R170 Alarm problems

[LadyPenelope]

LadyPs R170 200 is a reliable beast . Its a 2004 plate and still looking and running well so its been a while since ive had to post on here but alas another problem has arisen.

Its been parked up due to this pesky virus for several weeks and has only been started occasionally just to run it. Its parked forward facing on a drive that has an upward incline.

The other day i arrived home from work,went in the house and maybe 20 mins later LadyP said,can you hear that noise? Went outside and the mercs alarm was going full screech.

It was incredibly loud and i had to go get a pair of ear defenders to deal with it. Even with the alarm screeching i could unlock and start the car but no way could i silence the alarm other than pulling the battery.

Left it a while,battery back on,alarm fires again.

My initial thoughts are battery is low. It was new in April 2017 so should be in good nick. So ive got it on a battery charger.

Im pretty confident that when i put the battery back on ,the same problem will be there.

Ive checked the PSE pump and its good as new with not a hint of there ever being any moisture in the foam carrier etc.

Ive googled everywhere and there seems to be a lot of confusion as to where the actual siren is.

Where the battery sits, on the forward facing side of the battery is a square black plastic box. I think this is what is called the alarm control unit.

Ive read varying things that the siren is under the wiper grille,its in the wheel arch ,its in the engine bay.

Does anyone know where it is on a 2004 model uk spec? I feel ill have to unplug it just so that the car can be used and longer term maybe try a new siren module. Can ou still get them?

Im assuming its an alarm only and not an immobiliser otherwise i wouldnt have ben able to start the car??

Also does anyone know where the tow sensor is and what sort of device it is??


Thanks all...

 

[Uncle Benz]

It's under the wiper scuttle. You'll need to remove the wiper arms to get to it.

You can unplug it, it won't stop the car running.

 

[LadyPenelope]

Well today i investigated the issue. The car hasnt really been used since lockdown uk started and so i thought perhaps the alarm had misbehaved due to a low main battery.

Its interesting to note and a tribute to Mercs old school manufacture that this battery was new in April 2017 and it replaced the original Merc battery and this in a 2004 car so the original battery was 13 years old !

I hauled the battery out and put it on a 5amp charger for a while. This morning when i had some spare time, i thought id go looking for the siren unit.

At this point i'll say that it is possible to remove the siren without the hassle of trying to get the wipers off.



You will need a small torq but to remove the row of screws that secure the large black plastic air intake trim that covers the area between the rear of the bonnet and the lower edge of the windscreen.

Unclip the vac hose thats clipped along the front of this trim.

Now then if you ease the trim forward and up as much as you dare on the right hand side, using a torch you can see the siren unit down in the scuttle. Its just slightly right of the midway point across the width of the car.

Your chances of reaching your hand under here and unclipping the plug to the siren are slim to none.

It is however possible to unbolt the siren and ease it up and out over the front of the firewall so that it is now in the engine bay and you can unplug it !

As you stand at the front of the car look along the central line of the engine. Now look on the rear engine bay firewall under the vac hoses near the top edge. You should see a 13mm bolt. This goes through and bolts into a captive nut on the bracket that holds the siren in position. When you undo it, the siren is free to move. Carefully ease it over the firewall and into the engine bay. No wiper removal needed

Now then back to my situation. I refitted the battery and half expected the siren to fire up with its demonic screeching again. I had my ear defenders on just in case as the noise is painfully loud. If your thinking of playing with this, get some ear defenders and DONT risk hearing damage !

Well it didnt fire. Just a little bleep when the power was applied to the battery. I left it a while,fired the engine,all normal.

So now we are uncertain. We dont know if it misbehaved because the main battery was discharged,albeith there was power in it to start the car,,,or whether the siren module is intermittently faulty,,or whether its just something random that wont happen again.

LadyP was worried that it would start screeching when im not home as she would not be able to do anything with it. Accordingly ive unplugged the siren for now.

It looked in as new condition but the label on it had a date 18-09-2003 and the part number A1708202126 Q6

Even if i bought a new one from Merc,,would they have the part and when would it likely to have been manufactured? Surely not made in recent years unless its the modern equivalent part thats backward compatible.

Does the security system on the R170 as factory fitted also act as an immobiliser? ie is the key chipped?

Hope this info has been useful. By the way this is a RHD UK spec car.

 

[Uncle Benz]

You did well to access it with a reach around. I'd have removed the scuttle myself. I get worried about twenty year old brittle plastics

Immobiliser is PATS, chip in key.

 

[grahamcol]

You did exceedingly well there. My R170 is a 2001 230k model but l expect is identical in this area. Thanks for posting back and l feel this info is extremely useful for anyone who has similar alarm problems, particularly with reference to removing the siren unit without removing the wiper arms. I must remember this thread if ever l have similar issues.
I hope your car stays fixed.

 

[phillj]

The alarm siren has two internal Ni-Cad PCB mount rechargeable battery cells which have probably leaked due to age. Resulting in all sorts of weird alarm problems

 

[braunsteina]

Can someone tell me where the siren batteries are located? Thank you in advance

 

[Blobcat]

Can someone tell me where the siren batteries are located? Thank you in advance

Within the siren unit...

 

[brandwooddixon]

It's a common issue that the NiCds leak and damage the PCB, causing all sorts of issues.
I don't recall the sirens being that expensive, but times may change.

 

[braunsteina]

Within the siren unit...

Thank you I found a picture after I made the initial post.

 

[joderest]

On my Mercedes, the alarm unit also controls the indicators when you lock and unlock, not sure if yours the same.
When i had issues, mine to was located under the scuttle, what i have done is extended the wiring and fitted it in the engine bay, now its a 10 second job to unplug it and 2 mins to change it.
You could do this and show other half how to unplug it until you have proved one way or another that you need a replacement, i took a chance on a £15 second hand unit, 3 years later, its still working.
Oh, i also fitted it so that any discharge from the internal batteries runs out the unit and not onto the circuit board.