The OVP is the Over Voltage Protection relay which sits up behind the battery behind the black plastic cover under your windshield.

Its about 3 inches high and has a fuse on the top of it.

They are a known weak spot in the w124/w202's as they had a design fault from new. The replacements which cost about $60 to purchase give no further problems and take about 5 minutes to replace once you have removed the battery.

The OVP protects your ECU and other electricals from getting fried by too much voltage going through the system.

I'm kind of worried at how much money you may have spent on a new MAF/ ECU etc, as the OVP is often the culprit and is cheap to replace.

Good luck

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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