Alarm goes off at random

[tim.100]

I am going to buy a 97 C250 turbodiesel from my parents, taking delivery in September.

The alarm goes off randomly on some nights, whether it is parked at home or elsewhere.

The car battery has been replaced so we know it's good (it was the original item) but no-one has any sensible suggestions about what to check next.

Can the dealer download fault codes from this alarm, or is that no good if the fault is intermittent?

Regards,

Tim

 

[frog1520]

How often does it happen? I've had this on my W202 occasionally; theres nights when it starts, and i turn it off, and it starts again few hours later etc. If i go down and turn off the interior motion sensor for the night, that fixes it. Next night its OK. It might be a fly or something in the car!

Theres a rumour that leaving the exterior air vents open can cause it too.

Theres a few threads on here about this - so it might be worth doing a search for it!

Good luck!

 

[tim.100]

I did try a search - using 'alarm' and 'random.' Only 1 hit! I then posted my question, only to see the 2003 thread below get resurrected.

Suggestions from that thread, and your reply, seem to be:
- shut the air vents at night
- deactiviate the internal sensors
- replace the sensors (£450 at dealer)

The problem is quite frequent (twice a week) and occurs wherever the car is parked- so I can't blame anyone's cat!

If my (future) car is normal, then how come W202 alarms are not going off everywhere?

 

[Real_Hunter]

I had a similar problem with my C220 (N reg). The problem was eventually traced to switch in the bonnet. However my prbolem did not happen at nights only, but happened at random.

 

[tim.100]

Can you recall the steps taken to diagnose this switch?

With MB parts prices, I can't afford to replace bits in an effort to locate the problem.

Very grateful for your feedback.

 

[Real_Hunter]

I am afriad I did not diagnosed the probelm. It was done by a friend of mine. However I recommend that examine all reset switches physically and with a meter if possible.

 

[Simon]

When mine goes off more than twice in the night I open the boot and take out fuse No 7, that shuts it up. You'll have to remember to replace it though of course as it could affect your insurance.

Interestingly there were 3 spiders in the car yesterday which I'm sure must be the culprits.