500 SL Alarm............or not?

[Hartzpartz]

My 500 SL (1991) allegedly has option code 551 which should mean it has an alarm (described in the code list as "anti-theft warning system").

Now I don't want to sound daft but I can find any evidence that an alarm is fitted. I've tried locking the car with a window open, leaning in and opening a door from the inside, but nothing happens. Same apllies if I release bonnet or boot lid. There is an underbonnet switch but I can't see any visible interior sensors or flashing LED.

It has a tracker fitted but this was added a few years after it came off the production line.

Any ideas?

 

[BlackC55]

Is there an alarm siren under the bonnet?

 

[BTB 500]

Might use the car horn.

My '97 certainly has interior sensors, but that's MBSS (M-B Security System) which wasn't a factory option ... it was added to cars after delivery to the UK.

 

[stats007]

You should have the colour coded LEDs in the door handles and boot lock that flash green when the alarm is set. You don't have any motion sensors - it's purely based on switches for the various locks and igniton etc.

 

[Hartzpartz]

Yes, I have the red/green leds in all the right places and they operate as they should. No sirens or interior sensors fitted.

No special arming process I suppose (can't see anything in user manual)?

Assuming the system isn't working, does anyone know where the fuse/control module may be located?

 

[stats007]

The alarm module N26 is fed from the combination relay via fuse 5 in the boot. The module is located behind the central locking vacuum pump which is in the battery compartment.

 

[Hartzpartz]

Now that's interesting. There is a control module in the battery compartment with nothing plugged into it! I'll see if I can any get numbers from it.

Noticed it when checking the battery the other day. Didn't see any obvious wiring that had come a drift though.

Will investigate further and let you know.

 

[Hartzpartz]

I've just had another look and there is a white control box fitted above the vacuum control unit. The wiring plug had been disconnected and cable tied to another loom.

I've plugged it back in for now but using the method previously discribed (lock car with window open and lean in to release door from inside) nothing happens.

The only thing I do notice after doing this is that the red door LED flashes for about 5 seconds (i.e. for longer than normal) although I seem to recall this happening before (with the alarm module unplugged).

I've checked fuse 5 in the boot and that appears to be ok.

Unless the system takes a few minutes to arm, it appears the module may be faulty? I just hope it doesn't flatten my battery. I guess I'll find that out in the morning!

 

[Flash]

It would take 30/60 secs for the alarm system to arm itself!

 

[stats007]

If it flashes for that long (approx 10s) then it means it can't arm itself due to a fault in the system - possibly a sensor for the boot etc. You can get the fault code read from the alarm system which should tell you which it is.

 

[Hartzpartz]

The door/boot handle LEDs flash normally unless I simulate a break in. Then they flash red for 10 seconds when I unlock the car in the usual way.

Should this factory alarm system ever make an audible sound when arming?

As stats007 has suggested, I'll have to get someone to pull the fault code as it could be something simple that's causing the problem.

It's a bit suspicious that the alarm module was disconnected and the wiring cable tied out of the way, especially as this car has only ever been serviced by Mercedes and the previous owner doesn't appear to have been the type to leave any fault unrectified!

 

[Flash]

I would not be surprised if MB had something to do with it whilst working on the car...

 

[Setanta]

Hi guys

Just following on from this thread, which I found when looking for info on the 1992 500SL, I was also wondering about this since the owner of a carI am looking at says that he has no idea whether an alarm/immobilisor is fitted and certainly was not shown such when he bought the car.

It apparently also has the red and green leds in the door and boot - does this system actually immobilise the car and was it a factory fitment or indeed, how does the whole shebang work since I don't want to void an insurance policy if it does not have one (or, perhaps I should look for another car).

Thanks]

James

 

[stats007]

As far as I understand it the early cars just have an alarm. MY'93 had the starter lock out relay introduced to the alarm system. The MBSS immobilising systems were normally added by the dealer but weren't introduced until 1995.

 

[glojo]

I would not be surprised if MB had something to do with it whilst working on the car...

Hi Flash,
With the greatest of respect, that would make no sense unless the owner requested this be done and if I were the service manager then I would want something in writing? When filling out an insurance application I have always been asked if the car has an anti theft system. I'm not sure if it asks if the system is working but I bet the insurance company would weant to know if it has been deliberately disconnected?

We are discussing a prestige, high value vehicle (or it was once a higfh value machine )

Regards
John

 

[jaymanek]

The early R129's used the horn as the siren and yes if it has the flashing lights then it has an immobiliser...

 

[stats007]

The early R129's used the horn as the siren and yes if it has the flashing lights then it has an immobiliser...

The alarm LED indicator was introduced in '93 with the starter lock out relay - so that's the only immobiliser it has.

 

[Hartzpartz]

I can only assume the previous owner of my car was told the alarm module was faulty and did not wish to have it replaced and so just had it disconnected at the unit. The car was 1st registered at the end of 1991 and a tracking device was fitted (and invoiced by MB) in 1994.

At the moment I have left it connected with no adverse effects (touch wood!). Still not working though!

I have a friend with a Snap On scanner and so was hoping to see if a DTC could be pulled. Does anyone know if this is possible on early cars??

 

[stats007]

You can read and clear the codes using a makeshift LED and grounding the correct pins in the 38-pin diagnostic port - a any simple 'scanner' should work.

 

[Hartzpartz]

So you're able to attach a 12 volt LED to terminals in the diagnostics socket?

Are there different terminals for the different systems (i.e. ABS, Alarm etc.)?

Do you know which terminals do what?