Matt sent me this as a PM, but I thought it might be useful to post it to the forum in case anyone else experiences the same problem:
The alarm has two separate warning devices. A small horn, to the left of the battery (as you look at it from the front of the car) and an ear-splitting siren to the right of the battery, under which is mounted the tilt sensor.
If you leave a zone open when you lock the car (lid, door, etc) you get a single beep from the alarm horn. If there is a system fault, you get a double-beep from the alarm horn.
If the system is set and there are no faults, the LED in the centre of the dash will rapid-flash initially, and then after a few seconds settle down to a flash every two seconds. If there is a system fault, this will be a double-flash.
A chirp from the alarm siren is unusual, but I have had the same problem before myself. The siren and tilt sensor are an add-on to the Porsche system to meet Thatcham Catagory 1 requirements for the UK market. Therefore, on the early cars like yours (and my old C2 of the same age) the diagnostics have limited reporting of siren and tilt sensor faults. (Almost non-existent really).
A very common problem with the early 996s is that due to the location of the tilt sensor, it fills with water over a period of time. You should be able to see this, as the casing is translucent. I once heard from someone who thought it was a backup battery! Anyway, I found that this was the problem on my C2. To be honest, I disconnected it and the problem stopped, and I never bothered to reconnect or replace it.
I recall someone else had the same problem with the siren chirp, and it was caused by a failure of the backup battery in the alarm siren itself.
I would suggest disconnecting them (one at a time) to see if it solves the problem. Another feature of the alarm system is the battery theft warning, and the siren will go off when the car battery is disconnected. The same might apply to the alarm siren, so I would suggest you use the same battery removal procedure for stopping the siren going off: Make sure you have the radio code if disconnecting the battery! Turn the ignition on, but don't start the car. Turn off all loads you can (aircon, radio, interior lights, etc.). Leave the key in this position while you replace the battery (or disconnect the siren/tilt sensor in your case).
There is an energy-saving load shut-off after 5 days to conserve battery power. Part of this is that it puts the alarm into sleep mode, which disables the remote. I had EXACTLY the same problem with my 1998 C2. No matter what I did, I couldn't stop the alarm going off if I left the car locked for more than 5 days. The second I turned the key in the lock it would go off. In my case it wasn't a big issue, as my car is a daily driver, so it only happened when I went away on holiday.
Some people say that you can unlock the car with the key, then lock it again with the key (which wakes up the alarm), then unlock it with the remote. This works on my 2000 996T, but it never worked on my C2. If you find a way around it you have done better than me! I learned to be ready for it, and be quick about unlocking the door and turning the ignition on in record time.
By the way - I've never seen a lock in the passenger side door. Does yours have one? If so, someone must have fitted a LHD driver's door lock to that side.
If you need to contact me again, it might be easier by email, as I can then send you photos, etc. I have a good one of the location of the siren and tilt sensor, although you should find it on the forum if you do a search. My email is richard@richard-hamilton.co.uk.
Hope this all makes sense, and good luck.
The alarm has two separate warning devices. A small horn, to the left of the battery (as you look at it from the front of the car) and an ear-splitting siren to the right of the battery, under which is mounted the tilt sensor.
If you leave a zone open when you lock the car (lid, door, etc) you get a single beep from the alarm horn. If there is a system fault, you get a double-beep from the alarm horn.
If the system is set and there are no faults, the LED in the centre of the dash will rapid-flash initially, and then after a few seconds settle down to a flash every two seconds. If there is a system fault, this will be a double-flash.
A chirp from the alarm siren is unusual, but I have had the same problem before myself. The siren and tilt sensor are an add-on to the Porsche system to meet Thatcham Catagory 1 requirements for the UK market. Therefore, on the early cars like yours (and my old C2 of the same age) the diagnostics have limited reporting of siren and tilt sensor faults. (Almost non-existent really).
A very common problem with the early 996s is that due to the location of the tilt sensor, it fills with water over a period of time. You should be able to see this, as the casing is translucent. I once heard from someone who thought it was a backup battery! Anyway, I found that this was the problem on my C2. To be honest, I disconnected it and the problem stopped, and I never bothered to reconnect or replace it.
I recall someone else had the same problem with the siren chirp, and it was caused by a failure of the backup battery in the alarm siren itself.
I would suggest disconnecting them (one at a time) to see if it solves the problem. Another feature of the alarm system is the battery theft warning, and the siren will go off when the car battery is disconnected. The same might apply to the alarm siren, so I would suggest you use the same battery removal procedure for stopping the siren going off: Make sure you have the radio code if disconnecting the battery! Turn the ignition on, but don't start the car. Turn off all loads you can (aircon, radio, interior lights, etc.). Leave the key in this position while you replace the battery (or disconnect the siren/tilt sensor in your case).
There is an energy-saving load shut-off after 5 days to conserve battery power. Part of this is that it puts the alarm into sleep mode, which disables the remote. I had EXACTLY the same problem with my 1998 C2. No matter what I did, I couldn't stop the alarm going off if I left the car locked for more than 5 days. The second I turned the key in the lock it would go off. In my case it wasn't a big issue, as my car is a daily driver, so it only happened when I went away on holiday.
Some people say that you can unlock the car with the key, then lock it again with the key (which wakes up the alarm), then unlock it with the remote. This works on my 2000 996T, but it never worked on my C2. If you find a way around it you have done better than me! I learned to be ready for it, and be quick about unlocking the door and turning the ignition on in record time.
By the way - I've never seen a lock in the passenger side door. Does yours have one? If so, someone must have fitted a LHD driver's door lock to that side.
If you need to contact me again, it might be easier by email, as I can then send you photos, etc. I have a good one of the location of the siren and tilt sensor, although you should find it on the forum if you do a search. My email is richard@richard-hamilton.co.uk.
Hope this all makes sense, and good luck.
