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Disco alarm/immobiliser issues.


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Hi all, I'm having a bit of bother...

My alarm has gone a bit funny maybe 5 times in the past couple of years, never usually takes more than unlocking/relocking the drivers door to reset it. I should probably mention that the fob, even with a new battery no longer operates the alarm and hasn't for about 18 months.

All I need to do is lock the drivers door, indicators flash three times and its set and immobilised. Upon return, unlock the drivers door, indicators flash once and it starts on the turn of the key.

In the past week my problems have intensified. When I lock the drivers door, the alarm goes off (Horn and indicators). I quickly unlock the door and relock - unlock to deactivate. Usually successful first time but sometimes takes a few attempts to stop it sounding.

Most of the time when I try to start it its immobilised - no cranking, full dash lights etc. I have to get out and re-do the lock unlock bit - I'm now getting good at judging whether it will work or not by the indicator flashes or absences of - no flashes means still immobed, one flash means its probably ok. Sometimes this is easy, one attempt - other times its five minutes of trying and worrying its not going to work.

I've done a bit of searching and i don't think its a spider problem - its a 1994 300Tdi with a single button fob - if I'm reading right, they didn't have spiders.

Has anyone got any ideas what could be causing this or how I can fix it?

Perhaps more importantly, how can I bypass or deactivate the alarm and immobiliser completely (leaving the central locking working) until I get round to fixing it or replacing it? I have a feeling its going to leave me stranded at just the wrong moment...

Any advice appreciated,


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I've done a bit of searching and i don't think its a spider problem - its a 1994 300Tdi with a single button fob - if I'm reading right, they didn't have spiders.


I have the same year of vehicle, you are correct, no spider.

Your problem almost certainly is caused by a faulty Alarm ECU. This ECU also operates the central locking. The common cause in vehicles of this age is the ECU getting wet due to rain water leakage and subsequent corrosion of the printed circuit board.

It is possible to take the Ecu out and have a look inside for any obvious faults, It has been known for a bit of cleaning and soldering of connections to solve problems. Bit hit and miss. It could be as simple as getting hold of a used replacement ECU and trying that.

The idea of disconnecting the immobiliser function whist leaving the central locking operating should be feasible but I don't have a wiring diagram for this set up. If some one else can help with this I'll have a look.

The alarm ECU is behind the Glove box, passenger side, and is a green box mounted on it's side.

Hope this helps a bit

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It is possible to bypass the immobiliser and keep the central locking. Stu666 has had this done to his 1994 300tdi. Unfortunatly, it was done by a local garage so I cant help any more than to say it is possible.

not much help I know.

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im not sure exactly about this situation, but my immobiliser has stopped working after the battery ran out on my 200tdi, the keyfob does not unlock the car anymore. fortunately i left it unlocked when this happened, else it might have immoblised and i would have been a bit bummed. i now lock and unlock using the key in the drivers door which should theoretically lock/unlock all the doors, exept the locking mechanism doesent work in 1 door, however you can lock it by hand, maybe a connection problem. and the passenger door lock unit is buggered so it does unlock, but when you try to open it the lock falls down a bit, locking it, annoying the passenger and making the driver reach over to hold the tab up whilst the passenger opens it.

but hey at least i cant immobilise myself out if the keyfob has lost its code.


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