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06-26-2009, 02:30 PM
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OVP Relay -- When does it trip?
Last model (1985) W123, all W124, and I suppose W126 too have an Over-voltage Protection (OVP) relay to protect some of the electronics in the car from a failed alternator regulator. When the system voltage gets too high, a zener diode in the OVP goes into conduction, causing a short circuit that blows the built-in 10 Amp fuse. That cuts off power to the protected circuits.
Curious Jeremy wants to know: at what voltage does the zener go into conduction? 16 Volts? 20 Volts? Higher? Mercedes doesn't say in any spec I've ever seen and I've never read of anyone testing an OVP by turning up the voltage until the fuse blows or taking the OVP apart and putting the zener on a transistor tester. Anyone know?
__________________
 "Buster" in the '95 Our all-Diesel family 1996 E300D (W210) . .338,000 miles Wife's car 2005 E320 CDI . . 113,000 miles My car Santa Rosa population 176,762 (2022) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970 
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