Actually, its the overvoltage protection relay, and it keeps voltage spikes or a runaway alternator from frying the electronics. Cuts the voltage to all sensitive circuits when the voltage gets too high.

It also tends to get "tired" and cut the current when it's not supposed to as well, probably the reason you need to replace it. Usually just cracked solder joints, but can also be burned contacts.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!