PeachParts Mercedes-Benz Forum - View Single Post

EricSilver · #33 09-29-2008, 01:08 PM

For me, this is distilling down into a voltage issue. Whether it is an Over or Under voltage is unclear, but arrows point to the voltage regulator and/or battery.

My car was parked for 2.5 days this weekend; conditions were very humid with heavy rain on Sunday. Sunday night I correctly assumed the ASR issue would return, so I drove the car and made these observations:

ASR fault appears to be triggered by a change in voltage, whether from A.) An electrical load being applied, i.e. the brake lights; or B.) A diminution of voltage/current from the alternator, i.e. foot off the accelerator, and engine/alternator revs dropping below an approx 550rpm threshold, and concurrent drop in voltage/current.

ASR illumination is accompanied by the click/ping of a relay (OVP?), audible in the center dash blower area (but on one occasion it sounded like it was coming from behind me).

The fault took MUCH longer to clear this time and I initially thought, “It finally failed for good.” But then the Common Sense Fairy pointed out, “This is your first drive in over 2 days, it is the first time you are expereincing it at night so your head & tail lights are on, you have the blower motor running, and are drawing a large electrical load – not optimal conditions for trickle charging a battery -- assuming low battery voltage is the cause.”

When the condition finally cleared, after about 30 minutes of driving (compared to the typical 1-5 min), I felt the urge to hear some music and turned on the CD player. Approaching a red light, I applied the brakes, adding electric load, and click/ping went the relay and on came the ASR. Turned off radio, drove a bit further, restarted, and the fault cleared as usual.

Evidence suggests weak battery and/or worn voltage regulator. I don’t know how the stop light switch circuitry works, beyond illuminating the stoplights, so I won’t rule it out as a cause either.