I have a 2013 ML350 (W166) with the Bluetec Diesel. The brake lights are stuck on whenever the Ignition is on. I can make them function normally when the brake light switch on the pedal is unplugged. However this has other problems like cruise control no longer functions. I found a similar thread on the site but for a different car and my wiring harness does not match. Here is that link: https://mbworld.org/forums/e-class-w212/849180-brake-light-stays-all-time.html
My problem acts the same, however, of my 4 wires with switch unplugged I show battery voltage on 2 and a good ground on 1...I believe with switch unplugged that I shouldn't see battery voltage on either sensor wire. So I presume it has a short to power somewhere. But it would be very helpful if I had a wiring diagram to verify which should be power, ground, and signals.

I have replaced the brake light switch twice with an OEM switche then a rock auto switch. Also I tried a salvage ABS module. All with no change. I suspect the problem is the rear SAM has a short to power for the pedal actuated brake light switch signal wire.

Thanks, Neil

 

Check for water ingress in the rear luggage and spare tire area.

When were the main and aux batteries last replaced?

 

We drove the car for about a year with the switch unplugged so brake lights would function properly using the ABS pressure triggered brake light switch. From what I've read on the W166 the rear SAM is located behind the center console under the carpet.

I'll be replacing the main battery since the car has sat for 2 years due to a oil cooler leak. I finally found a shop willing to tackle that job. But I would like to fix the lights first since the oil cooler leak is a costly repair.

Is it possible a weak auxiliary battery could be the issue? The main functioned well for a year before parking it.

Neil

 

Sounds like this car has a lot of deferred maintenance and problems. I wouldn’t rule out rodent damage to the wiring if the car sat for 2 years. Read codes, throw parts at it and report back.

 

To be clear these issues are why the car was parked. It has been a real challenge to find a shop including the dealership to address either problem.

Does anyone have access to Alldata or other programs to provide the wiring diagrams? Wiring issues such as rodent damage are particularly difficult to diagnose without a diagram.

I do not think either signal wire of the brake light switch should be seeing battery voltage. However, without a diagram I can't be sure which SAM the switch communicates with. Some cars show rear SAM others show front SAM.

 

You can easily get pro-active to fix this brake light under 10mn!!

Simply scan your car to read status of brake switch inputs. One of the SAM is going to scream at you with a active fault. Nice and simple

If you are inquisitive, you can subscribe to alldata with your VIN for access to a challenging schematic. It has everything you need spread across half a dozen pages.
The pin out will be useful for testing.

I would concider a basic footwell GND near top of your short list.

 

You need WIS/EPC and a XENTRY system. User @BenzNinja can point you in the right direction.

WIS/EPC is the dealer-level service manual and parts catalog. XENTRY is the dealer-level diagnostic system.