BAS ASR light on
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Newbie
Joined: Aug 2019
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W202 C240
BAS ASR light on
Having read all the advice on the BAS ASR light there seems to be a number of solution but the symptoms to not seem to match my issues.
I have a W202 C240 and some time ago the BAS ASR light came on during each journey when I first put my foot on the brake and then stayed on. My garage rectified the problem with a new brake light switch. After the switch was fitted it came on two or three times but then seemed to rectify itself. Approximately 6 months later the light came on again but this time it is on all the time. The diagnostic says it is the brake light switch but this was replaced and it is still on. The wiring to the control module has also been checked and found to be OK and the rear brake lights operate correctly. The cruise control sometimes works and sometime doesn't and at the moment it does not.
I was told it could be a faulty earth connection onto the brake light switch but then I read that the switch contains two contacts, one normally open and one normally closed - no cable from the switch goes to earth.
It is my understanding that the cruise control will not activate if the brake light switch is faulty as it needs to be able to disengage the cruise when the brake is applied. This again points to the brake light switch.
The battery has been suggested but I am unsure of the voltages I am looking for. I have checked it with 2 devices each of which which gives a slightly different reading. A voltmeter on the battery itself gives 12.1 volts with the engine not running and the other device plugged into the cigarette lighter shows 11.9 volts. With the engine running it is 13.8 volts. Is this likely to be a problem? Are the wheel sensors likely to be the problem if the diagnostic is saying that it is the brake light switch?
What other issues will give a brake light switch error code?
Any help really appreciated
I have a W202 C240 and some time ago the BAS ASR light came on during each journey when I first put my foot on the brake and then stayed on. My garage rectified the problem with a new brake light switch. After the switch was fitted it came on two or three times but then seemed to rectify itself. Approximately 6 months later the light came on again but this time it is on all the time. The diagnostic says it is the brake light switch but this was replaced and it is still on. The wiring to the control module has also been checked and found to be OK and the rear brake lights operate correctly. The cruise control sometimes works and sometime doesn't and at the moment it does not.
I was told it could be a faulty earth connection onto the brake light switch but then I read that the switch contains two contacts, one normally open and one normally closed - no cable from the switch goes to earth.
It is my understanding that the cruise control will not activate if the brake light switch is faulty as it needs to be able to disengage the cruise when the brake is applied. This again points to the brake light switch.
The battery has been suggested but I am unsure of the voltages I am looking for. I have checked it with 2 devices each of which which gives a slightly different reading. A voltmeter on the battery itself gives 12.1 volts with the engine not running and the other device plugged into the cigarette lighter shows 11.9 volts. With the engine running it is 13.8 volts. Is this likely to be a problem? Are the wheel sensors likely to be the problem if the diagnostic is saying that it is the brake light switch?
What other issues will give a brake light switch error code?
Any help really appreciated
Junior Member
Joined: Dec 2017
Posts: 46
Likes: 9
1999 C230K
Does your car have OBDII? If a wheel sensor is bad it should give you a P500 code, or at least show up on the diagnostics you've run. You could try to reset the brake light switch by fully extending the plunger. When installed, the plunger should be all the way out. A lot of shops don't know this and don't do it when replacing the switch. Also, the brake light switch could be the wrong part number. Even though they're all on-off type switches, MB has different part numbers for different models and I've found they don't all work the exact same (possible different voltage levels).
Thread Starter
Newbie
Joined: Aug 2019
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W202 C240
My garage tell me it is just giving a brake light switch error and they cannot extract the more detailed information available on the later cars. I have tried resetting the brake light switch, as you suggested but the light remains on.
I assume they have fitted the correct brake light switch but for the cost of a switch it may a good idea to replace it again. The garage seem to be going towards the control module itself which they say is too expensive to consider buying new. I have looked around and second hand units are available but it seem that I will need to buy the control module, hydraulic block and pump together and use just the control module - assuming it easily comes apart.
I assume they have fitted the correct brake light switch but for the cost of a switch it may a good idea to replace it again. The garage seem to be going towards the control module itself which they say is too expensive to consider buying new. I have looked around and second hand units are available but it seem that I will need to buy the control module, hydraulic block and pump together and use just the control module - assuming it easily comes apart.
Junior Member
Joined: Dec 2017
Posts: 46
Likes: 9
1999 C230K
If it worked for 6 months it's probably the right part. When my BAS ASR lights came on I went to my local auto parts store (Autozone) thinking it was a simple replacement. The new switch worked for a little and then the ASR light came on. Carsoft showed a bad brake light switch. The Autozone switch looked just like my old one but it was the wrong part number. I got the right part number from EPC, put in the right part, and it's been fine since. What diagnostics are you using? It it's showing a bad switch it's either the switch or the wiring, especially since you said the light came on a few times. I would think that if the control module was bad the diagnostics would show it. You could try checking the connections to the control module and tugging lightly on some of your underdash wiring to see if the light goes off. What year is your car?
Thread Starter
Newbie
Joined: Aug 2019
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W202 C240
All the diagnostics have been done by my garage who have checked the wiring (twice) and say its OK but I will do as you suggest and tug lightly on the wires.
I have only started looking at it myself as the garage do not seem convinced about the cause of the problem. I have spoken to somebody else who has done a lot of work on Mercs and he is pointing me back to the brake light switch especially now that the cruise control is not working. He also suggested that these modules are very voltage sensitive. I have been trying to locate a wiring diagram but to date have been unsuccessful.
My car is a 1998 C240
I have only started looking at it myself as the garage do not seem convinced about the cause of the problem. I have spoken to somebody else who has done a lot of work on Mercs and he is pointing me back to the brake light switch especially now that the cruise control is not working. He also suggested that these modules are very voltage sensitive. I have been trying to locate a wiring diagram but to date have been unsuccessful.
My car is a 1998 C240
Junior Member
Joined: Dec 2017
Posts: 46
Likes: 9
1999 C230K
You're better off replacing the switch. If it fixes it, good. If it doesn't, at least you know it's not the switch, and it might be the wiring. The switches are fairly inexpensive and you'll have a spare. I'd rather replace a switch, and know if it's good or bad, than waste time looking for a wiring problem that may or may not exist. If you're going to a garage it's probably costing money on every visit. Good luck.
