100 Horn Sounds On its Own

Joined
Feb 20, 2009
Messages
21
Location
Redondo Beach, California
Gents - I have a 2000 UJZ100. Recently the horn began to sound as I pulled out onto the road after picking up my girlfriend. No amount of turning the ignition on and off or pushing the remote entry button on the key had any effect. I pulled the horn relay and we went on our way. The next day, I reinstalled the horn relay and all was well. Yesterday, at the carwash, on the way to pick up my girlfriend. The horn began to sound again. Again, no joy with the key. Pulled the relay and, on re-installing it today, it's back to normal. My question is this: should I get a new girlfriend?

Well no. That isn't the question. Pretty happy with this one, actually. Looking at the wiring diagram, it appears the horn relay is pulled to ground by two things: the steering wheel horn button and pin 12 on the Theft Deterrent ECU. I think it's less likely that the horn switch is intermittent than there's something going on in the anti-theft system. I'm curious about a couple of things:

1) does anyone know where in the vehicle the ECU pin 12 wire connects to the horn switch wire? The parts location diagram seems to place it behind the dash, which would make it a pain in the butt to isolate.

2) does anyone have troubleshooting notes on the Theft Deterrent ECU? If I assume for a moment that this ECU is the issue, what conditions would cause it to sound the horn, while still allowing the vehicle to start?
 
Joined
Apr 29, 2013
Messages
167
I think this may have something to do with the girlfriend, seems the vehicle only does this when it knows it's picking her up
 
Joined
Apr 16, 2006
Messages
1,288
Location
Roca Blanca
When I bought my '99, the horn fuse was missing. I replaced it. All was well. Last night (4am) the horn went off. The 100 was parked and locked. I couldn't see the fuse diagram, and had to pull the wires off the horns to quiet the neighbourhood down. Seems weird it would just go off on its own while not moving or running?

My wife was in bed with me, so maybe it was telling me to GET a girlfriend?
 
Joined
Mar 10, 2010
Messages
443
Location
San Diego
From past experience, water intrusion in the rear can cause electrical gremlins in the theft circuit. I've had several experiences where the horn just decided to blast on it's own, and not just on off on, but just continuous horn sound. Both times, it was because I had been previously driving in the rain, and then stopped overnight with the windows rolled down. I think humidity got into the rear of the car, and exacerbated whatever electrical issues are in the alarm circuit. I haven't yet pulled the rear interior apart to diagnose the issue, but since then, I've simply kept my windows always closed at night, and it hasn't happened since. As long as the interior is dry, I've yet to have a recurrence. Several forum posts have talked about getting water in the rear hatch and causing strange electrical behavior so you may want to poke around back there and see if you see any bad grounds, etc.
 

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