Light switch unlocks and locks doors in 350sdl - PeachParts Mercedes-Benz Forum

sba121400 · #1 08-03-2011, 04:11 PM

In my 90 350sdl, The light switch unlocks the doors when the headlights are turned on, and locks the doors when the headlights are turned off. Has anyone had this problem before, if so how do you fix it? Help Needed.
Thanks
Sean

w123fanman · #2 08-03-2011, 07:07 PM

Quote:

Originally Posted by sba121400

In my 90 350sdl, The light switch unlocks the doors when the headlights are turned on, and locks the doors when the headlights are turned off. Has anyone had this problem before, if so how do you fix it? Help Needed.
Thanks
Sean

I'm not familiar with that car but did this start happening after a repair or service? It wouldn't make sense otherwise, as these things wouldn't be on the same switch

compu_85 · #3 08-03-2011, 07:10 PM

Sounds like a grounding problem?

-J

sba121400 · #4 08-03-2011, 07:42 PM

Quote:

Originally Posted by w123fanman

I'm not familiar with that car but did this start happening after a repair or service? It wouldn't make sense otherwise, as these things wouldn't be on the same switch

I recently bought the car, and realized it once I got the car home.

sba121400 · #5 08-03-2011, 07:43 PM

Quote:

Originally Posted by compu_85

Sounds like a grounding problem?

-J

Thats what I was thinking, Do you know where these are grounded?
Thanks
Sean

ah-kay · #6 08-03-2011, 07:45 PM

This is strange. You may have a wire(s) pinched somewhere. Were there any accident before or any repair done?

I cannot tell you where is the problem but I would do the followings:

1) The door lock and unlock is done by activating a 12V signal to the vacuum pump. IIRC, the vacuum pump has a plug with 3 connectors. 12V to one wire will lock and then 12V to another will unlock. 12V wire is red, the other 2 wires are brown and yellow, I think.
2) Remove the plug or with the plug in place and use a DMM to measure the voltage of the 3 wires to chassis and write them down.
3) Ask someone to activate the light ON and measure them again.
4) Then turn light OFF and measure them.
5) There should be NO change in a good system. If it does change then note which wire(s) and then trace it backwards. The voltage change means the light tries to activate the lock.

Auto electric is not fun. Good luck.

btw: the vacuum pump is in the trunk, next to spare tire.

sba121400 · #7 08-03-2011, 08:00 PM

Quote:

Originally Posted by ah-kay

This is strange. You may have a wire(s) pinched somewhere. Were there any accident before or any repair done?

I cannot tell you where is the problem but I would do the followings:

1) The door lock and unlock is done by activating a 12V signal to the vacuum pump. IIRC, the vacuum pump has a plug with 3 connectors. 12V to one wire will lock and then 12V to another will unlock. 12V wire is red, the other 2 wires are brown and yellow, I think.
2) Remove the plug or with the plug in place and use a DMM to measure the voltage of the 3 wires to chassis and write them down.
3) Ask someone to activate the light ON and measure them again.
4) Then turn light OFF and measure them.
5) There should be NO change in a good system. If it does change then note which wire(s) and then trace it backwards. The voltage change means the light tries to activate the lock.

Auto electric is not fun. Good luck.

btw: the vacuum pump is in the trunk, next to spare tire.

I bought the car recently and didnt realize this till I got the car home, there was mice living throughout the car, but I have not found any chewed up wires, and have went through most of the car. I will try the vacuum pump and test the voltage.
Thanks
Sean

sixto · #8 08-03-2011, 08:14 PM

Is there an aftermarket alarm in the car? It might have been wired to flash the lights when locking and unlocking the doors. The headlight and door lock wires come closest in the driver footwell so that's where I'd start looking for an aftermarket alarm unit.

Sixto
87 300D

sba121400 · #9 08-03-2011, 08:23 PM

Quote:

Originally Posted by sixto

Is there an aftermarket alarm in the car? It might have been wired to flash the lights when locking and unlocking the doors. The headlight and door lock wires come closest in the driver footwell so that's where I'd start looking for an aftermarket alarm unit.

Sixto
87 300D

I don't believe that there is an aftermarket alarm in the car, It is completely stock. I will check out the drivers side footwell for loose wires.
Thanks
Sean

locry · #10 08-05-2011, 06:18 AM

yeah, i'd go with sixto's theory... might've had a keyless entry installed sometime

scottmcphee · #11 08-05-2011, 08:23 AM

Recently bought! Phone the seller and tell them you're not happy with this behavior and you want your money back unless they can help you make it right. Maybe they know what is going on.