Rear Door and Dome Light - PeachParts Mercedes-Benz Forum

Jareb · #1 09-21-2019, 12:48 PM

1985 W123 300TD

The rear dome light does not come on when opening either of the rear doors or the hatch. The rear dome light will work from the dash switch. I pulled both rear door switches and I am getting no power to the switch from the wires on either side. I tried with the lights on, with the car running and still nothing. Front dome light and front door switches work fine. Any ideas or am I missing something obvious here?

resto108 · #2 09-23-2019, 12:08 PM

my 82 is the same way, though I never troubleshot it.

Alec300SD · #3 09-23-2019, 01:02 PM

Quote:

Originally Posted by Jareb

1985 W123 300TD
Any ideas or am I missing something obvious here?

Either bad contacts on the door switches or, the wire from the hatch has failed due to repeated flexing over the years.
BTDT with my old Volvo wagon.

Jareb · #4 09-23-2019, 04:52 PM

Quote:

Originally Posted by Alec300SD

Either bad contacts on the door switches or, the wire from the hatch has failed due to repeated flexing over the years.
BTDT with my old Volvo wagon.

Thanks Alec. The switches are good. The 2 wires that go to each rear door switch are dead on both sides. I would think there should be 12V going to these same as the front door switch/wires. Only thing I can think of is there is a disconnect somewhere. Could be a broken/disconnected wire under the dash since both rear doors are affected? I haven't checked the hatch door wires but the hatch dome light does come on with the dash switch so it must have 12V going to it.

Wish I had a schematic of this circuit

Alec300SD · #5 09-23-2019, 05:58 PM

I believe you have two independent circuits to control the rear dome light.

One is the direct connection from the dash to the light (wires stay in in cabin).

The second circuit is activated by the rear door and hatch switches (hatch wires leave the cabin).
The three switches are connected together.
You have probably have a break in the wires going from the cabin to the hatch.

Hence no power to the rear door switches.

Diagnostic test: jumpering the wires going to the hatch switch (under the headliner) will bypass the broken connection and should restore power to the rear door switches.

Jareb · #6 09-23-2019, 07:36 PM

Quote:

Originally Posted by Alec300SD

I believe you have two independent circuits to control the rear dome light.

One is the direct connection from the dash to the light (wires stay in in cabin).

The second circuit is activated by the rear door and hatch switches (hatch wires leave the cabin).
The three switches are connected together.
You have probably have a break in the wires going from the cabin to the hatch.

Hence no power to the rear door switches.

Diagnostic test: jumpering the wires going to the hatch switch (under the headliner) will bypass the broken connection and should restore power to the rear door switches.

I'm not too good at electrical stuff but what your saying makes some sense to me. So, the 2 rear door switches and the hatch switch are on the same circuit and are wired together? Any break anywhere in the wiring between any one of the 3 switches would cause them all to be inop? The wiring between the 3 switches is under the headliner to the back? I looked for the wiring from the driver side rear door switch and followed it down behind the pillar with the seatbelt. It went under the flat plastic protector conduit on the floor and I stopped at that point. Where would I jumper power from to go to the rear switch? Are you saying if I get 12 volts to the hatch switch...the rear door switches will be hot? Like I said; I'm not a very good electrical diagnostician

Alec300SD · #7 09-24-2019, 12:25 AM

You should be getting 12 volts to each switch at all times.

Did you test for power to each switch by pulling it out of the chassis and connecting one lead of your voltmeter to the positive feed and the other lead of the voltmeter to a good chassis ground?

The ground (or negative feed) is usually a brown wire.
The positive feed would be the other colored wire connected to the switch.
On my W116s the positive feed is a red wire.

Jareb · #8 09-24-2019, 06:16 AM

Quote:

Originally Posted by Alec300SD

You should be getting 12 volts to each switch at all times.

Did you test for power to each switch by pulling it out of the chassis and connecting one lead of your voltmeter to the positive feed and the other lead of the voltmeter to a good chassis ground?

The ground (or negative feed) is usually a brown wire.
The positive feed would be the other colored wire connected to the switch.
On my W116s the positive feed is a red wire.

Yes, I tested both passenger and driver rear door wire. Put a meter between the 2 wires and got nothing. I did the same on the front doors to be sure I was measuring correctly and got 12V there. I did not test the hatch wires as that would require pulling the lock where the switch is buried and I knew that the light got 12V from the dash switch. Since they are all connected, I will do the hatch wiring although I suspect it is dead also since the light does not come on opening the hatch. Whatever wire supplies 12V to the 3 different switches is what I need to find and go from there? If there is 12V at that wire then there must be a break somewhere downstream?
Thanks
John

jay_bob · #9 09-24-2019, 07:20 AM

Most older Mercedes door switches are on the ground side of the circuit. It was done this way until the models in the mid-late 90s, when they started using SAMs and CAN bus to run things.

Meaning it goes battery + -> fuse -> bulb(s) -> switches -> ground.

If there are multiple bulbs, they would be connected in parallel, one side of each bulb connected to the fuse, and the other side of each bulb to the switches.

The switches would be in parallel with each other, one side of the switch faces the bulb(s) and the other faces ground.

The other thing to know is incandescent light bulbs act as a dead short when cold (off) and when operating (hot) the internal resistance of the filament increases. This can trick you when reading the circuit with a meter. LEDs are more problematic, the meter probably does not allow enough current to flow through the bulb to 'wake up' the internal electronics, so you may not read anything at the switch with LED bulbs.

When you put your meter on the switch terminals you will read 12 V to ground because the light bulb when off acts as almost a dead short. However once you ground the switch lead (by turning the switch on) it causes the current to flow through the light bulb and its resistance increases. At this point you will have 0 V to ground at both ends of the switch.

Never worked lights on a 123 wagon but have on a 123 sedan and a 124 wagon. MB usually kept things pretty much the same between the 123 and 124 in the body electrical so here is what I remember about how it works.

On my 123 sedan I had a two position rear light switch. The choices were always on or controlled by the door switches. It was basically wired in parallel with the two switches in the rear doors. Opening either rear door, or turning on the dash switch, completed the ground circuit.

The 124 wagon has a 3 position switch, always on, never on, or controlled by the doors. In the always on position, the ground is made locally at the dash switch. Never on, the ground path to the door switches is broken, so the light can never come on, and in the door controlled position, the path from the bulb to the door switches is completed through the dash switch, allowing the door switches to control the light.

In the 124 wagon tailgate there is a light switch that is part of the lock mechanism; pretty sure the 123 did it the same way. This is what turns on the light when you open the tailgate. The switch wire has to pass through the flex boot from the body to the tailgate. After many years of opening and closing the door the wires start to break in the boot. The wiring to the tailgate joins the body wiring at a terminal block behind the headliner in the left rear corner. On the 124 you don't have to drop the headliner to find it, it is behind a plastic trim piece that spans the back end of the interior. Not sure how they did this on a 123 wagon.

Hopefully this helps you figure out how the circuit works. I would get a copy of the wiring diagram and start tracing with a meter. Just keep in mind what you are measuring at the switches, it is not direct 12 V but rather battery voltage through a bulb filament.

Jareb · #10 09-24-2019, 12:24 PM

Quote:

Originally Posted by jay_bob

Most older Mercedes door switches are on the ground side of the circuit. It was done this way until the models in the mid-late 90s, when they started using SAMs and CAN bus to run things.

Meaning it goes battery + -> fuse -> bulb(s) -> switches -> ground.

If there are multiple bulbs, they would be connected in parallel, one side of each bulb connected to the fuse, and the other side of each bulb to the switches.

The switches would be in parallel with each other, one side of the switch faces the bulb(s) and the other faces ground.

The other thing to know is incandescent light bulbs act as a dead short when cold (off) and when operating (hot) the internal resistance of the filament increases. This can trick you when reading the circuit with a meter. LEDs are more problematic, the meter probably does not allow enough current to flow through the bulb to 'wake up' the internal electronics, so you may not read anything at the switch with LED bulbs.

When you put your meter on the switch terminals you will read 12 V to ground because the light bulb when off acts as almost a dead short. However once you ground the switch lead (by turning the switch on) it causes the current to flow through the light bulb and its resistance increases. At this point you will have 0 V to ground at both ends of the switch.

Never worked lights on a 123 wagon but have on a 123 sedan and a 124 wagon. MB usually kept things pretty much the same between the 123 and 124 in the body electrical so here is what I remember about how it works.

On my 123 sedan I had a two position rear light switch. The choices were always on or controlled by the door switches. It was basically wired in parallel with the two switches in the rear doors. Opening either rear door, or turning on the dash switch, completed the ground circuit.

The 124 wagon has a 3 position switch, always on, never on, or controlled by the doors. In the always on position, the ground is made locally at the dash switch. Never on, the ground path to the door switches is broken, so the light can never come on, and in the door controlled position, the path from the bulb to the door switches is completed through the dash switch, allowing the door switches to control the light.

In the 124 wagon tailgate there is a light switch that is part of the lock mechanism; pretty sure the 123 did it the same way. This is what turns on the light when you open the tailgate. The switch wire has to pass through the flex boot from the body to the tailgate. After many years of opening and closing the door the wires start to break in the boot. The wiring to the tailgate joins the body wiring at a terminal block behind the headliner in the left rear corner. On the 124 you don't have to drop the headliner to find it, it is behind a plastic trim piece that spans the back end of the interior. Not sure how they did this on a 123 wagon.

Hopefully this helps you figure out how the circuit works. I would get a copy of the wiring diagram and start tracing with a meter. Just keep in mind what you are measuring at the switches, it is not direct 12 V but rather battery voltage through a bulb filament.

Thanks JayBob for the explanation. I will pull the rear header panel{where dome light is} and see if the terminal block is there. I actually started to take it off today but after removing the screws, I couldn't get it to come off easily and I was afraid I'd break it by pulling too hard. Not sure if there is a trick to removing it. The front dome light on the wagon is a 3 position switch as you say...the rear light is 2 position: either on all the time or on when the rear doors or hatch is opened and off when they are closed. Are you saying that because the 3 switches {2 rear door and 1 hatch} are wired in parallel that a break in any of the light wires going through the hatch conduit would make all of the 3 switches inop? The one wire from the front dash switch must be intact through the conduit because that one will light the rear dome light. I'm pretty dense when it comes to electrical stuff so excuse the elementary questions.
Thanks
John

Alec300SD · #11 09-24-2019, 03:09 PM

For myself, when all else fails read the manual.

Page 8 on the attached pdf shows the courtesy light circuit.
https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/ETM/123%20turbodiesel_2.pdf

As I understand it, looks like the door switches merely compete the the circuit to ground.
Sounds like you have grounding issues.

Directly connecting any of the .75 BR/GR (Brown/Gray) wires to a good chassis ground should normally cause the rear dome light to illuminate.

(Unless there is a bad contact at the off position (door controlled position) of the rear dome light switch, or a break in the BR/GY wire from the rear dome light switch to the junction block
(C136 terminal 5), or from the switches to the junction block (C136 terminal 5).

If the rear dome light does illuminate when you do this test, then you probably just have corrosion in the switch(es) preventing activation of the light.
Twirling the spring loaded plunger of the switch(es) can temporarily improve the connection.

Jareb · #12 09-24-2019, 04:59 PM

Quote:

Originally Posted by Alec300SD

For myself, when all else fails read the manual.

Page 8 on the attached pdf shows the courtesy light circuit.
https://www.startekinfo.com/StarTek/outside/12265/disc_2/program/ETM/123%20turbodiesel_2.pdf

As I understand it, looks like the door switches merely compete the the circuit to ground.
Sounds like you have grounding issues.

Directly connecting any of the .75 BR/GR (Brown/Gray) wires to a good chassis ground should normally cause the rear dome light to illuminate.

(Unless there is a bad contact at the off position (door controlled position) of the rear dome light switch, or a break in the BR/GY wire from the rear dome light switch to the junction block
(C136 terminal 5), or from the switches to the junction block (C136 terminal 5).

If the rear dome light does illuminate when you do this test, then you probably just have corrosion in the switch(es) preventing activation of the light.
Twirling the spring loaded plunger of the switch(es) can temporarily improve the connection.

Thanks for the schematic Alec. I had to look at it for awhile but I think understand what you are saying. Three questions:
1. Is the location of terminal block C136 up under the rear header panel?
2. Are you saying that if the rear hatch switch itself is bad, then the circuit for the 3 switches {rear hatch and 2 rear doors}would not work at all?
3.I will do your ground check on the brown/gray wires but could it be that the hatch switch is bad and that is the problem or am I oversimplifying it?

It may take me a month but I will find this problem!

Alec300SD · #13 09-24-2019, 06:34 PM

1) Not sure where the terminal block C136 is located.
The blue "Component Location" button is not active on the pdf at the startekinfo site.
I do have the CD-Rom version of the W123 FSM at home and can look it up this evening.

2) Yes. However, I may be misinterpreting which switch is by the bulb (in the top square of the schematic).
Since it shows always on or off as the only positions, perhaps that switch may be the dash switch?

3)Yes.

Jareb · #14 09-24-2019, 09:09 PM

Quote:

Originally Posted by Alec300SD

1) Not sure where the terminal block C136 is located.
The blue "Component Location" button is not active on the pdf at the startekinfo site.
I do have the CD-Rom version of the W123 FSM at home and can look it up this evening.

2) Yes. However, I may be misinterpreting which switch is by the bulb (in the top square of the schematic).
Since it shows always on or off as the only positions, perhaps that switch may be the dash switch?

3)Yes.

I think the box at the top right is the actual dome light with the 2 position rotary knob. The smaller box below and to the left is the dash switch. The smaller boxes {3 in a row} below that is the hatch switch{left} and the two door switches{boxes to right of hatch switch}. So.... according to the schematic, a failed hatch switch would make all 3 inop?

Alec300SD · #15 09-24-2019, 10:44 PM

Quote:

Originally Posted by Jareb

I think the box at the top right is the actual dome light with the 2 position rotary knob. The smaller box below and to the left is the dash switch. The smaller boxes {3 in a row} below that is the hatch switch{left} and the two door switches{boxes to right of hatch switch}. So.... according to the schematic, a failed hatch switch would make all 3 inop?

Yes, a failed hatch switch would make all 3 inoperable, because there would be no path to ground.

You may just have a corroded connection at the dome light switch, or perhaps a broken mechanism in the switch.
Try unplugging the connections (remove the fuse first), and cleaning the mating surfaces with a good electrical contact cleaner.

Fingers crossed that it will be so simple.