The OVP relay is really just a fuse. It powers the systems involved. It is the ultimate in simplicity. If the system is functioning at all the OVP is obviously good.
The reason this isn't lightbulb clear is that KE systems work fine without electronic control. Cold starting is poor as cold enrichment is all electronic. Hesitations can be noticed as acceleration enrichment is electronically enhanced. There are more that one power leg of OVP which allows the combined symptom of ABS light. The ABS light comes on because the ABS control unit is not powered and turning the light off is one of the control units functions.
The way I test OVP is to monitor EHA current. No current probably no OVP. Any current, OVP is working.
The severe exception to this is the OVPs ability to act as a resistor to the control unit. I have caught OVP resistance causing a voltage drop (not complete shut-down) such that idle current (usually over 600ma) would drop to 350-450ma. This causes the idle valve to about totally shutdown the idle. Idle speed drops to 400rpm or so and dies. One of the common idle problems. This is a problem that won't happen with a good OVP or NO OVP. It can only happen when there is a voltage drop through the device.
Over the years the oVP has powered different systems and has many different syptoms. Only in the electronically controlled fuel systems does OVP become critical. An HFM system powered by OVP would be a sure nostart without OVP.
Bosch Master, ASE Master, L1
33 years MB technician