Car Electrical Ques: Parasitic Amp Draw

[911_Dude]

My old '94 Chevy S-10 is not a daily driver. But if it sits for more than a week the battery will be completely dead. Battery is 2-3 years old. Good volts with motor running.

I started checking for parasitic amp draw by pulling fuses and checking the amperage across fuse terminals. The biggest violator was the Horn fuse- About 150 mA. Another had 20 mA and the rest far less. I then confirmed the total draw by checking the amps pulled across the Pos battery terminal and Pos cable (cable disconnected and current running through amp meter). About 180 mA total.

Is 180 mA draw too much? It doesnt seem like it to me. Doing the math it seems like, ideally, a 600 CCA battery should last about 145 days. I just find it hard to believe the battery is bad because the truck has always done this, even when the battery was new.

[javadog]

That's about 3 times what I'd consider normal and I don't see any reason that the horn circuit should draw 150mA. Maybe a bad horn relay?

JR

[rick-l]

180 sounds very high.

Cold Cranking Amps (CCA ) and capacity are two different things.

Capacity is typically 90 Amp Hours. 90/0.18=500 hours = 21 days for it to be discharged. That for sure won't start the truck but I am not sure what capacity it takes.

[john70t]

Originally Posted by 911_Dude

Is 180 mA draw too much?

How to Find a Parasitic Battery Drain - 9 Easy Steps
"If the ammeter is reading over 25-50 milliamps, something is using too much battery power."

(assuming there are no fancy remote or radio systems)

Yeah. Check continuity of horn relay and/or try swapping it with junkyard.
Hopefully there are no frayed wires in the column.

[Arizona_928]

Too much work imho. Master battery cut off switch to the positive terminal

[dad911]

Originally Posted by AZ_porschekid

Too much work imho. Master battery cut off switch to the positive terminal

Until the 'short' gets worse and burns up the harness.

My money is on a mouse chewing the wires. Happens all the time in our older trucks, and just had to clean up some wiring in a hot tub.

Does the horn work? Anything else on the same circuit?

I think a relay draws about that much current, light bulbs a little more.

Could the horn relay be bad?

[911_Dude]

Good suggestions all. Thanks. I think John70t's amp hour estimate is more realistic than mine I dont think I did my estimate correctly. I just havnt had this issue before and wasnt sure what a good number for amp draw should be. Further research shows that the horns themselves are a common failure in these trucks. They rust out and short. Although mine seem to sound just fine. Ill check out that circuit.

As a side note, just this morning Im pretty sure the transmission pump went out on the S10 while my son was borrowing it. Sometimes maintaining 6 cars and 3 bikes is just a pain in the arse.

[1990C4S]

You need to fix it. Start by finding the horn relay. It is very unlikely that's the issue, but does the horn still work?

[Rusty Heap]

Check your grounds.......any resistance = a voltage drop.

[1990C4S]

Since you just posted that the horn works, it is really unlikely that the relay or the horn are the issue.

You may have a wiring issue...or an aftermarket device on the same circuit? Stereo, alarm, etc?

[john70t]

Originally Posted by 1990C4S

Since you just posted that the horn works, it is really unlikely that the relay or the horn are the issue.

They could both be happening, iirc.
1). Internally: The relay control circuit goes from the battery, to the fuse box(??), through the wheel switch, through the relay, back to chassis ground.
(not necessarily in that order)

2). Inside the relay are very fine windings to pull in the sprung bar, which closes the contacts for the load circuit.

All those small fine windings are a perfect place for a KEY-OFF short.
I had a fuel pump relay get too hot for restart and then randomly cut out while driving.

3). The load circuit(fat wire) just goes from battery, to fuse, to relay, to horn, back to ground.
Open/Close. Simple circuit.

I dunno. It's a easy check to eliminate from the search.
If you pull the horn relay, I wonder what the voltage numbers are in the contacts?
(Use a small spade connector and a double alligator clamp wire.)
Downstream shouldn't have any volts back-feeding into the circuit.

[1990C4S]

Yes, it could be the horn relay. It's just very unlikely given the scenario he described, there would have to be multiple failures. Impossible? No, but there are lot more likely places to look.

The relay and contacts apparently work. Like others have said, find and pull the relay.

[Arizona_928]

Originally Posted by dad911

Until the 'short' gets worse and burns up the harness.

My money is on a mouse chewing the wires. Happens all the time in our older trucks, and just had to clean up some wiring in a hot tub.

Does the horn work? Anything else on the same circuit?

I think a relay draws about that much current, light bulbs a little more.

Could the horn relay be bad?

Of course with a short, but a draw.. if it's a beater, I wouldn't put any more effort then is needed to get running. Anyways have you ohm the system?

[911_Dude]

Originally Posted by AZ_porschekid

Too much work imho. Master battery cut off switch to the positive terminal

I have certainly thought about doing that. Yes, the truck is a beater that is used only when I take on a house flip and random cargo hauls. Ill give it one more shot with trying to chase down the issue. Would not surprise me if its just a worn down wire insulation someplace. The truck has over 265k miles.

[Nostril Cheese]

Check the positive cable from the battery to the starter. I had something similar happen on an old S-10 with high mileage. Battery started going dead, lots of electrical glitches.

[1990C4S]

If the draw is consistent it is unlikely you have a worn or chafed cable...

A battery switch is fine unless your radio loses it's programmed stations, then you need to wire that separately.

[yetibone]

If it still has a Delco radio, take it out of the dash and see if it's a little warm to the touch. Even if it isn't warm, it could still be over drawing current through the memory wire.

Delco CS alternators can also cause a parasitic draw.

[Nostril Cheese]

Originally Posted by 1990C4S

If the draw is consistent it is unlikely you have a worn or chafed cable....

Like it would kill him to take a look.

I was pretty amazed with my old Blazer with the 2.8 V6. Got over 400K on it until the cam lobes wore flat, lol.

[rattlsnak]

Originally Posted by AZ_porschekid

Too much work imho. Master battery cut off switch to the positive terminal

The formula to check for parasitic draw is to take the reserve capacity of the battery and divide it by 4. This should put you inj the 20-25ma range for most batteries. Anything more than will drain your battery.

I have a 94 Z 71 truck that I had the same problem with. I only drive it once every 5-6 weeks or so and the battery was dead every time. I checked my draw and it was right at 23ma so I added a battery cut off switch as stated above and never had a problem since.

[javadog]

A battery will self-discharge, just sitting on a shelf, connected to nothing. Add a draw, any draw, and it discharges at a faster rate.

JR