Thread: horn going off when lock car

I think if the horn sounds it hasn't armed the alarm

I have a theory on this but can't vouch is 100% perfect but i couldnt find other explanation, the fact is that the bonnet switch's input has a different pattern than the door switches and dedicated input in the BCU, the door switch inputs are directly recognised by the BCU cos it's about open circuit or closed to earth input:

Originally Posted by RAVE - ALARM SYSTEM AND HORN(86-4-6 DESCRIPTION AND OPERATION)

The input from the door latch switches to the BCU is either zero volts or an open circuit. Zero volts indicates the door
is open. An open circuit indicates that the door is closed. When the BCU sees an open circuit, it pulls the input high internally

now, the gist is here IMO:

Originally Posted by "RAVE - ALARM SYSTEM AND HORN(DESCRIPTION AND OPERATION 86-4-9)

When the bonnet is closed the bonnet activated alarm switch is in an open condition and the input to the BCU is more
than 6 volts, in which case the BCU pulls the input high internally. When the bonnet is open the bonnet activated alarm
switch is closed and the input to the BCU is less than 2 volts.

i couldn't find the BCU's internal electronic diagram to see what kind of voltage monitoring setup is there to identify the difference between "less than 2" and "more than 6" volts but there are cases when such circuit could have a delay untill the main component is ''triggered" cos the voltage must rise within by missing the earth path and in this case if the BCU gets a lock command before that "more than 6V" signal was recognised it will interpret it as misslock at the bonnet and give the warning

i made tests on mine cos i was curious and i found out that this delay can vary between 30 and 90 seconds also an ignition on or unlock command from the fob would reset this thing that's why then there's no misslock sound

my final onclusion is to not bother too much with it as long as it happens only in short time after the bonnet was closed

The bracket that holds the bonnet switch is easily bent. Leaning over the vehicle and reaching in to work in the engine bay can sometimes cause you to lean on the bracket , moving it slightly. Then when the bonnet is closed it triggers a miss lock on the switch sounding the horn.
By bending the bracket back to the correct position will often cure the problem of the horn going off.
Cheers,
Mario

Originally Posted by Roverlord off road spares

The bracket that holds the bonnet switch is easily bent. Leaning over the vehicle and reaching in to work in the engine bay can sometimes cause you to lean on the bracket , moving it slightly. Then when the bonnet is closed it triggers a miss lock on the switch sounding the horn.
By bending the bracket back to the correct position will often cure the problem of the horn going off.
Cheers,
Mario

Hi, what you say it's perfectly true but then IMO the misslock on the bonnet would be permannet untill the fix, in this case it goes away after a while by itself or after ignition/unlock command which sends back to my theory though

Originally Posted by Roverlord off road spares

The bracket that holds the bonnet switch is easily bent. Leaning over the vehicle and reaching in to work in the engine bay can sometimes cause you to lean on the bracket , moving it slightly. Then when the bonnet is closed it triggers a miss lock on the switch sounding the horn.
By bending the bracket back to the correct position will often cure the problem of the horn going off.
Cheers,
Mario

That Mario is because you have a V8 running on BBQ fuel and because it is never running to satisfaction as a BBQ should , Thats why you are forever under the bonnet messing about trying to fix and or bending brackets
Sorry mate could not resist mate

Originally Posted by rangieman

That Mario is because you have a V8 running on BBQ fuel and because it is never running to satisfaction as a BBQ should , Thats why you are forever under the bonnet messing about trying to fix and or bending brackets
Sorry mate could not resist mate

Says he who doesn't check under the bonnet and blows a hose on his camping trip, so the mighty V8 had to be in pristine condition by regular maintenance to come an do a road side rescue

All this talk and it's a simple cause. It's programmed that way. It's a "tamper" (for lack of a better word) thing. Unlock and open bonnet, close and lock. Horn goes off.

It WILL ALSO go off if a door is not fully closed, the bonnet switch bracket is bent or if the volume sensor is sensing while attempting to lock.

I hate to contradict but IMO it's not like that, at least it's nowhere mentioned in any official document and it doesnt happen all the time, i mean the "Unlock and open bonnet, close and lock. Horn goes off." thing , first you can't unlock and open the bonnet cos you have to open the door first to release the bonnet

1. It WILL ALSO go off if a door is not fully closed, the bonnet switch bracket is bent
2. or if the volume sensor is sensing while attempting to lock.

about these ^^^ statements:
1. Correct ...provided it's set this way cos the misslock warning can be disabled in BCU...and i've seen some cases when it was
2. that's something strange though cos IMO a volumetric sensor can't feel anything while attempting to lock as the volumetric alarm will be active only after the lock command when the alarm becomes active