Over Voltage Protection relay....a common cause of hard starts and the relay is known to fail with age due to dry soldered joints...if your car is still on its original it will fail....its only a matter of time.

I now replace them as a matter of course every time I get a M104 engined Merc, as prior to that they always used to give me trouble.....easy to replace and cost about $60

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Current Mercs
R129 1994 300-24SL
W124 320E Turbo Technics Twin Turbo 350bhp

Previous Mercs
W124 1986 Quad Cam AMG Hammer, 45,000 miles
W124 300E Turbo Technics Twin Turbo 320bhp
W124 E500