The belt routing you describe is incorrect.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Yes sir, it sure is. I found the same in the manual. Will reroute it soon.

I’ve had this car for a long time and do all maintenance on it. I more or less know every nut and bolt on it, but have no idea when this belt was put on like that.

The belt routing is definitely not intuitive and if you have the 603 with the "large" water pump pulley, it's a proper PITA to get the belt between the pulleys to get to the tensioner pulley. Because it is possible to route the belt incorrectly and still have it drive the pulleys, it's a fairly common thing to do. The problem, as you found out, is that the crank pulley has a very small contact area and slips on the belt when the load increases. Given the aluminum head's sensitivity to overheating, it's a good idea to fix sooner rather than later.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Thank you for the comforting words
Makes me feel a little less stupid, but still quite.

I’ve changed the head twice during the years, so I do know about the over heating issues. It has a #20 now, that I hope will last. I have a #22 on a spare motor. I didn’ see it over 105C (red mark is 128, normal is 82, and the fan start at 102. Water boils at 100).

Long story short: rerouted the belt, now it charges very well. Maintains 14,4 at idle with lights on and fan at 2. At 1000 rpm it is above 14V with all consumers on.

I don’t know if I shuld cry or laugh. A bit of both I guess.