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Large Battery Drain

 
Old 10-05-2010, 09:07 PM
  #16  
Alan
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I'd suggest measuring the battery drain very directly with an ammeter in series with the battery ground strap. Ensure the interior lights are off.. see what current you have...

Its easier if you have an assistant - validate how much the parasitic current is reduced as you remove each fuse in sequence? You can also seperate the positive supplies at the battery to see if it is the ECU's, PSD etc (direct battery feeders), can also seperate the jump post connections to isolate all the CE Panel & Dash and seperate at the alternator and ABS to evaluate leakage to those.

Being methodical is the key. Starting by isolating at the Jump Post wil tell you if its even worth doing anything with the CE Panel. I'd start there, and follow with seperating all the battery feeders. This provides quick determination of where to go next.

Bear in mind that sometimes the results are not repeatable when you remove power - e.g. leakage is maintained after ignition on-off - only until power is removed (e.g. battery disconnect). So evaliuate for this case also..

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Old 10-05-2010, 09:21 PM
  #17  
Jim Chambers
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Alan, I understand "positive supplies" at the battery, but where is the "jump post"?

I assume CE is the central electrical (fuse) panel.
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Old 10-05-2010, 09:24 PM
  #18  
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Denny, yes it was something like 20 mV or less but can't remember exactly what the reading was.
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Old 10-05-2010, 10:24 PM
  #19  
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Originally Posted by depami View Post
That test could be valid IF the fuse were a high impedence component in the system. But, the fuse should idealy be "zero ohms", therefore, there would be no voltage drop on the "fuse itself". There could be resistance in the "fuse connections" or "wiring", but, that would not be detectable across the "fuse itself". So, any voltage drop would more likely be in wiring or connections and not detectable at the fuse alone.

Were your measurements taken on the "fuse", or, on the CE Panel prior to "the fuse connection"? That could make a BIG difference.

Ohm's Law says: E = IR
Hi Denny,
I was skeptical too. You can check the method by measuring fuses that are carrying a load and ones that you know are not. The voltage drop is small and varies by the size of the fuse, but it is definitely there and can be measured repeatably. It is really a yes/no result. You either have 0 milivolts, or you have 20 or 30 milivolts.
In one case you have no significant current flow, in the other you do. Most people have a digital voltmeter. Some do not have an ammeter. If you have both, you can confirm your voltage drop results by pulling the fuse and measuring the current with an ammeter in series with the battery cable. It is a real time saver and does not require an assistant to read curent at the battery while you pull fuses at the CE panel. Please feel free to use the conventional method, but voltage drop measurements can help you zero in on your problem. It only works on fused circuits where you can probe both poles of the fuse involved.
Good luck,
Dave
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Old 10-05-2010, 10:29 PM
  #20  
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Hi Denny,
BTW the measurement is taken on the fuse itself while it is in place in the fuse panel. If you look very closely at the top of a blade type fuse, you will see two tiny holes in the plastic case. You can insert a sharp pointed voltage probe into each hole and make contact with each pole of the fuse. The voltage drop measurement is taken in situ with the circuit in whatever state of function you wish to evaluate.
Good luck,
Dave
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Old 10-05-2010, 10:52 PM
  #21  
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Randy for a quick idea of the CE panel ,
get an IR gun and shoot all of the relays, a hot or warm one will give you an idea of whats still being powered after you turn off the key.
( NOTE this must be done after you shut off the car and let the relays cool a powered one will stay warm)
I had a blower relay sticking it could take out the battery in a day
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Old 10-05-2010, 11:44 PM
  #22  
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Originally Posted by Silver79 View Post
Denny, yes it was something like 20 mV or less but can't remember exactly what the reading was.
That test could be valid IF the fuse were a high impedance component in the system. But, the fuse should ideally be "zero ohms"! Therefore, there would be no voltage drop on the "fuse itself". There could be resistance in the "fuse connections" or "wiring". But, that would not be detectable across the "fuse itself". So, any voltage drop would more likely be in wiring or connections and not detectable at the fuse alone.

IMHO any fuse that has a resistance of more than a few milli ohms is not a very good fuse, but rather, a better resistor.

Where were your measurements taken? On the "fuse itself"? Or, on the CE Panel prior to "the fuse connection"? That could make a BIG difference.

Ohm's Law says: E = IR

Where:

E = Electrical Potential, measured in Volts - (consider this the pressure at your garden hose water faucet);

I = Induced Current flow, measured in Amps - (consider this the amount of water flowing through the garden hose);

R = Resistance to allow current to flow, measured in Ohms - (consider this the reluctance of the garden hose to allow flow of water - diameter, kinks, sharp bends, etc. - "resistance").

If one were to replace the fuse with a resistor of, lets say, 1 ohm (which is HUGE compared to a fuse), that would yeild a 1:1 voltage drop for any given current load. That would mean that a 1 Amp draw would show a 1 Volt drop on that resistor (unlikely if a fuse).

At a 1 Amp discharge, how long would it take a battery (AmpHourRating?) to go dead? If it were a 4 Amp (?) winndow or wiper motor, maybe so?

Sorry, don't intend to be a "DICK", just thought I would rant a bit and explain my earlier "vauge" statement.

Last edited by depami; 10-06-2010 at 12:56 AM.
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Old 10-05-2010, 11:52 PM
  #23  
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Originally Posted by Mrmerlin View Post
Randy for a quick idea of the CE panel ,
get an IR gun and shoot all of the relays, a hot or warm one will give you an idea of whats still being powered after you turn off the key.
( NOTE this must be done after you shut off the car and let the relays cool a powered one will stay warm)
I had a blower relay sticking it could take out the battery in a day
THIS is EXCELLENT advice!

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Old 10-06-2010, 12:51 AM
  #24  
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I've been in the electronics field for 30 years and have often wondered "how do they test a fuse?"!

Once they test it to it's limit it is shot, isn't it?

I think I may have just learned something tonight!

***** Thank you ALL for that! *****

Perhaps it is done by: Current flow, Voltage drop and Temperature!

Maybe the resistance does change enough to measure a voltage drop.

I have never tested this theory, I have just assumed a fuse was zero ohms.

But if it is taxed to it's limit, it must certainly heat up an gain resistance.

Perhaps they are designed to "run warm" and be at the "verge of failure" before they "actually fail".

Thank you Mrmerlin )

This 928 forum never ceases to amaze me!


Last edited by depami; 10-06-2010 at 01:17 AM.
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Old 10-06-2010, 02:07 PM
  #25  
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Originally Posted by depami View Post
...I have never tested this theory, I have just assumed a fuse was zero ohms. :banghead....
Well think some more - if a fuse were zero ohms why would it ever blow?

A fuse blowing is not a spontaneous act - it is caused by the fuse element heating up and ultimately melting - that is the "Blow" here.

It can only self heat if its dissipating power...

Why? - it can only be because it has a resistance and drops voltage across the fuse element (Power = Volts x Amps).

So there will always be some voltage dropped across a fuse that is passing current - but it will usually be quite small.

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Old 10-06-2010, 02:10 PM
  #26  
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Originally Posted by Jim Chambers View Post
Alan, I understand "positive supplies" at the battery, but where is the "jump post"?

I assume CE is the central electrical (fuse) panel.
CE - yes - Jump post is the Jump Post in the engine bay - under plastic cover on LHD passenger inner fender wall - just behind Radiator.

This connects the alternator (and starter & battery) to the CE panel via a paired feeder. It just unbolts (but be careful to do this with the battery off and isolate the ends berore turning back on)

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Old 10-06-2010, 09:39 PM
  #27  
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the IR gun test will work , most of the relays will be about 65 deg and the warm one will show 85 to 120 F.
BTDT

BTW its probably a relay sticking as opposed to a circuit being fed .
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Old 10-06-2010, 09:47 PM
  #28  
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Hi Stan,
I like the IR gun idea. It probably would have spotted my sticking rear window defrost relay.
Thanks,
Dave
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Old 10-06-2010, 09:50 PM
  #29  
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BTW Stan,
Once you try the voltage drop at the fuse method, you will kick yourself for all the work you could have saved. In case the relay temps do not tell the tale.
Good luck,
Dave
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Old 10-06-2010, 10:00 PM
  #30  
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I guess I am a bit dense, but don't understand "voltage drop" at the fuse. Drops from what to what? I have never done the test, but I would think that a fuse with no voltage would read "0" volts and one which was carrying power would show some voltage. Help me out please.

When my car sits for about 10 days I have a dead battery. I like the idea of checking relay temp. What will an IR meter cost?
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