Checking the OVP relay
After my 300E's repair (O2 and OVP), the mech gave me the OVP to play with. Checking is easy. Look at the schematic on the side. Check the diode paths with an ohmmeter (low resistance forwards; backwards they are high ohms). Next check relay for *open* (roughly infinity ohms.) When I tore my old one open, and played 12V on the relay coil, the ohms through the relay contacts went from zero, to infinity, then zero to 1.5 megs (!!!), then zero then infinity. It is this 1.5 megs that caused the electronics problems. Relays sometimes get what I call whiskers across contacts (crud, microscopic metal, dirt, etc...) And no way to open/clean/reassemble this charmer! Just replace.