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#1
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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#2
06-26-2009, 02:46 PM
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measure it yourself
If it was me in need of that information I would measure it myself. If you have an adjustable powere supply and a voltmeter then you can do it yourself. At least it will tell you the specification of the unit you have.
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2010 ML350 Bluetec 2012 Mustang Convertible
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#3
06-26-2009, 03:21 PM
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Lord Dorwin said it best . . .
"But wheah's the necessity? It seems an uncommonly woundabout and hopelessly wigmawolish method of getting anywheahs." -- Lord Dorwin, Chancellor of the Empire, in Foundation, by Isaac Asimov [1951].
But if you insist . . . I put my spare OVP on a current-limited power supply and turned up the voltage until the zener went into conduction at . . . 22 Volts! Much higher than I expected. OTOH, the diagram on the side of the OVP shows the zener and it is labeled "Z22." I thought it was just a part number but looks like it is the zener's rated voltage. Jeremy
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"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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#4
06-26-2009, 04:31 PM
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Note
It also should pop if the car is reverse poled during charging or jump starting.
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Linear Mode
