I had this happen to me last year. My light would blink on randomly while driving. I have concluded that the GP relay was bad. Around the same time I was also having charging issues with alternators going bad. Since I replaced the GP relay with one from Fastlane I have not had any issues.

My conclusion was that the relay would come on and stay on and took out my alternators. I'm not 100% certain but it has been about 6 months and none of the symptoms have come back so it seems that a new GP relay did indeed solve the issue.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD