How to unset alarm/immobiliser??

[Bolide]

1995 W124 E300 Diesel estate. Ordinary locking (not RCL). The car has the Mercedes security system fitted that has an alarm & immobiliser. I set mine off accidentally and now can't unset the immobiliser

There's a blank switch on the dash with a red warning light that flashes now when the ignition is turned on. The engine won't turn over, central locking doesn't function but everything else seems OK

There's no apparent alarm unit under the bonnet - I think it just sounds the horn when the alarm goes off - and there is, to my knowledge, no alarm or immobiliser deactivator key

The manual says "When the vehicle is unlocked the warning system is switched off". Not, it seems, in my case...

Any ideas? I'd quite like to go to Silverstone tomorrow!


Nick Froome
www.w124.co.uk

 

[Bolide]

Update

Quick update; Fuse C has blown and blows when replaced even with the ignition off. C feeds, amongst other things, the central locking. 16A fuse pops almost immediately

Interior lights stay on with doors closed even with interior lightswitch in "switched by doors" position

Immobiliser is still locking out the starter motor. At least that's what I assume - no ignition system, it's a diesel

Does anyone have a complete wiring diagram? The Haynes one is useless. Car has electric seats, climate aircon, towbar, premium sound system, cruise and elec steering column so the electrical system is complex


Nick Froome
www.w124.co.uk

 

[softtop]

wires to boot lid

I have a coupe and I fixed up a number of broken down wires in the plastic trunking going from the car to the boot lid. I had to cut open the trunking to locate the offending wires. Could be similar shorting going on - there must be a similar arrangement in an estate boot lid?

Also you may have a key hole to disable the alarm altogether - mine is at the back of the glove compartment - you need the special key.

 

[george shand]

Immobiliser.

Alarm control unit is normally behind the metal footplate under the carpet on front nearside,there are 3 fuses clipped together which when removed i would assume would disable the security system.

 

[Bolide]

Immobiliser immobilised

It's interesting how different this 1994 W124 is to my old 1987 one. It seems that every time MB brought out a new electrical subsystem they'd create a complete loom for it and add it to the "original" car. This goes against modern practice but makes it a lot easier to fault-find!

I found a wiring diagram on the bootleg MB US CD I bought off eBay. I've never used it as it's designed for Windows and I use a proper computer. But the diagrams are in the form of PDFs and it's possible to navigate through some of the interface on a Mac - enough to find what you're looking for

The feed to the starter motor solenoid comes from the start position of the ignition through the gear selector lockout (so you can only start in P or N) via the make contacts of the ATA (Anti Theft Alarm) relay. If the ATA is activated the relay isn't tagged, the starter feed doesn't go live and the starter won't activate

The ATA relay coil has permanent live feed and an earth is provided by the ATA Control Unit N26 under the passenger footwell next to the passenger Airbag sensor. So if N26 doesn't give an earth to the ATA relay your car doesn't go. And if you manually earth the correct pin on the correct plug going to N26 the starter will work on the key

In my case, with N26 plugged in, fuse C would blow almost immediately. With it unplugged C would survive and with a manual earth on the ATA relay the car would start

I haven't located the fault with the ATA yest but on the CD there's a checklist of tests to verify the sensors are working OK. I'll work through this checklist tomorrow and then replace N26 if everything else is OK

From this it seems it'd be quite easy to put your own ATA bypass switch in (bypass by earthing the relay or by bridging the contacts) or a permanent lockout (by cutting the relay earth or putting another switch is series). This would best be done near the relay but it's not behind the instrument binnacle as described on the CD

ATA is suprisingly complex. It checks the following:

* front driver's doorlock
* front passenger's doorlock
* tailgate lock (or boot on saloon, coupe & cabriolet)
* bonnet (via switch on offside edge)
* radio removal sensor
* operation of parking brake
* towing sensor (may not be fitted post '87)

The system arms itself about 15 seconds after the ignition is switched off and the doors locked. Subsequently, if any of these sensors are activated, ATA shuts off the starter feed (on a diesel) and sounds the horn

From what I've seen I think it'd be possible to add more sensors or to use the ATA to trigger a siren (or have no audible alarm) and/or make it trigger a paging unit. It seems like a well thought-out system but is probably well understood by expert car thieves

I think the clue as to whether you have ATA or not is the vertical pushswitch on the offside edge of the bonnet halfway back. I've seen it on many late (93 onwards) W124s I've looked at. There appears to be no on-off switch for the system, so it is a liability in that sense, but it seems easy enough to fault find if you have & can read a circuit diagram

Hope this is of use to others


Nick Froome
www.w124.co.uk

 

[fletchf1]

re; immobilise the immobilser??

Dear Nick Froome, I have noted that you once posted a thread describing an immobiliser problem that you had with a w124 diesel, siting the ATA as the problem. Well I too think that I have an immobilser problem with my e300d, it will crank over but not start. So I would like to immobilse the immobiliser, or in fact the entire merc alarm system (I have a switch on the lhs under the bonnet). I am a long time member of this forum and have owned two late w124's for several years, so hoped you might have some advice on how to disable these systems and get my car going again. My car has many extras and the mercedes alarm system which has worked until now. I recently changed the stereo, do you think this could have disturbed somthing? Or could parking on a hill starve the diesel pump? I have tried locking the doors-unlocking-starting, and i have tried the infra red key fob son and off, to no avail. Any clues appreciated, Christian. (christian.fletcher@lotusf1team.com)

 

[television]

Components are failing now due to the age and in most cases the only option is to remove the UK fitted immobilizer. The early 129 cars as well are getting many failures of the original alarm that works in conjunction with the extra fitted one, and expensive parts at some £600.

It may be worth looking at some modern systems that some members have used for the locking.