The OVP is not a relay, but rather a multi-circuit, circuit breaker. At least that's my impression of it's purpose.

This device has received as much attention here as any item I can recall.

You mention a 420SEL, but do not mention the year. I do not know if this applies in your case, but 300SELs from 88-89 used a fuel pump relay that was near the battery. 90-91s had this relay incorporated into the MAS unit.

Your best bet now would be to hit the SEARCH facility where you'll find a wealth of knowledge.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"