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Old 01-20-2022, 02:03 PM #1
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Help please: Cannot localize parasitic load by removing fuses

I’m a bit stuck. I have a parasitic load of 2.8-2.9A.

I’ve done the usual debugging - pulled every fuse, one by one, and then pulled ALL of them. (Yes, in both the engine compartment and under the dash.) Except for the PITA 140A alternator fuse.

I also removed the non-OEM cables (HID headlights for example).

There are two major cables from the battery positive. One I assume heads down to starter. The other goes into fuse box. The parasitic load is definitely on the fuse box cable.

What would be the easiest way to determine whether it really is the alternator?

It sounds like getting to the screw _under_ (WTF?) the 140A fuse is ridiculously hard.

Unhooking the alternator would work, but appears to be a bit of a pain - it is accessible but not easily accessible, and it is not obvious how to unhook it. Any advice?

Are alternators a common source of parasitic draw?

One more factoid… this started after a detail job which included an engine bay cleaning. I suspect water got into something. Anyone run into this? Any known places to drain and/or jet out with compressed air? The fuse box itself seems like an obvious place to jet out. Anywhere else you might suggest?

Any other ideas/suggestions/words of wisdom or advice?

2017 SR5 Premium, in case that matters.

Thanks!!
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Old 01-20-2022, 02:25 PM #2
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Ran into the same problem (different brand of vehicle) after removing what I thought was the source of the drain (and it was, a bad radio) the battery proceeded to continue to discharge but would stop discharging at 7-8V. Enough to run my interior lights weeks after it would no longer be enough to start.

I had to think outside the box. That same persistent, parasitic drain had caused a bad cell in my battery. It would continue accepting charge indefinitely from the “smart” charger and displaying the “charging” amperage.

Replaced battery and all was well.

Now, if you HAVEN’T identified the source of the drain I wouldn’t be so quick to jump to “bad battery” but it would be within the realm of possibility.


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Old 01-20-2022, 02:32 PM #3
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How are you measuring the parasitic load?
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Old 01-20-2022, 04:11 PM #4
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Quote:
Originally Posted by 2021nightshade4x4 View Post

Now, if you HAVEN’T identified the source of the drain I wouldn’t be so quick to jump to “bad battery” but it would be within the realm of possibility.
I don't _think_ it is the battery...

If I leave the battery hooked up, after 5-7 days, it is dead, or close to it.

If I leave the battery unhooked, when I return and hook it up, engine starts right up.

FWIW, I brought vehicle to local auto parts store, and they ran battery and alternator tests and said all good.

So if it is the battery, it is a VERY weird one.
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Old 01-20-2022, 04:18 PM #5
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Quote:
Originally Posted by cw8us View Post
I don't _think_ it is the battery...

If I leave the battery hooked up, after 5-7 days, it is dead, or close to it.

If I leave the battery unhooked, when I return and hook it up, engine starts right up.

FWIW, I brought vehicle to local auto parts store, and they ran battery and alternator tests and said all good.

So if it is the battery, it is a VERY weird one.

Ah ok thanks for the details. Mine, when left hooked up, would refuse to start 1 hour after coming off the charger. Just wanted to run that by you.


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Old 01-20-2022, 04:21 PM #6
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Quote:
Originally Posted by Jetboy View Post
How are you measuring the parasitic load?
Unhooked negative battery cable from battery. Put multimeter, set to "DC amps" and zeroed out, between battery cable end and battery terminal. Started pulling fuses.

There are a lot of wiki pages describing how to do this. One I just now found, Measuring Parasitic Draw, give a LOT more detail than others, including waiting an hour. It also includes this gem:

"The most difficult part of making a parasitic draw measurement is establishing a “shunt” connection through your DVOM prior to disconnecting the terminal cable from the negative battery post."

I did not wait an hour, or maintain connection. I think that only matters when trying to find small loads (say 100 mA). Given my large load, even after pulling all fuses, I don't see how waiting for everything to sleep would help.
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Old 01-20-2022, 05:55 PM #7
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I don't know your engine, but if it's like every other your alt has a bunch of electronics that could go bad, built in regulator, diodes that could short. Typically they're pretty easy to access, 1 plug for circuitry and a single 10 or 12mm nut to disconnect the main wire. Make sure you leave your battery neg disconnected when working on it or you might have to access that pita fuse after all...
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Old 01-24-2022, 02:04 PM #8
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Moved thread to a more appropriate place...
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