Originally Posted by Cigar Havana

I had a similar problem on a 89 420 SEL. Battery would go dead, sometimes after 2 weeks, sometimes after 1 week, sometimes after 2 days. The final time it died was on the highway, going 100 KPH, with heater on, lights on, radio on, seat heater on (it was -22c outside). When the battery started to die, the cassette player turned off automatically, next the SRS light came on, and then various warning lights came on, then the headlights started to dim.

The problem was the alternator. It was OK when it was ice cold, but after running for some time, it stopped charging. It tested good when I took it in for a test, at Canadian Tire.

I replaced the alternator, I have had no problem since replacement. The voltage at the battery terminals is now 12.8v with the car off. When idling, the voltage at the battery terminals is 14.2V

The alternator that failed was only 1.5 years old. The brushes were in excellent condition, 1/16th inch shorter than brand new. I believe my old blower motor blew the alternator, drawing too much amperage. The shaft was very difficult to turn, compared to the new motor. I do have a clamping amp meter, so I could provide current draws on the 420, if you are interested.