I changed the belt (it was dry rotted and slipped when wet anyway), but the problem remains.....I think it is likely a bad voltage regulator, and I want to see if anyone on the forum can validate this guess...

It glows dimly at idle and gets brighter as I press the throttle.

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT