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Can you pull fuses and relays to result in zero battery drain?

 
Old 02-07-2019, 12:32 PM
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TheOtherEric
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Default Can you pull fuses and relays to result in zero battery drain?

I'm guessing No.

I was measuring to see if I had a battery drain ( I did not -- Yay!) and discovered I couldn't pull any combination of fuses and relays to get anywhere near zero current draw. Seems weird, so let's ask the experts. TSB 9402 says we should have 16mA draw with no aftermarket stuff connected, consisting of:
1) 10 mA window relay, fuse 4.
2) 3 mA clock, alarm diodes, fuse 12.
3) 1.7 mA Sunroof or cab top, fuse 13.
4) 1.4 mA DME, fuse 35 (but for 993, the DME is with the clock, fuse 12).

Yet I can't manage to kill that "window relay" or sunroof current draw by pulling fuses/relays in the fuse box. Are there fuses/relays elsewhere to solve the riddle? I pulled every single fuse & relay... still seeing over 15 mA.

Granted this question is largely academic; I'm just trying to understand our wiring better.
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Old 02-07-2019, 02:37 PM
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ToreB
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You probably have a faulty door switch, keeping the sunroof/windows active.
Normal current draw is about 25mA. Here's a guide on how to measure the battery current.
Cheers,
Tore
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Old 02-07-2019, 02:38 PM
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mike cap
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I remember checking my car's current draw once with the car powered down, hoods closed, doors closed but alarm set and the current draw was 50 mA 'ish. 16 mA seems low but maybe it's with out the alarm set?
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Old 02-07-2019, 02:59 PM
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TheOtherEric
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Originally Posted by ToreB View Post
You probably have a faulty door switch, keeping the sunroof/windows active.
Normal current draw is about 25mA. Here's a guide on how to measure the battery current.
Cheers,
Tore
A faulty door switch would add 100+ mA, right? I'm getting a current draw of only 20 mA, so I don't think there are faults. (when I reinstall the fuse to my aftermarket alarm / keyless entry, it adds another 28 mA but that's beside the point).

As an academic exercise (curiosity), I just wanted to re-create the TSB's claimed current draws, but I could only get the 20 mA down to 18, with the difference all due to Fuse #12 (clock, DME). Nothing else made a dent, which seems weird.
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Old 02-07-2019, 04:23 PM
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pp000830
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I have a funny story about parasitic electrical losses. When I first purchased my 96' 993 it made no warning buzzer sound when one left the lights on or the door ajar or keys left in the ignition and I just inferred it didn't come with one. After leaving the lights on running the battery down a couple of times I purchased an aftermarket electronic solution that has a piezo buzzer, worked great but slightly increases parasitic load as it is active when the ignition is off. Years later I noticed on the factory wiring diagram that there are components to provide this warning and they were simply not working in my car. The aftermarket solution seemed to work fine so I left it. That was in 2006 and it is still working, go figure?
Andy
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Old 02-08-2019, 05:30 AM
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IXLR8
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Originally Posted by TheOtherEric View Post
I'm just trying to understand our wiring better.
Best done when using wiring schematics. Then you'll know for sure.

BTW, not on our 993s but on newer cars with CAN BUS, you measure the drop across the fuse without taking it out. That is why you see the tabs exposed at the end of the fuse. You need a good meter that reads in the mV.

You use a Current Consumption Matrix chart to determine the drains. VW has a Technical Bulletin out if you Google it.


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Old 02-10-2019, 01:10 PM
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Actually, ALL blade fuses can be probed as described in the tech bulletin (attached). While there's nothing technically wrong with inferring current through a fuse this way, it's not particularly accurate as fuse resistance isn't tightly-controlled during manufacture, nor is it temperature stable. Accuracy will get worse when troubleshooting leakage-level currents, where voltage drops can be a fraction of a millivolt, as the chart in the attached shows. The real value of this technique is it's non-invasive because pulling a fuse to insert a meter can temporarily change the very current being measured. It's also good for relative measurements (did the current change when I did x?) or telling the difference between 1/2 amp and 5 amps. But use caution at low currents.
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