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Old 02-18-2021, 08:04 PM   #1
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Default 2006 Sprinter Brake Lights on all the time

I came home from running errands the other day and I noticed the brake lights were on my Sprinter like someone was pushing on the brake pedal. Fortunately there was not anyone in the camper van, the lights were on all by themselves. I disconnected the battery and for good measure I also replaced he brake light switch (even though old one seemed to work as it should). Reconnected the battery and lights remain on.

I checked all the fuses, all test good. When I remove fuse 8 the lights go out, which I do understand either.

Has anyone else had this problem?

Help!
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Old 03-04-2021, 04:19 PM   #2
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Quote:
Originally Posted by Rtnmead View Post
I came home from running errands the other day and I noticed the brake lights were on my Sprinter like someone was pushing on the brake pedal. Fortunately there was not anyone in the camper van, the lights were on all by themselves. I disconnected the battery and for good measure I also replaced he brake light switch (even though old one seemed to work as it should). Reconnected the battery and lights remain on.

I checked all the fuses, all test good. When I remove fuse 8 the lights go out, which I do understand either.

Has anyone else had this problem?

Help!
Do they stay on when the switch is disconnected?
AND
You said ".. which I do understand either... "
Do you mean do or don't understand?
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Old 03-04-2021, 04:33 PM   #3
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Is fuse 8 identified as the fuse feeding the brake lights.(Owners manual?, etc?)?
If not, what is it supposed to feed?
Might it feed the Aux power to the trailer plug (for example) suggesting the trailer plug is shorted.
We need more information from you to troubleshoot.
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Old 03-04-2021, 04:57 PM   #4
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Quote:
Originally Posted by Rtnmead View Post
I came home from running errands the other day and I noticed the brake lights were on my Sprinter like someone was pushing on the brake pedal. Fortunately there was not anyone in the camper van, the lights were on all by themselves. I disconnected the battery and for good measure I also replaced he brake light switch (even though old one seemed to work as it should). Reconnected the battery and lights remain on.

I checked all the fuses, all test good. When I remove fuse 8 the lights go out, which I do understand either.

Has anyone else had this problem?

Help!
When you say ".. (even though old one seemed to work as it should).."

Did you check if the ajustment was to the needed depth such that the switch is indeed disengaged when the brake pedal is in the relaxed position?

Try unplugging one of the wires to the switch.
If brake lights go out, adjustment is incorrect.

Use a meter or trouble probe; see if power is or is not occuring through the switch.

Is brake pedal not reracting all the way and thereby keeping the switch engaged?
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Old 03-04-2021, 07:28 PM   #5
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Brake light switch has one normally open contact one normally closed. For brake lights and cruise disengage. Easy to think it is fine when they both act the same. There is actually a travel adjustment inside the switch which will fixit until you can get a new one. Dealer cost is stupid $70 Aliexpress about $12cad.
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Old 03-04-2021, 08:25 PM   #6
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Dealer switch comes from Germany
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Old 03-12-2021, 04:56 PM   #7
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now on the second new switch. one replaced by me, one replaced by the dealer. interesting comment on the inclusion of the cruise control cause it quit working too. In other words, it seems the brake light problem does disable the cruise control.
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Old 03-14-2021, 01:20 AM   #8
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Default 2011 Sprinter (2012 Forest River) with same issue

I think it's CAN bus problem. The CAN bus turns electrical things on and off, and the decoder in each light bulb decides if it's been told to turn on and off.

Mine only dimly lights up a few LEDs on one side, rather than the whole brake light.
And sometimes they come and go, so it's not the main brake switch.

I don't know how the CAN bus addresses any individual light bulb, but try swapping the bulbs with the other side and see if the problem follows a bad bulb.

Did someone replace your CAN bus bulbs with a standard bulbs or LEDs without CAN bus?


Edit: I just checked mine, and today they are off. Maybe because it's been cold, or maybe because some days more moisture condenses inside the lens cover. It's a mystery so stay tuned!
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Old 03-14-2021, 04:19 AM   #9
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I think it's CAN bus problem. The CAN bus turns electrical things on and off, and the decoder in each light bulb decides if it's been told to turn on and off.

Mine only dimly lights up a few LEDs on one side, rather than the whole brake light.
And sometimes they come and go, so it's not the main brake switch.

I don't know how the CAN bus addresses any individual light bulb, but try swapping the bulbs with the other side and see if the problem follows a bad bulb.

Did someone replace your CAN bus bulbs with a standard bulbs or LEDs without CAN bus?

Edit: I just checked mine, and today they are off. Maybe because it's been cold, or maybe because some days more moisture condenses inside the lens cover. It's a mystery so stay tuned!
Sprinter light bulbs are NOT directly CANbus controlled. They are plain old bulbs that are controlled by an ECU, which monitors their current consumption in order to detect failures. The reason LEDs often don't work properly is that they don't draw enough current for the ECU to think that it is operating properly.
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Old 03-14-2021, 04:38 AM   #10
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Quote:
Originally Posted by Road Toad View Post
Do they stay on when the switch is disconnected?
AND
You said ".. which I do understand either... "
Do you mean do or don't understand?
Sorry for the delay in response. I meant to say I do not understand why removing fuse 8 caused the brake lights to go out. the dealer said the fuse box under the steering column had a short and they replaced it. Brake lights were working as they should until yesterday when I did a 100 mile test drive, when brake light came back as well a check engine light, disconnected the battery, checked that all fuses were seated, reconnect battery the next day. After reconnect brake lights and check engine are working as they should.
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Old 03-14-2021, 06:22 AM   #11
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Sprinter light bulbs are NOT directly CANbus controlled. They are plain old bulbs that are controlled by an ECU, which monitors their current consumption in order to detect failures. The reason LEDs often don't work properly is that they don't draw enough current for the ECU to think that it is operating properly.
That's useful information. And now it's more puzzling why a few LEDs on my rig glow sometimes. It's not a reflection from the sun, because I see them at night!

And my brake lights are not Mercedes parts. They're something Forest River installed during the RV conversion to a Class C.

Ugh... ECU sounds like a much more expensive fix.

I just noticed the OP is in Washington State. So was I when my problem first started. Both of our issues might be a moisture problem. Maybe it's time to dry out some connectors.
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Old 03-14-2021, 02:50 PM   #12
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And now it's more puzzling why a few LEDs on my rig glow sometimes.
This is very common. It is caused by the presence of a small continuous current being provided to the bulb. Too small to matter with incandescent, but enough to make the LED glow a bit.

There are two main reasons for this: (a) the use of solid-state relays instead of relays in the ECUs. Some of them have slightly too-low impedance, allowing some leakage. (b) A small intentional current, which permits the detection of open-circuits even when the bulb is off.

Either way, the fix is the same: You need a small (maybe 10K) bleed resistor in parallel with the LED. You can buy automotive LEDs with this feature built in, or you can do it yourself. The commercial ones are often called "CANbus LEDs", but they really have nothing to do with CANbus--it is marketing hype.
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