BAS ASR Light - Brake Light Switch Issue?
 

Wife was driving home tonight on icy roads, hit brakes and the BAS/ASR Light came on and stayed on.

Specifics:
98 E300
No battery issue
No OBDII Codes
Never had problem before and no other recent problems/changes.
Would not reset with the turning the steering wheel side to side.

I read a suggestion that it is likely a brake light switch, $12. Any thoughts?

Turn off the car and turn back on...is the light still on???
If not...hit the brake...does the light then come on???
if so ..its the $14 switch...just did mine last week...easy...15 minutes..

No, the light comes on when I turn on the car. Interestingly, if I start the car while holding the brake down the light does not come on until I release the brake. Also, while holding down the brake, if I hit the ASR switch the Yellow Triangle warning light comes on, and if I hit it again (still holding brake pedal down), then the Yellow Triangle warning goes off. Finally, I let up the pedal and I get the light.

Sounds like a power issue to the ABS/ASR unit. I would start with the relay located under the black plastic cover on the unit itself. Usually if the brake pedal switch is the problem you would have a code in the traction computer for it. If the scanner used only checked the engine computer it may not show up. Make sure you check the traction computer!!

Thanks BENZ TEK

I only have the generic OBDII Scanner and it is showing no codes when connected to the port by the drivers knee. Can I use this scanner for the traction computer or is it time for a real MB Mechanic? If I can use it on the Traction Computer (BAS/ASR module I assume) what is the procedure?

Cruise control does not work.

Is the relay in the main fuse box in the engine compartment? Should I try a relay replacement and brake switch replacement before the dealer?

OBD-II does not expose the BAS/ASR system. Cruise control not working is likely related.

Before you do anything, replace the brake lamp switch. OEM ONLY! Be sure to pull the plunger on the new switch when you install it.

Thanks, Matt, will do. I've pulled the dash apart.

I pushed the light switch plunger in and then pulled it back out. No joy, I will replace it as soon as I get the part. I don't feel bad replacing it since it seems to be a 'consumable'.

I may call me independent MB Tech to get a read on the modules. Also considering a Carsoft reader. I'll start with the switch, the car is functional.

Probably the pressure release switch. On your car I believe it's on the bottom of the booster asmbly, and part of the brake controller system.

This release switch has to coordinate with the brake light switch on the brake pedal. If more than 50ms off, it will flag BAS. WIS will show the correct problem. If it's the release switch, I don't know of a fix other than module replacement. They can be had at junkyards if you know what you're after.

If you're talented with a diode scope, you can touch resolder the module TTL attach points to the original board and save $800

Note that newer MBs don't have a brake light switch at all, instead using pressure averages for computer controlled braking.

Note2 One time I replaced 4 brake light switches before I got a good one

Solved for now
 

Installed the new brake light switch and the BAS/ASR lights are off for the first time in 5 days. Most likely the problem is solved. I'll leave the dash apart a few days.

Problem/Symptoms were as follows:

98 E300

No battery issue

Not in limp home mode

No OBDII Codes (checked engine codes only)

Never had problem before and no other recent problems/changes.

Would not reset with the turning the steering wheel side to side.

Cruise control would not work.

BAS light comes on when I turn on the car. Interestingly, if I start the car while holding the brake down the light does not come on until I release the brake. Also, while holding down the brake, if I hit the ASR switch the Yellow Triangle warning light comes on, and if I hit it again (still holding brake pedal down), then the Yellow Triangle warning goes off. Finally, I let up the pedal and I get the light.

Light was always on when starting the car except if I held foot on the brake.

Tried to shim the old switch (pulled out stem and put a 1/32" shim between the switch and the pedal), no affect.

Brake lights always functioned correctly.

Thanks Guys - always figured it was a 90% chance it was the switch. Shopforum works again.

I tore apart the switch. It looks like the square gold color switchplate on the right completes the brakelight circuit which is the larger connector. The one contact is oxidized so I thought it might be the problem.

The failure was the small gold contact that broke off of the contact in the bottom of the switch housing. The silver contact in the lower right makes contact with it. Evidently my circuit was never closing (small connector).