I know this has been covered hundreds of times before but I'm struggling to find any information regarding my system.

Does anyone have, or know where I can get hold of, a wiring diagram for the alarm and imobiliser on a 1996 300TDI auto with the electronic diesel control (I believe the immobiliser is built into the ECU so no spider unit, can anyone confirm this?).

The story so far...

The alarm packed up last night (2 button fob type), no flashing led, no unlock or lock from the fob, no central locking when unlocked/locked with the key. initially when I got in it to leave work it started up and drove home fine.

Went out to it this morning unlocked it, popped the bonnet and started working on my winch as the motor was playing up (but thats another story...) Went to start it up a couple of hours later and nothing, wouldn't crank, just the red key symbol in the dash, which i've had before and resolved by pressing the unlock on the fob, but now as the alarm is down the signal doesn't get back to the imobiliser therefore she remained immobile, also making sure that the key fob is close enough to the reader (I have learnt that the transponder chip is in the remote, not the key in these systems). mild panic sets in at this point...

Read up on the emergency key code that can be used to clear the immobiliser if the fob is lost and tried that (managed to find the code writen in the book), tried that, no luck, i'm assuming again that the alarm needs to be working in order for this procedure to work? Really starting to panic at this point, the fear of having to cycle to work was unbareable.
In a last ditch attempt to make something happen I disconnected the battery for 10 mins, layed the fob on the dash near the key hole hoping it might notice it and clear the immobiliser and reconnected the battery, still just the red key flashing at me. I'm not 100% what I did next but a combination of taking the battery out and back in the fob, pushing the unlock button and a few other things I then managed to get it to start?? I have currently taped the fob to the dash in a hope that it will remain un-imobilised until I leave work at 10.

This may or may not be related but last week the hazard lights and horn packed up, despite there being 12v in all the right places, I am assuming a joint somewhere has water damage therefore high resistance, I have not got round to searching for the joint to repair it yet, is the alarm on the same circuit maybe?

Any help greatfully recieved, heres hoping I'm not walking home at 10!

Okay, firstly the immobilising is carried out by the security ECU with an EDC engine, the ECU also looks after the central locking along with the alarm, if nothing is working surgests the ECU has lost its 12v supply therefore the engine is immobilised by default.
If either the vehicles or the fobs battery is disconnected the system requires the system to be synchronised by pressing the lock button on the fob a least 4 time, that info is in the handbook.
So leaving the fob on the dash won't leave a permantly remobilise engine as your system is 'passive' therefore will immobilise on its own, you may have a faulty fob, battery, battery connections or pcb could be faulty, assuming you now have a 12v supply to the ECU.

Many thanks for the quick response, I forgot to mention that I have tried the 4 presses on the remote, which appeared to do nothing. I'm thinking that the security ECU has lost its 12v, I have a hunch it is fed from the same supply as the horn and hazards (fuse 4 under bonnet, 30A). Is this ECU in the passengerside foot well, behind/above glovebox?

As for the passive immobilising, looks like I could be walking home later

Done a bit of digging and found out that it is the 10AS alarm/imobiliser fitted and managed to find a pin-out for the connectors, at least now I can make a start testing what I have where. I will report back how I get on

Although rare, it's not unknown for the ECU to fail.

Hoping for a power supply failure, I'd have a look now but as feared it didn't want to start so it's sat in the works car park and I had to walk the mile and a bit home (I know, I should walk all the time, but I'm laaaaazy!)

Recovery arranged for tomorrow evening, so will do some investigation tomorrow night hopefully, will keep this thread updated in the hope it helps someone in the future

Well, after an evening and a morning of head scratching and going round the houses, I finally found the issue, the main power feed to the alarm module had failed due to a corroded connection behind the passenger footwell kick palnel. So all up and running again.

As mentioned, I went around the houses rather, I was convinced that the issue was linked with the hazard lights and horn not working, and once I'd got at the alarm, I was pleased to find a wire of the same colours in the loom, bingo I thought, so I cut the wire and ran a temporary jumper direct off the battery to test it out, however, still no life from the alarm, no central locking, still imobilised etc. At this point I disregarded the pin out I'd found online for the alarm module as the wire i'd just connected didn't tie up with it so I assumed the diagram was for a different type of unit.

With this having not worked I convinced myself that there must be another feed on the same line somewhere, so I set about opening the main loom in the driverside footwell and tracing the wire from where it comes though the firewall to find a joint in it. Having opened most of the looms under the dash I eventually found the joint about 4 inchs to the left of the alarm unit in the passenger side footwell! As I removed the tape around the connection all of the wires just fell out of it, totally corroded. Pleased I was getting somewhere I cut all the wires back and re-joined them. And yet, still no alarm... however the hazards and horn now work.

Getting rather dis-heartened at this point, I starting looking along the lines of converting it to a mechanical pump so I could work around the imobiliser or having the ECU reprogrammed to remove it.

Eventually I went back out to the truck for one last look, I studied the pin-out for the alarm again and realised that it was correct, I just hadn't been looking at the right pins (looking at the plug from the wrong side) and discovered that although the feed I had repaired did certainly go to the alarm, all it was powering was the hazard light flash when the system is armed, disarmed etc. I then found the main 12v into the alarm on the diagram, located it on the plug, put a meter on it and nothing. 0v. I traced this back to another cable joint right next to the first one I'd repaired, and, like the first one, it fell to bits.

I repaired this joint, and bingo, everything springs back into life, truck starts first time (well, second, after I'd remember I'd unplugged the ECU when wire hunting!). Phew! massive sigh of relief.

Hopefully my rather long winded thread for what was, in the end, a very simple fix will one day help someone else out of a hole, I really did struggle to find information relating to the alarm system in the EDC Disco's.

I won't post the pinout for the 10AS unit as it is readiliy available online (once you know what your looking for!)

Well done for getting it sorted and well done for taking the time to post details of the solution to help others

thank you , I May need that one day