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1995 MGF Mpi
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Discussion Starter · #1 ·
I'll be working on a friend's Switzerland-export 1997 F this winter, doing a number of little projects for him.

One thing he asked me to look into is the horn/alarm going off when the car is locked. He can lock the car, walk away and minutes or an hour later the horn starts blowing and the lights flash, the alarm has gone off. It seems to be sporadic, it doesn't always happen but does happen more times than not. My initial thought is that the problem is originating in the door latch, maybe one of the microswitches in the lock assembly is dirty/not making good contact. I will also check all the grounds related to the door latch and the 5AS.

Has anyone else ever experienced this type of issue? Any suggestions as to what could be causing it?
 

Rover Lifestyle
Rover 200 Rover 600
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It is known for such thing to happen, but it is not always the same culprit.

As you said first thing to look is always the door microswitches. It might end up that you will need to replace it to fix it. If he has problems with interior lamps not turning on when door closes then microswitches are most likely candidate.
However, before ordering new lock you need to look into other possible situations:
  • Does it have microwave volumetric sensor? If he does, try to disconnect it and see if something triggers it (in some cases it was faulty microwave of neighbour next door whichnstart trigering its perimeter sensor; or nearby antena for mobile phones where peoducing interferance which triggered the alarm);
  • You need to check boot and bonnet switches if they work correctly and not creating shorts of some way in csrtain circumstances (like water dripping onto them and making circuit)
  • As you said, need to inspect the earth points
 

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The horn on my car sounds randomly like this when the voltage of the battery is very low.

It鈥檚 kind of a warning, but of course this makes the condition worse lol
 
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i got fed up with mine going off for no reason and when it fancied it. I live on a housing estate, so not neighbourly behaviour- I could feel the looks! Most around here have cars they dont own or summer convertables in their garages. It only takes one in the neighbourhood to like working on old cars and it pulls the tone down...I digress.
I added a relay and wired the horn live only when the ignition was on. Yep, it overided the security but It stopped the horn going off at 3 in the morning. And the TF was less conspicuous ...
 

Registered
1995 MGF Mpi
Joined
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Discussion Starter · #5 ·
It is known for such thing to happen, but it is not always the same culprit.

As you said first thing to look is always the door microswitches. It might end up that you will need to replace it to fix it. If he has problems with interior lamps not turning on when door closes then microswitches are most likely candidate.
However, before ordering new lock you need to look into other possible situations:
  • Does it have microwave volumetric sensor? If he does, try to disconnect it and see if something triggers it (in some cases it was faulty microwave of neighbour next door whichnstart trigering its perimeter sensor; or nearby antena for mobile phones where peoducing interferance which triggered the alarm);
  • You need to check boot and bonnet switches if they work correctly and not creating shorts of some way in csrtain circumstances (like water dripping onto them and making circuit)
  • As you said, need to inspect the earth points
Yes, the car does have the microwave volumetric sensor. My friend has mentioned that when the car is parked on a street and a large truck goes by it, it can set off the horn blowing. He also mentioned the spontaneous horn blowing when parked in a busy area. Whereas, it very rarely blows when parked in they driveway. So, maybe it is related to the volumetric sensor.

Can the volumetric sensor just be disconnected and still allow the remaining bits of the alarm system to work?
 

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Can the volumetric sensor just be disconnected and still allow the remaining bits of the alarm system to work?
Yes it can. Alarm system will just not get triggered from it and that is all.
 

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I believe you can disarm the volumetric sensor by locking the car with the key rather than the remote.
Yes, that is correct. But if you disconnect it, you removed it for good.
 

In the Garage
MG TF
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Is there an old microwave oven being used close by the car? Leaky microwave ovens are known to set the alarm off. My garage is under next doors flat and the TF鈥檚 alarm stated to randomly go off and in the end it turned out to be their microwave.
 

Registered
1995 MGF Mpi
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Discussion Starter · #11 ·
I don't know the age of the microwaves in the area, I can ask my friend to knock on the doors of his neighbours and do a survey of the age of microwaves, might as well ask for the brand name as model too, just to be complete. :)
 
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