Parasitic Draw

[76/85 Truck]

Hello all! My new battery keeps dying after about 3 days in my 85 k20. I've tested the draw, and I get about 0.180A. I started pulling fuses and found the Horn/dome fuse to be my draw. My horn works normally, and the dome light is completely disconnected.

I'm fairly good at electrical but this one is really getting me. Does anything else run off that fuse? I have an aftermarket radio and tach but those don't seem to be the issue.

Appreciate any help!

[DBLSPRT68]

You said your dome light is disconnected? Is it disconnected at the light bulb or the door switches? The action of the dome light starts at the door switches or light switch, and ends at the bulb. Try that, then go to the horn? Pull that fuse for a few days and see if it still kills the battery. Then you know where to go from there.

[homemade87]

I think what I would do is unplug the relay for the horn and see if the draw stops . If not look to the dome circuit . I think those 2 things only share power source . This may give you a direction to go .

[76/85 Truck]

Door switches are disconnected, as well as the harness connector behind the seat for the dome light. I unplugged the horn relay also, no draw change.

[hatzie]

There are three rows of accessory plug-in connections across the top of the 1985 fuse block where various optional sub-harnesses were plugged in by GM--(Factory/Dealership) or previous owners. The GM systems will have obsolete keyed connectors to keep you from plugging the harness into the wrong slot.

According to the 1985 wiring diagrams the DOME/Courtesy-Floor-Lamp is the ORANGE wire in the Lower Left accessory plug-in position on the 1985 fuse panel. Un-plug it and see if your parasitic draw disappears.

Circuit 140 also powers the HAZARD lamps...

[76/85 Truck]

This trucks wiring has been messed with a lot before I got a hold of it. I followed the dome lamp wiring up to the fuse block and I see a connector with an orange wire plugged in to the upper left. Disconnected it, no change in draw.

[hatzie]

That likely rules out your dome lamp wiring.
The Horn relay makes no difference too.

Since the Hazards are on Circuit 140 too I'd pull the flasher relays and see what difference that makes... I've read about some electronic flasher relays causing a parasitic draw.

[Dead Parrot]

A lot of aftermarket radios have a memory wire that the install manual will say to connect to an always hot source.

Does your truck have one of those under hood service lights? Don't remember when they started putting those in, just remember seeing some at the pull-a-part and getting one.

Any chance a PO installed some kind of LowJac device? Would likely be hidden and connected to an always on source.

[andyh1956]

Unplug your alternator & try that. If it has been sitting a while it can get that white corrosive powder in it that will conduct when damp.

[76/85 Truck]

Thanks for all your help! Found the source of my draw. My aftermarket tach was plugged in to one of the "B" terminals on the fuse block. Dug up the diagram for the rear of the fuse block (note the image is reversed because you're viewing the fuse block from behind) and you can see where the B terminals are on the same circuit as the Horn Dome fuse.

I have .001A draw now. Perfectly within reason.

Moved the tach battery positive lead to a switched ignition terminal on the fuse block. Hope these images can help someone else too.

Thanks again!

[andyh1956]

OK then it was "On" all the time.