Losing hope

Diodes don't shut off. It's not a switch. A diode is basically an electric check valve. It allows voltage to flow in one direction only. Example - light emitting diode, install it backwards no light. Install in correct polarity lights bright. That's a diodes basic function. If yours did fail it could have been allowing voltage to flow both directions. Creating low voltage drawdown.

GoToGuy said:

Diodes don't shut off. It's not a switch. A diode is basically an electric check valve. It allows voltage to flow in one direction only.

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Easy there, Tiger

It allows current to flow in one direction only... typically. Voltage doesn't "flow".

This image illustrates the relationship of a typical, functioning diode... provided the diode hasn't failed. Your "check valve" remark jibes with a diode's allowance for current flow.

"Failure" may mean it's shorted and conducts in both directions.

"Failure" may mean it's open-circuited and conducts in neither direction.

I always had good luck with the old tail light bulb method. Get a taillight or parking light WITH 6" or so of the wiring off an old junk car. You can also buy a bulb and socket with pigtail. Unhook the positive battery cable, put one side of the bulb pigtail to the cable, the other side to the battery positive. If something is shorted it will light the bulb. Pull one fuse at a time until the bulb goes out. When it goes out you've found the problem circuit. Easy peasy.

Testing the fuse box is the one place where an incandescent bulb test light is really the superior performer. I like multimeters, but not in this case. I don't care what the current is, I care if it's enough to drain the battery. I don't like LED test lights because just about any current will light them up.

Get a quilting t-pin and put it in the ground clamp. Take a picture of the fuse box with your phone so you know what fuse goes where. Yank all the fuses at once and you can check every circuit for parasitic draw in less than a minute. Then refer to your picture and put all the fuses back in.

Parts store test lights stink. The Lisle is good.
www.amazon.com/Lisle-28400-Heavy-Circuit-Voltage/dp/B0002BC06E

If you insist on a multimeter, don't spend $20 on the cheapest multimeter you can find. Spend $45 and get a true Fluke meter.
www.amazon.com/Fluke-101-Multimeter-Resistance-Capacitance/dp/B00HE6MIJY

Remodelmaster said:

I has a similar issue with my Suburban a few years ago. My battery would drain overnight and have to jump start it the next morning. The short story is, it turned out to be the vanity light on my sun visor was staying on draining the battery. I disconnected it and haven't had an issue since. Your issue could be something that simple.

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Had the same thing, took me too long to figure that one out

PWC Repair said:

I always had good luck with the old tail light bulb method. Get a taillight or parking light WITH 6" or so of the wiring off an old junk car. You can also buy a bulb and socket with pigtail. Unhook the positive battery cable, put one side of the bulb pigtail to the cable, the other side to the battery positive. If something is shorted it will light the bulb. Pull one fuse at a time until the bulb goes out. When it goes out you've found the problem circuit. Easy peasy.

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A variety is handy... taillight bulb, headlight bulb, interior bulb (like a 193). Use the proper bulb for the suspected load, or try them each.

If he's searching for a low-amperage parasitic load, he'll likely need a DVM.

Remodelmaster said:

I has a similar issue with my Suburban a few years ago. My battery would drain overnight and have to jump start it the next morning. The short story is, it turned out to be the vanity light on my sun visor was staying on draining the battery. I disconnected it and haven't had an issue since. Your issue could be something that simple.

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Yup this is a common issue with these trucks that have the deluxe sunvisors. Happened to me on my Burb, and more recently on the crew cab. And good, nay, even usable visors are like hen's teeth in the yards in Houston area.....got a couple of them at the yard, and the mirror cover on one broke on the way home!

Been having same problem with my 91 for months, finally got a knife switch shutoff designed for marine use, takes 30 seconds to pop hood and turn off - no more getting stuck....
Still need to track it down but at least getting us through til warmer weather to tinker.

My slow draw, several days to pull the battery down, was the glovebox door. Sometimes it shut tight enough to shut off and then ease out and turn on. The doors become warped over time and cause the switch to ease on sometime. My solution was to pull the bulb

HHoldred88 said:

My slow draw, several days to pull the battery down, was the glovebox door. Sometimes it shut tight enough to shut off and then ease out and turn on. The doors become warped over time and cause the switch to ease on sometime. My solution was to pull the bulb

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Yes this happens a lot on the '95-up interior, because of the way the glovebox is made. Had it happen on my Burb, because the dashboard is in multiple pieces and the glovebox door is damaged too. So you have to be careful opening and closing it, to be sure that it's properly closed. That bulb being on, is like a dome light, and will pull a battery down in a few days.