W202 Odd alarm issue please help

Page may contain affiliate links. Please see terms for details.


Active Member
Dec 3, 2016
Chesterfield Derbyshire
W210 280 elegance
Hi all I have a 97 w202 c200 auto. I haven't owned the car all that long and it has super low mileage.
When I first got the car, I noticed that the alarm didn't function at all. That ended up being the fuse in the boot which is now replaced and the alarm now functions when opening a door if it's locked!

The issue I'm left with is puzzling me so I am kind of hoping that you guys can help!

The first issue is that the red led in the rear view mirror doesn't light up. Even when you open the door and press and hold the unlock side of the panic lock button (the manual method to deactivate interior protection)... the green light works when process is repeated!
The car has ultrasonic sensors inside on the A pillars and the passenger side one has a red led (doesn't light when system is armed).... I had to remove nasty tape residue and bluetack from these when purchased!

It has a touch key port (which I am not going to state where) and this also seems to do nothing also!

Is it possible that someone has unplugged something or is it more likely to be something that needs to be replaced??

As I said earlier in the post... the alarm functions in every way that it should except for the interior protection and red led in the mirror!

What the heck am I missing??? Please help.

Thanks guys in advance
All that seems wrong is the Red LED is dead.
If you lock yourself inside the car with the remote, stay completely still for 30 seconds to allow the alarm to arm, then move your arms about, this should trigger the alarm as the motion sensors detect movement, the red LED on the motion detector should flash as you move.

What type of remotes do you have?


Ok just been out and sat in the car for 10 minutes!

Got in after unlocking with the remote, shut door, started car.
Then turned off the car and open the shut the drivers door (as if getting out). Locked the car with the remote and stayed inside the car as suggested .... after 15 minutes nothing! Opened the door after I forgot all about setting the alarm and set it off.

The red light on the rear view mirror does nothing. Also the left side sensor on the a pillar has a red indicator on it.... that also does nothing. I'm still just as perplexed as before.

I have no idea if there is a control box for this anywhere and somebody in the past has disconnected a plug or cut a wire?. I have had a look under the left side carpet where there is a few Ecu looking things on the back of a metal tray that you can remove.... this all looks normal except for some wd40 sprayed there by previous owner (likely because of the drain hole being blocked that I have now sorted). I have cleaned away the wd40 already.

So what the heck am I missing??? Is this so glaringly obvious that I am blind and can't see what is right in front of my face?

The good side is that I know for sure the ultrasonic jobies are not working. If these are somehow wired into the indicator (red light) on the rear view mirror I am a little closer but no idea how to start tracing the fault correctly.
As far as I know the Red and Green LEDs are just a visual conformation that the car has been locked and that the immobiliser is armed, if any of the LEDs are not working the alarm system will still function as normal. The fact that the motion sensors were covered in tape/Blue tack may indicate that the previous owner had some false alarm issues and the were trying to cover up the motion sensors to prevent false alarms.

If you sit in the car and lock it with the remote then the alarm will be armed and the immobiliser will also be armed, so, after locking the car from the inside you should not be able to start the car… because the immobiliser is armed on locking. (disarmed on unlocking)

What alarm stickers are in the back window, see here… https://www.google.com/search?q=mer...ved=0ahUKEwjLi4OL6_7QAhWLL8AKHScEDF8Q_AUIBygC

Just went and tried to start the car while locked...

Get in and lock the car with the remote .. wait for a couple of minutes and then put key in to try and start it.
The green light on the interior mirror flashes rapidly and the alarm siren wails once... then the car starts but the hazard lights flash 3 times!

I guess that isn't normal behaviour?
That’s a scorpion alarm, the can be troublesome at times, yours is 20 years old.
Not normal behaviour, the hazards should flash only when locking and flash when unlocking.

When the Green light was flashing, the Red light was also flashing… if it had been working… Red and Green flash alternately when the ignition is turned on so as to show that the immobiliser is not disarmed… the immobiliser is disarmed when the car is unlocked with the remote.

That car shouldn’t have started while it was locked and immobelised, so, some changes to the alarm system may have been made by the last owner, are you in a position to ask the last owner what work was done on the alarm system?

Was the fuse blown or was it missing from its slot?

I do actually have a contact number for the previous owner and the address so contact may be possible.
I had a dig around on the forum through the early hours and found a picture of what the wiring should look like in the passenger footwell.... I am starting to suspect that some of the wiring is missing or unplugged and tucked away.

The fuse (#4 from memory) in the boot had been removed. I can only presume by whoever has messed with the alarm.
Last edited:
The may have messed with the alarm for a reason, it may have been faulty and stopping the car from being started. Might be best to leave well enough alone, the car is after all 20 years old.

In the video below you see 2 black boxes, the small one is the immobiliser, the big one is the Scorpion alarm brain box.

The guy in the video is remaking the Green wire connection so as to bypass the Scorpion alarm system.

Have a look see if the Green wire looks like it was cut and then rejoined.
Pictures are useful… if you can put up some of the passenger footwall area like in the video below



Mine doesn't look as the one in the video at all!

This is how mine looks....

I even looked under the carpet....

Nothing here either .... well except for my Ecig.

I have noticed that there is a couple of white relays sat under the washer bottle. Included in these are a standard 10amp fuse and a very large red fuse (unsure on rating as I can't see the top of it) ... on this small relay bank there is black insulating tape wrapped around it so someone has been in there before I guess?

I would like to either sort it out so that the system works (prefer this option) or remove the system completely and have a really good thatcham 1 alarm fitted (saves me more than the extra premium on insurance not having one fitted due to area)
As you suspected, the unit I was looking for lives under the binnacle as seen here....

At least now I have found the issue. Well the issue that is rendering the red mirror light and interior protection sensors useless.
My most hated thing of all, cut wires!

Not knowing why this was done in the first place I can only speculate that it was an immobiliser fault or some kind of failure internally within the immobiliser unit??

The biggest question here is what have they actually done and is this beyond repair? I have tried to find any trace of the cut wires other ends in the loom and nothing is present (that I can see).

Thank you so much for all of you're help with this.... this place is amazing :thumb:
The last owner probably doesn’t know EXACTLY what was done to the system, it’s the person/garage that did the work, that is who you need to speak to, if that is at all possible. Failing that and if you are intending to replace the system then the alarm installer should be able to work out what’s going on with the system… or are you intending to do a DIY replacement?

Is that the immobiliser below at the red arrow?


Either way the system is unreliable at that age and was “tampered with” / fuse removed, for a reason.

Not 100% sure as which is the immobiliser unit ... some information I have been given suggests that the immobiliser is within the alarm module ( below and to the right of the arrow with the cut wires hanging out).
Rather than mess around with what's already there I want to replace it all. I'm not sure yet whether I want to diy or pay a professional. Either way it's at least £117 for the replacement alarm alone. Would be nice to have it all working with the key as the original system does though.
That’s the factory fitted immobiliser below, if it has cut wires then the Scorpion alarm has been integrated into the factory fitted immobiliser. May be a similar set up to the guy in the video above in post #10.

That car came with the factory fitted immobiliser, the MB dealers then fitted Scorpion alarms, the Scorpion alarms (module) would also have an immobiliser BUT it may or may not have been integrated with the Mercedes factory fitted immobiliser. That’s where the cut wires come in.


Last edited:
Brilliant stuff. I will take the panel off again and have another look at the modules under there. At least the panel is easy enough to take off :thumb:

I will report back later.
I seem to have now removed the alarm box from the system without any catastrophic problems.

The immobiliser is right up behind the dash clocks as seen here in this picture...

And the main loom for this goes to what I would describe as a distribution box or similar as shown here....

The top connection goes to the immobiliser box and the lower connection goes to the alarm box behind the lower panel. Also there is a section of the lower connection wires that disappears behind the dash as can be seen (wires to siren??).
I couldn't find any trace of the cut wires at all in any of this mess either.

I removed the alarm box altogether because I don't like twisted together wires without proper connections. Made sure I pulled out the fuse so that the circuit is not live also..... fire waiting to happen in my opinion!

I'm also a bit concerned about this earth wire that has obviously been tampered with....

So... what else do I need to do for complete removal? Do I just unplug from the distribution box thing and chase it all back to the siren? Or is there more to it?

I have tested the immobiliser by removing one of the key blades from one of my keys.... I can happily say that it functions correctly. No chip key, no start!
At least that's a good thing.
It now appears the Scorpion alarm is in no way involved with immobilising the car, as was demonstrated when you were able to start the car while it was immobilised/locked...post#7.

The mirror Red LED is not working, maybe replacing or re-soldering might fix that, see thread below.

Regarding removal of the whole system, I would leave the junction box alone, I think you just need to unplug/remove the alarm module, siren and motion detectors.
There will be an alarm pin switch for the bonnet, located perhaps at the bulk head.
The wires that disappears behind the dash would be wires going to the motion detectors and the siren.

The Brown earth wire was cut and rejoined so not an issue.


Users who are viewing this thread

Top Bottom