Thoughts on built-in alarm - PeachParts Mercedes-Benz Forum

Jassper · #1 07-20-2003, 07:01 AM

Hello everyone.

Here are some of my thoughts (and some questions) on the built-in alarm.

As this is not another useless general thread of no meaning where everyone can post their opinions without actually knowing anything important - like what oil in engine, what rubber on wiper, how to polish chrome bumpers etc I don't expect answers but still I thought you might want to share my findings with me.

Right. The 124 300D 2.5 Turbo has a built-in alarm. There is a switch with 3 cables in the door right at where the key goes in that activates the alarm. When the key is turned clockwise the central locking locks the doors and the alarms gets armed. When the key is turned anticlockwise, the switch disarms the alarm and central locking unlocks the doors. When locked with a key, when one opens the door the alarm activates and sounds horn and flashes lights. So far so good. Also, this is an important feature - when the key is held for more than 3 seconds, all windows and roof will close.

Now here is the problem - an external alarm. I have an external alarm fitted in the car. It is by far better than the built-in alarm. Apart from the locking function to electric circutry it also allows me to remote lock and unlock the car which is an advantage.

The trouble stems from interaction between these 2 alarms. You can make the new alarm remotely un/lock the doors and arm the car. There are no arguments between those 2 systems because you don't lock the doors with a key - thus the built-in alarm does not arm itself. It is like the car had never been locked for the built-in alarm.

Problems start to arise when you want to use the very helpful feature of having your windows and roof closed when you remotely lock the car. Then the new alarm also operates the 3 wires that run to the door lock - when remotely locked it holds the wires connected for 12 seconds for all windows to close.
But with that the built-in alarm also gets armed.

When unlocking, it holds the wires also to give the built-in alarm feeling the car was unlocked with a key so it disarms.

The real problem is this : it doesn't work. Every now and then (sometimes yes and sometimes no) when remotely locked the built-in alarm goes off for no apparent reason. Also when unlocking and opening the door the alarm goes off. I have tried various ways to overcome this but at no avail.

The situation is like this - you can have an external alarm fitted and it will work perfectly. However, when you want it to also close windows the built-in alarm will go off.

How am I to overcome this? I tried to switch the built-in alarm off. I thought unplugging the plug underneath the passenger floormat (right to the red airbag plug) will help. I unplugged it but the built-in alarm goes off still. So, doing this does not turn the alarm off.

Is there a way to completely switch the alarm off?

Also I am afraid that turning it off will render the window-closing feature unoperational.

I thought that simulating the turning of the key with the external alarm would work - it does the same as a real key does - closes 2 wires to let the system know the car has been locked. But it does not. I am afraid the problem may lay in that there is a time gap counted in miliseconds between physical locking with a key and when the central locking starts working.

It means that when I lock the car with a key then I turn on the switch at the key in the door a few miliseconds earlier than the central locking unit gets a signal to lock the doors. This is the way it is supposed to work.

When I lock the doors with remote (and try to close any windows open), exactly the opposite happens - the remote unit on alarm gets activated by the central locking unit first and THEN it closes the wires. It is only a matter of a fraction of a second difference but I think this is what causes the alarm go off every now and then. Sometimes it locks and closes perfectly, maybe depending on what singnal was faster that moment.

Anyhow, my question is following - does someone know how to turn the built-in alarm completely off? As if it wasn't there.

Would there be a small gadget on market that could delay electric signals for a few miliseconds?

I would have given the car to a MB shop to have it solved but they have no idea also. They said to bring the car in and they would try. So they would keep fiddling with it the same way I did, trial and error. I don't feel like paying several hours of work on something with no clear result.

wjm · #2 07-22-2003, 12:21 PM

Jassper,

I'm very interested in your issue. I will assume that you bought the vehicle with this alarm, otherwise you would have returned it to the person who installed it, right? Also, after some searching, I see you 300D is a 1992, right?

I just installed keyless entry into my vehicle. My 94 E420 also has the convenience feature that closes the windows upon lock. In order for it to work, a Timer Relay had to be installed to continue sending power in order to close the windows. You probably have this as well. Because of the convenience feature, you need to wait at least 10 seconds (or maybe more depending what the timer relay is set to) between uses of the remote... otherwise crazy things happen like you are seeing.

My recommendation is disable the factory alarm. The factory alarm should have 2 plugs on it, a longer one and a shorter one. I'm not sure what you lose if you disable both plugs... so you'll need to test. Better would be to cut the factory alarm "ARM" wire which "should" be the yellow wire with blue stripe on the smaller plug. Don't cut it there, as it will be hard to reconnect. I believe you can get to that wire from the passenger footwell along the door side. You'll need to carefully cut open the wire sheath. Also, the factory "DISARM" wire should be the green wire with blue stripe... should you need to cut that one as well (although I don't see why you would). Again, you'll have to experiment.

Is there a red light on the dash that blinks when the alarm is on?

Anyway, please return with your findings. I will be replacing my keyless entry system with a keyless/remote start alarm shortly (when I decide which to buy) and installing myself. Can't stand the thought of Best Buy or Circuit City getting into my car. I will probably disable the factory alarm the same way... so I await your findings.

Jassper · #3 07-23-2003, 04:12 AM

Hello W!

"I will assume that you bought the vehicle with this alarm, otherwise you would have returned it to the person who installed it, right? "

I bought the vehicle with the factory alarm (of course) but I had the external alarm installed after I bought it by an electrician, a very wise person who does alarms. He isn't MB qualified though.

"Also, after some searching, I see you 300D is a 1992, right? "

Correct.

" Because of the convenience feature, you need to wait at least 10 seconds (or maybe more depending what the timer relay is set to) between uses of the remote... otherwise crazy things happen like you are seeing. "

There is a time relay - after remotely locking the car the external alarm also activates the relay that holds the locking wires for 12 seconds. Then it releases the wires. Also, when unlocking the car the external alarm holds another 2 wires, simulating a key unlocking thus setting the factory alarm off. That for approx 2 seconds.

"My recommendation is disable the factory alarm. "

I thought of that but didn't know how to. Also I was afraid that with disabling the alarm the window feature would also get disabled.

" The factory alarm should have 2 plugs on it, a longer one and a shorter one."

Where are the plugs? I haven't actually searched for the central unit of the facotry alarm. What I fiddled with was the door switch for key - 3 wires and a plug underneath the passenger, right by the red airbag plug. I had that unplugged but the alarm still activated.

" Better would be to cut the factory alarm "ARM" wire which "should" be the yellow wire with blue stripe on the smaller plug. "

Where would I find that wire and that plug?

" Don't cut it there, as it will be hard to reconnect. I believe you can get to that wire from the passenger footwell along the door side. You'll need to carefully cut open the wire sheath. Also, the factory "DISARM" wire should be the green wire with blue stripe... should you need to cut that one as well (although I don't see why you would). "

Not a problem cutting it. I can always resolder and insulate it. I am afraid though that cutting the arm/disarm wires will result in having the window feature inoperative as well. Have you tested that? I remember that when I somehow disarmed the alarm tit didn't go off (good) but neither it closed the windows (not as good). Are the factory alarm and window closing 2 separate entities?

"Is there a red light on the dash that blinks when the alarm is on?"

No.

" I will probably disable the factory alarm the same way... so I await your findings. "

The electrician goes for a holiday this Friday for a fortnight, new findings in 2 weeks I am afraid. However I have a feeling that with the disabled alarm you lose the window feature. I will get back to it after he returns because I want to have it operational. Will keep posted.

P.S. Gave a call to local MB service. The guy said it would be no problem but I think he had no idea and said a general 'yes come over, we'll look at it'. If the electrician won't make it workable I will give the vehicle to MB.

wjm · #4 07-23-2003, 07:58 AM

Jassper,

The alarm is a black box under the passenger footwell. Remove both matts and then the black metal cover and you'll see it. In many cars, the alarm will say "Becker" on it. It is to the left of where the red plug is. You'll see that the wires will also run down the right side of the vehicle through a black wire box. this is where you can follow the wire and cut from.

The window closing feature is handled by the convenience feature module which is underneath the rear seat. So, that is different than the alarm... although they do work together somehow.

Again, I'm not 100% sure... but you'll need to experiment to see if the arm wire cut will affect the window feature. Since you seem handy with an iron, should not be a problem.

Jassper · #5 07-23-2003, 10:20 AM

Hello W!

"The alarm is a black box under the passenger footwell. Remove both matts and then the black metal cover and you'll see it. In many cars, the alarm will say "Becker" on it. "

Aww my! Thanks for the info. I've come across that box and somehow thought it was used as alarm for the on-board radio (going off should someone take it away), since it had Becker on it. Also, I kept thinking I knew why a Mercedes cost so much since they used an entire box full of electronics to protect a radio

I'll follow your advice today and will let you know.

Rick Miley · #6 07-23-2003, 10:38 AM

The radio is indeed wired to the factory alarm. The box mentioned above monitors the doors, hood, trunk, and the radio. I once had a loose radio in a 190E that would set off the alarm while driving down the road. That was quite a sight!

wjm · #7 07-23-2003, 10:49 AM

In my car, there is only 1 wire that I know of that can control the radio/alarm. It connects to the bottom/middle of the radio. Anyone have any idea what this was for?

All I know is that I've replaced my radio and my alarm has never gone off.

Jassper · #8 07-23-2003, 11:56 AM

Right!

I searched for the yellow/blue wire underneath the passenger seat. There are 2 of them actually of the same colour, while the smaller plug actually seems to return to the driver's side and not go along the passenger side.

As they were 2 equal wires I decided it would not hurt not to cut them but try to unplug both plugs from the alarm first. I did so - both plugs out. Did a test - opened window and locked the car with a key and then opened the door from inside - the alarm did not go off. Happy happy joy joy. The alarm seems to be disabled. The car starts OK and drives OK, I don't think there are any other (important) devices affected with the disconnected alarm.

I connected the 12 sec time relay (powered off the new alarm box) to the window closing wire and it works! When I remotely lock the doors all windows and roof do close. When I unlock the factory alarm does not go off. This seems to have solved the problem.

Thanks for all your help!

P.S. W you are right - a procedure must be remembered. When locking the doors and unlocking them immediately within the 12 second interval one must wait for at least 12 seconds because when locking again after that, the windows go down instead of up.

wjm · #9 07-23-2003, 12:09 PM

Great. Glad everything works.

You are right about the window down thing. You can test this manually. Open your windows.. then lock the door with the key.. holding it a bit to let the windows close. Then, after they are closed (you don't even have to wait till they close), turn the key the other way and the windows will open.

Do me a favor. Test on occasion thoughout the diffent times whether your 1-touch window down works. That is, hold down the driver window button all the way and let go. Window should go down all the way. Sometimes, mine does not work.

Jassper · #10 07-23-2003, 12:16 PM

"Do me a favor. Test on occasion thoughout the diffent times whether your 1-touch window down works. That is, hold down the driver window button all the way and let go. Window should go down all the way. Sometimes, mine does not work."

I already tested that. It works. Yours seems to be an electric relay problem - the one-touch go-down works only with the key in a certain position (control lights on), when it is not in that position you can still wind the window down but have to hold the button all the time. This behaviour would indicate a relay is employed. I have disassembled almost everything on this car but this so I can't tell you where and what to look for in particular.

wjm · #11 07-23-2003, 12:25 PM

Thanks.

So with the factory alarm plugs "unplugged", your aftermarket alarm (I assume) is not affected, right?.

Just curious... what alarm did you put in?

Jassper · #12 07-23-2003, 12:36 PM

So with the factory alarm plugs "unplugged", your aftermarket alarm (I assume) is not affected, right?.

Just curious... what alarm did you put in? [/B][/QUOTE]

The aftermarket alarm has never been affected and worked fine without any problem since it had been installed. That was 2 years ago. I only didn't use the window-closing feature as it always amde trouble with the factory alarm. Now with the factory alarm disabled it works just as before but now it also closes the windows.

I was deciding between Meta and Protech alarms. Metas are far advanced but also more expensive. I went with ProTech AX 8000. The disadvantage is that Protech have an extremely weak horn, when the alarm sets off you hardly hear it from beneath the hood - unlike Meta for instance.

However, I don't use it as an alarm as much but as a convenient remote lock/unlock. With all windows closed now.

Grdenko · #13 07-26-2003, 04:35 PM

My factory alarm went crazy on me too, but only when i unlock the trunk. the alarm isnt disabled when i unlock the car from the trunk, so i just dont do that anymore. the car alarm also has a separate horn from the one you use on the road, i disconnected that horn, but the lights still flash.

mostly id just like to thank you for mentioning that you can close the windows and roof with the key. ive driven this car for 6 years and never knew that. i thought that was only something that has recently been put in new cars.

wjm · #14 07-26-2003, 06:18 PM

Quote:

My factory alarm went crazy on me too, but only when i unlock the trunk. the alarm isnt disabled when i unlock the car from the trunk, so i just dont do that anymore.

Hmm... not sure how it's supposed to function in your year, but something seems wrong.

Quote:

the car alarm also has a separate horn from the one you use on the road, i disconnected that horn, but the lights still flash.

Why? You've now disabled the most important part of the alarm. Do you have an aftermarket intstalled?

Quote:

mostly id just like to thank you for mentioning that you can close the windows and roof with the key. ive driven this car for 6 years and never knew that. i thought that was only something that has recently been put in new cars.

That's funny. You can't imagine how many owners STILL don't know this.

Anyway, regarding aftermarket alarms, and after MUCH research, I've narrowed it down to 2. The Hornet 554t or the CompuStar 2WSS-AS. Both are 2 way pager alarms with keyless entry and remote start.
I like the Hornet because it is made by DEI, the largest and arguably best maker of alarm systems around and the pager tells you the internal temperature of the vehicle. The Compustar, though, has a range of 6000 feet using spread spectrum technology. If you are a self installer, the Hornet can be had for around $165 on ebay, while the Compustar can be had for around $250.

Grdenko · #15 07-26-2003, 06:47 PM

no, i dont have an aftermarket alarm, and i was just thinking that since ive gotten out of the habit of unlocking the car from the trunk, i should just hook the horn back up

if anybody knows why my alarm goes off when i unlock the trunk, id like to know, but its not really a big deal.
thanks

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