Hello

First, there is no inline fuse with the brake light powering.
Second, you may not have voltage on the right brake power connector, if the wire is shorted somewhere. I assume you are measuring the voltage at the connector or the metal strip that brings power to the bulb with the bulb inserted.

1) make sure you use the correct bulb type . Switch bulbs with bulbs holders around, and see if the problem moves with the bulb+bulb holder.

2) make sure that the bulb is properly inserted. Sometimes an incorrect insertion will hold the bulb in the bulb holder socket, but will cause problems like yours. Sometimes it also causes the 3rd brake light not to work (I am not saying this is the cause, but it may be related).

You should also measure the resistance between the chassis ground and the right brake wire connection with the key out. It should not see a short circuit.

3) fix the third brake light. Check the bulb and check that the assembly contact springs that slide on the flat contacts are not burnt or corroded. I recently had to re-shape the spring contacts so that they have better contacts with the base metal strips.

The headlight control module (where the headlight knob is) generates the power for the brake lights, and if it sees a short in the line, it turns off the voltage for protection.

 

Hello

First, there is no inline fuse with the brake light powering.
Second, you may not have voltage on the right brake power connector, if the wire is shorted somewhere. I assume you are measuring the voltage at the connector or the metal strip that brings power to the bulb with the bulb inserted.

1) make sure you use the correct bulb type . Switch bulbs with bulbs holders around, and see if the problem moves with the bulb+bulb holder.

2) make sure that the bulb is properly inserted. Sometimes an incorrect insertion will hold the bulb in the bulb holder socket, but will cause problems like yours. Sometimes it also causes the 3rd brake light not to work (I am not saying this is the cause, but it may be related).

You should also measure the resistance between the chassis ground and the right brake wire connection with the key out. It should not see a short circuit.

3) fix the third brake light. Check the bulb and check that the assembly contact springs that slide on the flat contacts are not burnt or corroded. I recently had to re-shape the spring contacts so that they have better contacts with the base metal strips.

The headlight control module (where the headlight knob is) generates the power for the brake lights, and if it sees a short in the line, it turns off the voltage for protection.

Thanks for your help, I will measure the resistance without the key in. I swapped out the brake light bulb and socket from the driver's side that works, and it did not work on the passenger's side which is when I moved on to check the wiring. MB doesn't use Lucus wiring do they?
Thanks again, I'll keep you updated.

 

Okay, so lastnight I was set to start looking for a short in the brake light wire, when I decided to test it again with only the running lights on (I just turned the headlights on), because the brake light light bulb does work with the lights on. So I found the suspect wire does have power with the lights on, just not when the brake pedal is pressed. This means the wire is good and has no shorts! Now, would this be a faulty brake light switch then? I mean the driver's side brake light DOES work, but can a bad switch cause one light not to work but allow the other one to work? Most of the switches I've worked with are either all or nothing.
Thanks,
Scott

 

I assume you have no "defective bulb" message with the tail lights on, but it appears when you depress the brake pedal ?

The brake light switch does not directly power the brake lights. It generates a closed contact to the headlight switch module which in turn applies power to the brake light bulbs individually. If you have no closed contact from the brake light switch, you would have no brake lights among other many indications (BAS, ESP, ABS etc.).

I would check for the common ground connection to the chassis from the right light assembly for loose / corroded connections.

Also what happens when you press the brake pedal with the tail light is NOT turned on ? Do you still see the voltage on the brake wire when the brake is NOT applied, and no voltage when brake is applied ?

 

The light bulbs are all new, so that's not the problem. The 'defective bulb' message is displayed when I put the car into gear or turn the headlights on. I can however sit and press the brake pedal with the car in Park and the message does not come on. This message has been doing this ever since I bought the car. On that note, the small front headlight on the passenger's side only works with the brights on or when I lock the car. I have replaced that harness with a mew one, but it didn't fix anything. I was told the 'defective bulb' message was coming from the headlight. At this point, I'm thinking that the problem may lay in the headlight switch if that supplies power to the brake lights, and the headlight on the same side isn't operating properly. This car is turning into an electrical and vacuum nightmare (yeah, I have vacuum issues too).

 

Does the passenger inner light work now (the small 6w bulb) ?
If it does and you do not see any problem with the lights in the front or at the tail, make sure that the licence plate lights work with headlights on. There are two. If one goes, then you get the defective message with one or both bulbs bad.

You need to run the engine for the defective light message when brake is applied (I think).

There is really no relationship between the headlights and brake lights. Each bulb is independently powered and monitored.

Have you checked the ground connection around the tail light assembly ?

 

All of the bulbs in the right tail light work, so I don't think there is a grounding problem. The only one that does not work s the actual brake light.

The front inner right side headlight still does not function properly., which is why I am wondering if the switch is bad. I realize each bulb is independent, but if they are all controlled by the same switch, that might be the problem.

I will check the licence plate lights. I think they work, but I'm not sure if they do or not.

One day I will get this thing working again, and it will be glorious!

 

The front inner right side headlight still does not function properly., which is why I am wondering if the switch is bad. I realize each bulb is independent, but if they are all controlled by the same switch, that might be the problem.

The headlight wiring insulation gets brittle over time. I had the same problem. I replaced the small bulb at the passenger side, and it still did not work. Somewhat the wire that powers the small bulb was broken. I repaired it and it worked. Some people have had problems with the bulb not working, even with a new bulb, and intact wiring. There is a fusible link in between one of the wires to the small bulb socket, and it can open up. In that case it needs to be replaced with a suitable fuse, or the whole new headlamp wiring harness.

Regarding the brake light issue, as I said the power is supplied from the headlight switch module (actually called Illumination Control Module or ICM). The ICM is not just a switch but it has lots of electronics and solid state switches and protection circuits against shorts. It measures current drain for each line, and compares it against the thresholds. So a short , or an open in the line is signaled as the defective bulb to the Instrument cluster. It is quite possible that the circuit for the non-working bulb is bad, but I would make sure first that the problem does not lie somewhere else.

 

Hi mrboca,
I have a '97 E320 sedan. Only 65K miles. About a month ago, "Lamp Defective" message came up from time to time and it would disappear when I restarted the car.

About a week ago, this same message appeared on the console whenever I pressed the brake pedal. I checked all the lights around the car, all good except the right/passenger side brake/stop light. I found the smaller/lower filament burned in the bulb. So I replaced it with a 7225L bulb as the old one at O'Reilly. Nonetheless, the message persists.

So I switched the good left bulb/socket to the right side, and it burned out the lower/smaller filament. Then I experimented with combination of light bulbs and sockets at both sides. None of them worked. Am puzzling what should I try next? Any advise will be appreciated.

Thanks,
William O.

 

Hello .

So you put a good bulb from the left side, and installed in the right side, then the brake light filament burned immediately?

It must be from old age. Always replace bulbs in pair.

I suggest start with a new pair of bulbs, and make sure the bulbs are inserted to the bulb holder sockets properly. They are DC offset indexed bulbs, and sometimes you think it is inserted (and held by the socket) properly, but the contacts at the bottom of the bulbs do not touch the correct spring loaded contacts.

Install the sockets with the bulbs into the receptacles.

Then turn the headlights on. You should see the tail lights turned on with the same intensity at both sides. You should not have "lamp defective" warning. If you do, then the bulbs were not inserted properly.

Then turn the headlights off, and have someone press the brake pedal (with key in position 2), and observe the brake lights. You should not have any lamp defective warning and both lights should illuminate at the same intensity. Make sure the center brake light is also turned on (this will not trigger the lamp defective warning). If one of the brake lights is not turning on, then you have a problem with the receptacle in the brake light assembly, which is quite common (broken plastic studs that hold the contacts in place).

 

Hello .

So you put a good bulb from the left side, and installed in the right side, then the brake light filament burned immediately?

It must be from old age. Always replace bulbs in pair.

I suggest start with a new pair of bulbs, and make sure the bulbs are inserted to the bulb holder sockets properly. They are DC offset indexed bulbs, and sometimes you think it is inserted (and held by the socket) properly, but the contacts at the bottom of the bulbs do not touch the correct spring loaded contacts.

Install the sockets with the bulbs into the receptacles.

Then turn the headlights on. You should see the tail lights turned on with the same intensity at both sides. You should not have "lamp defective" warning. If you do, then the bulbs were not inserted properly.

Then turn the headlights off, and have someone press the brake pedal (with key in position 2), and observe the brake lights. You should not have any lamp defective warning and both lights should illuminate at the same intensity. Make sure the center brake light is also turned on (this will not trigger the lamp defective warning). If one of the brake lights is not turning on, then you have a problem with the receptacle in the brake light assembly, which is quite common (broken plastic studs that hold the contacts in place).

Hello mrboca,
Followed your instructions with two brand new 7225L bulbs, pushed/fitted the bulbs into the socket, visualized solid connections, and reinstalled the sockets into the receptacles. .

Turned the headlights on with engine off. I observed both tail lights turned on with the same intensity

. With that I assume the light bulbs, sockets, receptacles and their connection are all good.

Then I turned the headlights off, turned the key to 2nd position (which asks you to fasten seat belt, then shows yellow temp/miles/time). Signal works on both sides and No lamp defective warning. Pressed on the brake, the passenger brake light is still off

. Sigh!? What other step should I try before I take it to the MB shop?

Thanks,

William O.

 

If the socket+bulb switchover gives you the same results, the problem is likely a bad contact area in the tail light assembly receptacle.

With someone pressing the brake pedal with key in position 2, hold the installed brake light socket at the passenger (non-working) side with your fingers and gently tilt it towards and away from the receptacle, and observe if the brake light momentarily comes back as you tilt. If it does, you have a contact issue with the receptacle.

Take a picture of the contact area, and then gently sand the contact areas at the receptacle. In some cases the plastic posts break or melt away with the raised contact temperature, and this causes a bad or no contact with the socket tab.

Have a look at the picture.

 

Hi mrboca,
Alright, I went to O'Reilly auto parts store this morning for a new socket which I suspected of the culprit. The assistant manager, Pam ordered it and said it would be ready later this afternoon. At the store, with Pam's help, I decided to switch these tail brake lights/sockets and have her checked these tail brake lights while I pressed the brake. The "Lamp Defective" sign persisted with the passenger brake light still failed. So I asked her to wiggle the socket a bit and see if that might help with a bad socket/receptacle connection. Then, all of a sudden, she said both brake lights were on and I turned the engine off and on again, and that nasty warning disappeared for good. Keep my fingers crossed.

(edited)

Thank You Very Much!! You are the BEST!! Really Appreciated your help. This really makes my Day!!

William O.

 

I am so happy for you too . Thanks for reporting back. At least you know where the problem area is.