Hello again! I have had intermittent issues with my BAS/ASR light coming on. I was able to clear the issue with restarting the car. Now it is persistent so I replaced the Brake light witch with a Genuine Mercedes Brake Light Switch, and made sure to pull it out all the way prior to installing it. I also did a resistance check prior to install. 0 ohms extended, 0L when depressed. The BAS/ASR light comes on the MOMENT you touch the brake pedal, and the all 3 brake lights do not work. I also locked the wheels left to right multiple times. I'm convinced it has to due with the lights. The #2 fuse in fuse panel (driver side interior) is good. What else can I do? I have big trip coming up and this is the last thing in need. I have heard the Illumination control panel**?? can play into this? Could it be a K40 Relay? I'm at wits end and I can't find anything on the whole internet to help, the information drops off after brake light switch replace and/or the issue resolves itself.

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1999 E300D OM606, 278,000 km (Daily)
1984 Camry Turbo-D, 319,000 mi (Sold)
1992 300D OM602, ~450,000 mi (Sold, Blown Head)
1999 F250 7.3 Powerstroke, 243,000 mi (Sold)
1996 VW Passat TDI 110,000 mi (Sold) "The first Diesel"

Couple things to add after the drive home from work. Got I the car started it, touched the brake pedal and the BAS/ASR light came on, and saw that the brake lights weren't working. I noticed the voltage after start up (via scangauge II) was about 12.4 with AC on Max and headlights on. Right as I got home I noticed the brake lights were working (high mount reflection in window tint) and the voltage was now 13.2. I got home turned off the car, turned it on and Bam NO MORE BAS/ASR light and brake lights work! I'm sure the moment I hit the brakes tomorrow the lights with come back and brake lights won't work. Is my alternator not doing it's job? The battery is not even 5 months old from date of manufacture. Could less than 13 volts cause the BAS/ASR to assert?

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1999 E300D OM606, 278,000 km (Daily)
1984 Camry Turbo-D, 319,000 mi (Sold)
1992 300D OM602, ~450,000 mi (Sold, Blown Head)
1999 F250 7.3 Powerstroke, 243,000 mi (Sold)
1996 VW Passat TDI 110,000 mi (Sold) "The first Diesel"

https://www.justanswer.com/mercedes/7aqn3-2001-e320-mercedes-brake-lights-not-work-car-stops-fine.html

This is for the ML model and rely for this has a different number , but it explains more about the relays operation
https://www.justanswer.com/mercedes/7xb0p-hi-ml-nnn-nnn-nnnnmercedes-three-brake-lights.html

The relay is tied into the cruise control / abs /bas/asr and the gearbox shift lever interlink.

What year and model?

The W210 seems to have problems with underhood grounds, some have reported engines not running and other ills so I'd start there. Look for brown wires on a stud, remove and clean the eyelets and stud. I don't know how many are under the hood but there should be at least 2 studs.

12.4 at idle is low , are you getting this number from scan tool or at the battery? If getting from a scan tool, what module is reporting the voltage?

It is possible to have low voltage through a scan tool and good voltage at the battery if a ground has high resistance.

1999 E300D. I'm getting 12.2 v at start up, charges to 13.2 by the time cars at operating temp, but never past 13.2 with AC on, 13.3 with it off. The reading is via obd2 port scanner tool. I will put a mulimeter on it later today. I clranned the grounds all around the car (they were all still clean and shiny) this morning the conditions are back, no brake lights and the BAS/ASR light is back. I took a trip equal to last nights but it won't clear like it did. Only difference is last night it was 94° now it's 102°. I wonder if it's a loose connection or bad solder on the K40 relay.

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1999 E300D OM606, 278,000 km (Daily)
1984 Camry Turbo-D, 319,000 mi (Sold)
1992 300D OM602, ~450,000 mi (Sold, Blown Head)
1999 F250 7.3 Powerstroke, 243,000 mi (Sold)
1996 VW Passat TDI 110,000 mi (Sold) "The first Diesel"

I have had an issue once with a w124 having no dash warning lamps other than battery light,no glowug light or abs etc etc , the cause was badly worn brushes in alternator regulator with one of them being almost unable to make contact with slip ring.
May be worth just checking yours.

I'd need to see a wiring diagram. 96-7 cars used wires to the rear lights but 98 ish and up used CAN bus for many functions. I don't know if there is a CAN module for rear lights on 98 +.

CAN dramatically reduces the amount of wiring in a car. As an example, each door has a CAN module to run the glass / locks / alarm / side view mirrors / lighting / power seat switch. Run 2 wires for CAN communication , power and ground then everything works by sending a message rather than having a wire for each function.

Furthermore, wiring shrinks in count and gauge at the window and other switches. The power window switch now has 2 wires for CAN, power and ground.

I've had various electronic modules develop a sensitivity to high / low voltage resulting in various odd behaviors.

I think I have determined it is the alternator or k40 relay. The reason I say k40 is because when it's really hot out a bunch of items from bas to ASR to ignition to blower act up really strangly. Thr BAS/ASR clears up and the lights work after I drive the car for over about 20 min and the battery is reading 13.2 at the scangauge off the obd2 port. The battery terminals read 13.8-14.0 when running. The obd2 reads low when I first start up, like 12.2v. in reality the actual voltage at the battery is about12.8-13.0. why does the ECU read .6 volts lower? (The climate control unit reads no higher that 12.3 ever, as low as 11) Also my blower motor is surging and never seems to get to full power, sounds like my blower resistor is in store.

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1999 E300D OM606, 278,000 km (Daily)
1984 Camry Turbo-D, 319,000 mi (Sold)
1992 300D OM602, ~450,000 mi (Sold, Blown Head)
1999 F250 7.3 Powerstroke, 243,000 mi (Sold)
1996 VW Passat TDI 110,000 mi (Sold) "The first Diesel"

Well I replaced the entire blower motor assembly (thank you flEbay) and the BAS/ASR light has stayed off.....until this morning. Car started up fine no issues, drove 8 miles to the gym, shut off, worked out for an hour, start the car, no lights, tap the brakes....bam Lights on again. Restart the car to no avail. Let the voltage read 13.6 at idle tap the brake peddle and the light asserts. I'm lost guys.

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1999 E300D OM606, 278,000 km (Daily)
1984 Camry Turbo-D, 319,000 mi (Sold)
1992 300D OM602, ~450,000 mi (Sold, Blown Head)
1999 F250 7.3 Powerstroke, 243,000 mi (Sold)
1996 VW Passat TDI 110,000 mi (Sold) "The first Diesel"

What controls the brake lights? Get a wiring diagram and find out what has to happen for the brake lights to come on. Are they triggered directly from the brake pedal switch, or do they go through a relay or SAM module elsewhere in the car? You've already replaced the brake switch, now you need to go further down the line. You likely have a faulty control module or a fractured wire somewhere that's making an intermittent connection.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Took a look at the wire diagram and the brake light switch goes to the light switch/fuse panel, and the wired from the brake lights go to the light switch/fuse panel as well. I have looked at the fuse and its good but the voltage is non existent when I hit the brake pedal so it has to be another bad switch, I've gone through 2, one was a cheapo knock off, and the other was MB switch from the dealer. Next ill try to reset the connectors to the electronic brake controller. Ill also check the brake booster switch ( a TSB said if the O-ring is not present to replace the Brake Booster....

__________________
1999 E300D OM606, 278,000 km (Daily)
1984 Camry Turbo-D, 319,000 mi (Sold)
1992 300D OM602, ~450,000 mi (Sold, Blown Head)
1999 F250 7.3 Powerstroke, 243,000 mi (Sold)
1996 VW Passat TDI 110,000 mi (Sold) "The first Diesel"

You can try replacing the brake light bulbs. The system is very sensitive and mine was triggering intermittent bas/esp errors when I had a headlight bulb that was going out. I didn’t find it until the bulb blew out. Changed it and had no further problems. Since yours seems to be triggered only when you hit the brake pedal it must be something in that electrical system. It is also possible that you got a bad brake light switch from MB. I don’t recall ever seeing a procedure for testing them - just replace.

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07 E350 4Matic wagon, 06 S430 4Matic, 99 SLK 230
retired W210 (x3), W124, W126

Already changed all the bulbs with the correct wattage bulbs on the entire car. I will reorder the Brake Light Switch, but does anybody know if it can fail intermittently or would it just fail period? Also, I will be changing the voltage regulator, the voltage is never stable. it jumps all around from 13.3 to 13.9 at idle with all accessories off. never have I seen 14 volts (reading from the OBD2 Port) I will re-clean all the grounds again and hope that helps. I will also clean the Positive terminal in the engine bay.

__________________
1999 E300D OM606, 278,000 km (Daily)
1984 Camry Turbo-D, 319,000 mi (Sold)
1992 300D OM602, ~450,000 mi (Sold, Blown Head)
1999 F250 7.3 Powerstroke, 243,000 mi (Sold)
1996 VW Passat TDI 110,000 mi (Sold) "The first Diesel"

Hang a incandescent test light ( or vehicle marker light ) on both sides of the brake light switch. One should be on while the car is running and the other when brake is pressed.

Drive around to test.

Is the switch properly adjusted? If the plunger does not extend enough you won't have brake lights. Just be be sure, is this a switch that mounts to the brake peddle or a brake booster mounted switch under the hood?

I know that BAS Brake Assist has a booster mounted switch but i have not researched this system.