I was woken this morning at 4.am by the horns on my car blaring. The security system switch under the centre console was in the OFF position. I don’t use the remote fob to lock the car because the casing broke years ago and I just lock the car using the key, this allows the red LED on the dash top to flash continously while the car is left unattended BUT without the alarm being armed. I’ve used it like this for about the past five years without any problems. The only way I could get the horns to stop was to disconnect the battery. Today I removed the 'Intrusion control box No DBC 11688 from under the passenger seat and checked it inside and also the DBC 1000 box near the front R/H side that says 'Horn, screen wash etc on it, both look dry and clean inside. As soon as I touch the Battery earth cable to the battery post the horns will sound continously ?? and I’m stuck as to what to try next. This problem has occurred following about a week of almost monsoon like rain, and as the car lives outside on my driveway I can’t help feeling this is somehow connected to the problem but I could be incorrectly summising that.
Has anyone had a similar problem ? I would welcome any advice in sorting this out, my car cannot be used until I find out what’s causing this. Many thanks in advance for your responses 
No, never had that happen and my sympathy goes out to you 100% - what a wake-up call!
I had that happen (alarms going off) when I opened a rear door and the intrusion sensor at the headliner triggered even though I have the alarm switch in the off position in the armrest. Most disconcerting!
I suppose you could remove the horn connection for the moment if you wanted to drive the car?
Thanks for the reply Larry,
It wasn’t just a wake up call for me, It also woke most of the neighbours so I’m not very popular at the moment I’ll try your suggestion and unplug both horns, hopefully the car will still run and allow me to drive it somewhere else for advice if I can’t get to the bottom of it.
Casso - the relay for the horns is in relay module ‘D’ behind the right headlight in the engine bay. There are IIRC 4 relays on the PCB within each module. The horns are #3. Open up relay module D and see if relay #3 is stuck closed (on) or drowned.
Hi Mike, yes I believe that relay module ‘D’ is the one marked DBC1000. I’ve already removed it and looked inside, all 4 relays look fine with none of them stuck or closed and it is nice and dry inside too. The weird thing is this occurred without any other interference to the car. It had been sat unused on the driveway for 3 days prior to this, I’m at a loss as to how the horns have come to activate themselves, especially as the alarm was unarmed, ie switched off ?
When the rear door sensor decided I was an intruder, my alarms went off also, even though the alarm was switched off so I dare say there are perhaps levels of what is understood by the system as “intrusion”?
That’s interesting, I’ve never had the alarm go off when the switch is set to ‘OFF’ not even when I’ve left the car with a window open a few inches, I just assumed ‘OFF’ meant the alarm system was disabled. Also, in the past whenever the alarm was activated the horns would sound an On & Off rhythm with about a second between each blast, so it was beep…beep…beep, but this is a continuous BEEEEEEEEP with no break as if I was holding the horn button down. I’ve had a belly full for today to be honest, sometimes I hate this car !!!
I got the same feeling this week when my heater again failed to put warm air anywhere but the foot vents and the back seat.
Casso, point of clarification: is it the security system sounder (as Jaguar calls it) or the two horns (as acivated by the steering wheel buttons) or all of these that are sounding? The wiring diagrams will be of help once we have an answer to this question.
Good point Mike!
When my alarm(s) went off it wasn’t just the “horns” I heard but a selection of war-whoops and various other extremely annoying loud noises.
Hi Mike, I wasn’t aware the security system has it’s own designated sounder ? Today I disconnected the two horns and re connected the battery, all was quiet so I’m assuming the problem is in the horn circuit rather than the security system, also the car fired up first turn of the key so again I’m ASSUMING the security system is working ok. I can’t find another horn on the car that would be used as a security sounder but I’m guessing Jaguar would have deliberately sited it in a hidden position to prevent a thief from simply disconnecting it, have I got that right ?
I’m editing this post Mike, since I first posted it half hour ago I had a neighbour knock on my door to ask me if I know my sidelights keep flashing ? I went out to look and sure enough although the horns are not sounding the side lights and rear lights are coming on. They flash a bit, stay on for a bit then turn off before repeating the cycle. So the problem is not just the horn circuit. Do you know if all XJ40’s have a seperate alarm sounder or did some just rely on the road horns ? I’ve disconnected the battery again until I can investigate further.
Casso - Apparently there is the alarm sounder which is different from the horns …
A big thanks Larry, so seeing as the car is quiet with the normal ‘road horns’ disconnected looks like I can assume the problem is not the security system but a fault in the horn system. Hopefully that knowledge will make tracking it down a lot easier, but not today, it’s raining like a car wash outside and I’m not in the mood to start probing it today
Oh No, spoke too soon Larry read my edit to Mike Stone’s reply. Arrgh I hate this car.
It’s just an idea but you could try connecting a meter to the disconnected horn circuit and keep an eye on the reading while you pull each of the relay modules etc …
Casso - according to Jaguar publication S65/93, Electrical Guide XJ6 Sedan Range 1993, the security system ‘sounder’ is located in the engine bay on the left wheel arch. I am embarrassed to share this picture showing my dirty engine bay, but here goes:
It is, as you can see, near behind the left headlight.
Because it is the horns (not the security sounder) that is faulting a look at the wiring schematic is in order:
As you can see, the horns themselves are grounded. Voltage fed to them via the yellow wire will cause them to sound. That voltage must pass through (be fed from) the horn relay in module D as previously discussed. For the relay to be activated in order to feed said voltage the relay must be provided a ground via the green/blue (GU) wire. This can happen in several ways: (1) via the steering wheel horn buttons (switches in the diagram); (2) via the security system ECU by providing a ground via the green/blue (GU) wire that is found in connector LB23B, a 16-wire blue connector found on the left blower motor housing bracket; or (3) via a wire grounding out unintentionally due to broken/faulty insulation for instance. So hopefully checking each of these possibilities will yield an answer.
Perhaps the first thing to check would be the horn buttons/switches. Here’s a closer look at that piece of the wiring diagram:
The green/blue (GU) wire that is connected to pin 9 of the aforementioned connector LB23B runs to the steering column/steering wheel and, when either horn button is depressed, provide a ground for the horn relay. Disconnecting LB23B and checking the GU wire for continuity to ground should show NO continuity unless/until one of the horn buttons is depressed. If you have continuity to ground without a horn button depressed you have found the faulty bit of the circuit.
If the fault is not there, the next thing to check would be for continuity to ground through the security system ECU. Here is the relevant part of the wiring diagram for this bit:
Pin 2 in connector LB23B with a blue/green wire is for the ‘Sound Horns’ signal from the ECU. According to pub S65/93 the Inputs and Outputs in the security system are:
As is noted the ECU will cause the horns to sound (make them Active) by grounding the wire - the aforementioned green/blue wire found at pin 2 in LB23B. So again, checking this wire for continuity to ground should reveal whether this is the problem. If the security system is not activated there should be no continuity to ground.
If the fault is not with the horn buttons or wiring thereto, or with the security system ECU or wiring thereto, the next suspect would be an inadvertently grounded wire (the green/blue wire found at pin 10 in LB23B). There should be no continuity to ground.
This is a lot, and hopefully some of it at least is accurate. 
If there are any of you out there that know better than I about this type of thing please chime in and set the record straight. But this is my novice take on it. Hopefully it is of help.
Mike I am greatly indebted to you for taking the time and trouble in making such a detailed post 
If the rain holds off tomorrow I shall follow the procedure you have outlined for checking the various circuits starting with the horn buttons.
I’m not in my comfort zone when dealing with the electrics on this car, to be honest I can’t even understand the diagrams in the Heynes manual BUT your post and explanation seems clear, concise and straightforward and gives me a bit of confidence I’ll know what I’m doing
Thanks again Mike.
You are welcome, Casso. My advice is free and worth every farthing you pay for it! Keep us posted as to how you make out with this.
So I’ve been out there all day today, removed the glove box and bolster and then unplugged every plug from every control box that I could see and cleaned and inspected every one, including opening them up and looking inside, then I used a hair dryer to make certain everything was bone dry. Next I followed the diagnostic route you outlined starting with the horn circuit, there is no continuity to ground in the GU wire until either horn button is pressed. There is also no continuity to ground from the wire at pin 10 in LB23B.
I reconnected the battery and started the car and let it idle for around half an hour then switched off and locked all the doors and boot and sat staring at the car for an hour while I had a cup of tea. There was no flashing of sidelights and no rear lights came on so it’s looking better. Then I went for a short drive and locked the car again when I returned and looked out every few minutes and no side lights flashing or rear lights coming on. I have NOT reconnected the horns yet, I want to do that final test AWAY from the house, just in case the neighbours kick off if they start sounding again, so I’ll try that tomorrow, it’s too dark now. Something else I noticed which may be related - when I returned after the short run ( under a quarter mile) the engine cooling fans seemed to run for much longer than usual, maybe four to five minutes ? Is it possible the relay for the fans is sticking ? And if so could that trigger the security system to sound the horns ? I couldn’t identify which relay controls the engine fans, the Haynes manual shows it to be at the front R/H side somewhere but I couldn’t find it. Do you know if that relay is inside a control box with some others or is it one of the single square type ?
To test the horns without driving the neighbors nuts you might try stuffing a small rag into the horn openings to mute them.
Good Idea, I never thought of that !