You know your OVP needs replacement when...
 

For my 1990 300E, I recently needed to replace my OVP because:

1. It suddenly ran rough (missing like it wasn't getting enough fuel)
2. The ABS light come on at the same time
3. The push button at the Diagnostic Trouble Code plug would not turn on the red led light when depressed

Upon inspection of the OVP, the two fuses at the top were all intact. It looked brand new, it did not rattle. I replaced it with a more used looking one that rattled... and worked like a champ. Everything above was solved!