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Discussion Starter · #1 ·
My neighbor's 2006 Pilot battery dies overnight if the horn relay is left in place. When it's removed the battery holds its charge fine for any number of days. The alternator tests good and the battery is new and there are zero other electrical problems. Does anyone have any experience with battery relay faults like this one?

I thought the horn would sound if the relay closed, and that the power drain was due to a shorted (closed) relay. But no horn is sounding even though current is being drawn.

I'm thinking there are multiple problems, e.g. wire to horn relay is always hot, there's a relay problem that drains current without energizing the horn that I don't understand, or that the horn is defective so it won't produce sound when energized... along with one of the first two faults.

Any suggestions about troubleshooting this beyond these would be very helpful.

TIA,
Mike
 

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A few things to try:
  • Swap the relay for an equivalent known good one.
  • Check all your wiring. Use a multimeter.
  • Try powering the horn directly with wires that bypass the relay.
 

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Kind like plplplpl is saying, I wonder if the horn itself does not work. If so, maybe an alarm is going off all the time but you can’t hear the horn.

Does it have an aftermarket alam.

I would probably do a parasitic Voltage draw test to help narrow down the problem. Disconnect the horn and see if the voltage draw goes away. But, I would also thing that if the horn were drawing power all the time it would be hot to the touch.
 

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If you pull the relay you can perform a test to determine if it is stuck closed, if you have a multimeter. It still would not explain why the horn isn’t sounding if the relay is stuck closed though. That would be a separate investigation.
 
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Discussion Starter · #6 ·
thanks Huntertn and road2cycle, I'll look for an aftermarket alarm and test the horn. I tried using my meter as an ammeter in series with the + battery cable and I saw really low current being drawn, I think it was less than 10 mA, which was confusing because that should be enough to kill a battery over night, so the current draw that is killing the battery has to be intermittent, making it more tricky tof ind.
 

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10 milliamps is not enough to cause a problem. Did you let the car sit for about 15-30 minutes and see if the amp draw changed. Usually the amp draw will drop after the car goes to sleep when the systems shut down. This does not happen right away. But I have never heard of one increasing over time. But it is worth checking.

Once I had an Accord that would continue to try and lock itself for hours after I got out of it. I only figured it out one day because I walked up to the car and it was trying to lock the drivers door.
 

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Discussion Starter · #8 ·
huntertn, good catch with the lock. I wish I could rig a tester that showed current drain over time, I'm sure somebody makes them but I would have to build one maybe using an Arduino or run a VB6 windows program on my laptop and leave it running but I'd have to figure out how to build a current monitor that I could read over USB. I think I'll test the horn with 12v, and check the relay by replacing it or using a voltmeter and running 12V to the coil that closes it, or just saw it open and look inside while the car is running, test the horn 12V signal to the relay to see if it's always hot, etc. An always on 12V signal to the horn relay and a broken or unplugged horn would explain maybe. Maybe the previous owner unplugged the horn because they couldn't figure it out and then sold it. I don't know the history and the owner isn't car savvy so I have better questions for him and his wife now. Thanks!
 

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Discussion Starter · #9 · (Edited)
I found something interesting, but just for the sake of thoroughness:
Symptoms:
Horn works normally, Horn relay is new, Battery is new, Battery goes dead overnight with horn relay connected
No other electrical symptoms exist that we're aware of, No after-market alarm system that we could find.
I inlcuded a photo of the horn relay and the relay socket, with numbers photoshopped in.
The horn relay has 4 connections. 2 for the coil (1-2) and two for the contacts to handle the horn current (3-4)
  • the two connections for the coil (1-2) measure 0.08 ohms
  • the two connections for the horn current (3-4) measure open when the coil is not energized
In the horn relay socket 2 (two) of the connectors have 14V:
  • one of horn coil wire connectors (at the top right of the picture, labeled #2) has 14V! wtf?
  • one of the current carrying switch connectors (#4) has 14V (as expected)
So #2 is getting 14V (with no relay plugged in) even when the horn switch is not being pushed inside the car, so current appears to be draining through the horn relay activation coil, but not enough to close the coil and make the horn blow.

I've never seen that before. Has anyone seen how the horn switch is constructed on the steering wheel? Is it possible there is a resistive element that could pass current to the horn relay coil, enough to drain the battery but not enough to close the relay? That doesn't sound very likely.

I think the alarm is malfunctioning. Does this sound like something that can be repaired or is the alarm simply faulty? And if they can't afford to replace the alarm, does anyone know which wire on the alarm carries the horn signal? I'd like to just cap that off. Also, where is the alarm?
IMG_20200830_160939826.jpg IMG_20200830_160445427.jpg
 

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From what I have seen with lots of car electronics they are ground/negative switched. That could explain why you measured 12v at one side of the coil. I suspect pressing the horn button on the steering wheel causes the other side of the relay coil to tie to battery negative/chassis/ground.

One thing that did not make sense in the relay resistance measurement is that the coil is only 80 milliohms. I would expect the coil resistance to be much greater since only 10’s of mA should be required to energize the coil. I would have expected the coil resistance to be in the 100’s of ohms.

I’m also surprised that the relay is normally closed. That means with the relay plugged in that 12v is being fed to the horn when the relay is not being energized. Something isn’t adding up.
 
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Discussion Starter · #11 ·
From what I have seen with lots of car electronics they are ground/negative switched. That could explain why you measured 12v at one side of the coil. I suspect pressing the horn button on the steering wheel causes the other side of the relay coil to tie to battery negative/chassis/ground.

One thing that did not make sense in the relay resistance measurement is that the coil is only 80 milliohms. I would expect the coil resistance to be much greater since only 10’s of mA should be required to energize the coil. I would have expected the coil resistance to be in the 100’s of ohms.

I’m also surprised that the relay is normally closed. That means with the relay plugged in that 12v is being fed to the horn when the relay is not being energized. Something isn’t adding up.
Thanks for your reply. I didn't ohm the 0V side of the coil to ground but it sounds like I should now. I thought I measured the coil resistance correclty, I'll check it again. If I understand the relay connectors correctly to be 3-4 it is normally open, I stupidly wrote 0 ohms, I screwed that up, I'll fix it now, that should have read open. Sorry for the mistake.

.
 

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Discussion Starter · #12 ·
Well to complicate things the Honda battery died again with the horn relay disconnected, so it turns out it wasn't the horn relay causing the issue. We pulled the radio and Rear AC fuse (no clue why it says Rear) and we'll just start working our way through the fuses I guess. Sporadic current drains are difficult to diagnose. Any clues about what to look at first would be appreciated.
 

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A good number of parasitic drains reported on this forum turned out to be either aftermarket alarm systems or the radio. As you said, it can be a challenge to pin down, but maybe those are some good places to start.
 

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Leave each fuse in place. Put the voltmeter in mVdc mode. Measure voltage across each fuse with the Pilot off and key out of the ignition. The fuses with the largest voltage draw across them are the ones I would pull first.

There is a rear AC fuse for the rear blower motor.
 
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Discussion Starter · #15 ·
Thanks plplplpl and road2cycle, I'll see what happens with the radio fuse pulled (we can't find an aftermarket alarm anywhere) and then do the voltmeter check if it still has a problem. Appreciated.
 

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FOUND IT!!!!! it was the GPS, we just disconnected it and the problem was solved. Very annoying, took a lot of trial and error to find it.
Since the 2006 doesn't "natively" have a GPS, that must have been some sort of add-on, right?
 

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Discussion Starter · #20 ·
Kind like plplplpl is saying, I wonder if the horn itself does not work. If so, maybe an alarm is going off all the time but you can’t hear the horn.

Does it have an aftermarket alam.

I would probably do a parasitic Voltage draw test to help narrow down the problem. Disconnect the horn and see if the voltage draw goes away. But, I would also thing that if the horn were drawing power all the time it would be hot to the touch.
Apologies, I thought it might apply to your situation too, I wasn't aware that 06 had no gps, apologies. I'll delete the post if I can.
 
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