can cause this problem -- when the car's turned off (and key's taken out of the ignition), the power window circuit remains active (to let you roll up your windows) until you open your door. If the door switch is dead, the power window circuit will remain active and will slowly drain the battery.
Old 11-03-2010, 08:42 AM
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Have the battery load tested to see what it's cranking power is.

W
Old 11-03-2010, 08:56 AM
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Tell us if your baby is a daily or a weekend warrior?

Our cars drain the battery 3-6% daily if they sit, so if a weekend warrior you need to disconnect the battery when your baby is sitting and waiting to be riden.

Autozone will test the battery for free, in the car.
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Old 11-03-2010, 09:04 AM
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Quote:
Originally Posted by DRACO A5OG View Post
Tell us if your baby is a daily or a weekend warrior?

Our cars drain the battery 3-6% daily if they sit, so if a weekend warrior you need to disconnect the battery when your baby is sitting and waiting to be riden.

Autozone will test the battery for free, in the car.
3-6% daily??? I can leave my car for weeks without a problem and that is with an Odyssey 925 dry cell.. and my clock stays accurate too!
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Old 11-03-2010, 09:50 AM
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Quote:
Originally Posted by GaryR View Post
3-6% daily??? I can leave my car for weeks without a problem and that is with an Odyssey 925 dry cell.. and my clock stays accurate too!
You are lucky, but have you used a multimeter to really know how much or if any drop in charge after "weeks".
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Old 11-03-2010, 09:52 AM
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look at your radio/wiring. Most newer radio's consume power even when they're off, also check glove box light. it's easy to have it on without noticing it.
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Old 11-03-2010, 10:53 AM
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Battery Date Info

Quote:
Originally Posted by DRACO A5OG View Post
Tell us if your baby is a daily or a weekend warrior?

Our cars drain the battery 3-6% daily if they sit, so if a weekend warrior you need to disconnect the battery when your baby is sitting and waiting to be riden.

Lead-acid batteries stored with electrolyte slowly deteriorate in a process called sulfaton. Ideally, car batteries should be installed within one year of manufacture. The manufacturing date is printed on a sticker on the battery, usually with an alphanumeric code. The first character is a letter that specifies the month (A for January, B for February and so on); the letter "I" is usually skipped due to its potential to be mistaken for the number 1. The second character is a single digit that indicates the year of manufacturing (for example, 8 for 2008). So a battery manufacture code that says C9 would mean it was manufactured in March, 2009. I think AC Delco batteries reverse the code, so in this example an AC Delco battery manufactured in March, 2009 would have a manufacture date code of 9C.

Even though you may have purchased the battery a year and a half ago, it may have been sitting on the store shelf and/or in a warehouse up to a year or more longer than that. The 84-89 Carrera battery size (BCI 49) isn't the most popular, so it's not beyond imagination that your battery could be 2 to 3 years old -- check the date. FWIW, Interstate Batteries says it is their policy to recharge batteries in their warehouses or on their dealers' shelves every 3 months to keep them fresh -- that tells me it's not an uncommon occurrence.

When installing a new battery give it a top up charge at a low rate with an external battery charger; that will tend to maximize battery life and minimize the load on the vehicle charging system.

BTW, ~15 V DC is the voltage level where sulphation that may be present is driven from the plates back into the electrolyte solution. That's why DRACO is asking whether your car is a daily driver or weekend warrior. If it sits, sulphation happens faster and takes longer to reverse.

A battery disconnect helps preserve a charge so you have enough cranking amps to start your P-car after it's been sitting a while so good advice there. But a disconnect does nothing to stop or reverse sulphation (not to mention making you lose your radio presets and clock settings). I much prefer a trickle charger instead. Maintains cranking amps AND improves the health of your battery.

Of course you should still check for what might be draining your battery, and by how much; Aurel and ROT 911 are spot on the money w/that advice. A decent digital multimeter is less than 30 bucks and -- especially w/P-cars, in my opinion -- one of the most useful tools you'll own.

Just my $.02, but hope it helps.

D
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Old 11-03-2010, 11:03 AM
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Lil Red

Search the archives here. You are sure to find lots of discussion about this issue. Better yet, perhaps, go to the forum where the 964s hang out. Those cars often had this problem in spades, and their owners often routinely mount a battery tender to their battery, and plug it in when they put the car in the garage for the night.

The normal approach (whether before or after you decide if your battery is OK or is self-discharging at too high a rate) is to put an ammeter in series with the battery (positive or ground, doesn't matter) and see what the discharge rate actually is. A few milliamps is normal. I can see the clock tick when I use an older style needle instrument.

Then turn on the glovebox light to get an idea of what an unwanted current draw is. Most ammeter parts of multi-meters can take that current.

How does yours compare? Let's assume you think it is high (you can find posts with people's ideas of what high vs normal is). Now you want to find the culprit. Anti-theft devices seem likely suspects, as might aftermarket stereos. So you pull fuses (noting what they control) until the draw drops. Then look at what that fuse protects.

Walt
Old 11-03-2010, 11:07 AM
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It is a weekend car so maybe just disconnecting the battery after use on the weekends...could help. I do need to check the date because when I went to buy it the shop had an issue finding one so that is a good point about how long it may have been sitting before my purchase.
Old 11-03-2010, 11:27 AM
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Our 911s use the Group 44 battery.

I wonder if a diode is bad in your alternator? Put a multimeter between your ground and the terminal, then have someone click the front bonnet switch (passenger side, next to the light itself) on and off to get a sense of what draw that takes. Then hold it in, and note any draw.
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Old 11-03-2010, 11:48 AM
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Quote:
Originally Posted by Jdub View Post
Our 911s use the Group 44 battery.
Huh. I've never seen a Group 44 battery, or a reference to Group 44 in the BCI descriptions. Who makes yours? I'm intrigued . . .

I'm no battery expert; I've just been shopping for a new battery recently. FWIW Group 49 is ~14 3/8" x 6 7/8" x 7 1/2" (L x W x H); sometimes cross-application Group 93 (~14 3/8" x 6 7/8" x 6 7/8") is used instead.

D
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Old 11-03-2010, 12:34 PM
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Quote:
Originally Posted by darrin View Post
Do you have power windows and does your dome lights turn on when you open your door?

A bad door switch (that doesn't turn on the lights) can cause this problem -- when the car's turned off (and key's taken out of the ignition), the power window circuit remains active (to let you roll up your windows) until you open your door. If the door switch is dead, the power window circuit will remain active and will slowly drain the battery.
+1 this was my problem when I first got my 911
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Old 11-03-2010, 04:12 PM
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Darrin's observation is correct.

For your model year....the "standby" current drain should be 16mA....if it's more it might be the door switch or other parasitic drains. First order of business---> what is your existing current drain rate? Oops..didn't read all the responses....Walt Fricke's suggestion is spot-on.
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Last edited by Wil Ferch; 11-04-2010 at 09:49 AM..
Old 11-04-2010, 09:47 AM
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The problem child I will check her out tomorrow to figure out what is wrong. Thanks for helping you guys have really helped me out a lot!

Old 11-04-2010, 03:38 PM
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