W202 Central locking gremlins - help needed.

[Cudavanti]

Hello, hope this is the right part of the forum to post this in. I'm having so issues diagnosing the problem with my 1995 C280. I guess the easiest thing to do is explain what happens and hope someone can point me towards a remedy.

FYI, I have already replaced the batteries in the remote, didn't solve it sadly.

As you know, normally you press to unlock, the green light on the door handle/mirror flashes. You press to lock, the red light flashes. What usually happens (but only 90% of the time, the other 10% it works as should) is that both red and green flash at the alternately for about 30 seconds.

Sometimes after this, even though the door pins are up and all doors are unlocked the light continues to blink red as though still locked. If I physically open the door the immobiliser kicks in and the indicators start flashing. Other times it goes though its 30 seconds of red/green and finishes with green. I can open/start/drive.

This is doing my head in and means I can never lock the car for fear of being stuck somewhere with the immobiliser activated. I've taken everything valuable out of it, but even in a low crime area this is not ideal. Has anyone got any tips? Could the key itself be defective? If seems the car always responds to me pressing the button, but not always in the way I'd hope!

 

[Dec]

There is a key hole in the passenger door handle for emergency MANUAL locking/unlocking, it may have a round plastic weather bung in the lock key hole, if you remove this bung you will see the key hole.

Test your fob SEVERAL TIMES to see if it ever fails the test below.



The rules are as follows;

-Press button once, car unlocks and immobiliser is disarmed, Green LED flashes, car will start when key is turned to position 3.

-Press button once, car locks and immobiliser is armed, Red LED flashes,

-Red and Green flash ALTERNATLEY means fob batteries are low, however car usually locks/unlocks and starts.

-Red and Green flash TOGEATHER, WHEN KEY IS TURNED TO POSITION 2, means the immobiliser hasn’t disengaged so car wont start.


Dec

 

[Dec]

Further to the above.

When you lock the car, the immobiliser is engaged and the alarm is armed, you are then opening a car that has its alarm armed and so it goes off (door locks failed to lock). When the alarm goes off the indicators start flashing.

I think your problem is that the vacuum pump in the boot is not locking all the doors/boot and fuel flap, all of the time, when you lock or unlock the car you should hear the vacuum pump whine for 2 or 3 seconds, if the vacuum pump doesn’t run when you lock/unlock then the door pins won’t go down.

However if you can hear the vacuum pump running but the door pins don’t operate then there may be a vacuum tube leaking or missing/disconnected.

The vacuum pump is located inside the rear driver side wing in a foam box.

Check your fuses too.

Dec

 

[Dec]

The rules are as follows;

-Press button once, car unlocks and immobiliser is disarmed, Green LED flashes, car will start when key is turned to position 3.

-Press button once, car locks and immobiliser is armed, Red LED flashes,

-Red and Green flash ALTERNATLEY means fob batteries are low, however car usually locks/unlocks and starts.

-Red and Green flash TOGEATHER, WHEN KEY IS TURNED TO POSITION 2, means the immobiliser hasn’t disengaged so car wont start.


Dec

Correction to post number 2 above.

The rules are as follows;

-Press button once, car unlocks and immobiliser is disarmed, Green LED flashes, car will start when key is turned to position 3.

-Press button once, car locks and immobiliser is armed, Red LED flashes,

-Red and Green flash TOGEATHER means fob batteries are low, however car usually locks/unlocks and starts.

-Red and Green flash ALTERNATLEY, WHEN KEY IS TURNED TO POSITION 2, means the immobiliser hasn’t disengaged so car wont start.


Dec

 

[Cudavanti]

Thanks for the advice so far guys. I took the key apart and cleaned all the contacts and made the batteries fit more firmly. Now the key is giving a strong signal so no more alternate red/green. However, the car is flashing red-green simultaniously aout 90% of the time I lock and unlock it.

I stood with my ear to each door lock and the boot while conducting the tests. They all make the appropriate noise and seem to work ok. Also it's still on occasion flashing red AFTER it has unlocked, all pins up etc. I can physically open any door and the boot but of course the immobiliser kicks in as soon as I do.

What are your thoughts? Is this likely to mean I need a new immobiliser and if so, how complex is it to replace? Can I fit a second-hand one?

 

[Cudavanti]

Correction to post number 2 above.

The rules are as follows;

-Press button once, car unlocks and immobiliser is disarmed, Green LED flashes, car will start when key is turned to position 3.

-Press button once, car locks and immobiliser is armed, Red LED flashes,

-Red and Green flash TOGEATHER means fob batteries are low, however car usually locks/unlocks and starts.

-Red and Green flash ALTERNATLEY, WHEN KEY IS TURNED TO POSITION 2, means the immobiliser hasn’t disengaged so car wont start.


Dec

Sorry I hadn't refreshed the page since yesterday when I posted my reply above. Having read your corrected post, does this mean since I have red/green flashing together, I have a crappy key? I only have one key for the car and have already enquired with Mercedes the cost of getting a new one. They want 280€, is that similar to the cost in the UK?

 

[Dec]

I believe it’s about? £230 in UK so €280 is near enough. I don’t know about the key, it could also be a crappy vacuum pump.

Any time my red/green LEDs flash simultaneously, I change the fob batteries and that cures the problem, in my car, when the LEDs flash simultaneously it is always for 30 seconds, either when I lock or unlock , however this never stops the car from starting, locking or unlocking.
Could your fob batteries be out of date … even though the pass the camera test?

How long, in seconds, does the vacuum pump run for when you lock the car?
How long, in seconds, does the vacuum pump run for when you unlock the car?
… you will hear a whine noise from inside rear wing for about 2 or 3 seconds when locking and unlocking, if you keep the fuel flap open while you lock/unlock you will be able to see the fuel flap locking pin shoot out/in.

The flashing red AFTER it has unlocked, could it actually be flashing Red/Green simultaneously (for 30 seconds) but the Green LED is failing to light?... does both the door and mirror LEDs do the exact same flashing sequence?

When you press the fob button you arm the alarm, it would seem the vacuum pump is sometimes failing to lock all doors/boot/flap, opening a door under those circumstances will set off the alarm, the alarm knows the doors are all closed but it doesn’t know whether the are locked or not so opening a door when the alarm is armed will trigger it.

On the positive post of the car battery check that ALL wires are tight.

Dec