Since the OVP is a surge protecting power providing relay for a number of systems its overall operation can be verified by the fact that any of the systems are working.
A number of simple observations can tell a lot. Had a 500SEL on Tuesday that had the ABS light on and no power and was idling at 1500. My service writer handed me an OVP and the keys. Took all the fun out of figuring it out. In his case the car also had poor power because of a system yours doesn't have but the ABS light will be on (if the bulb isn't burned - view in the bulb check key on engine off). Your cars idle valve and KE controller are also powered by the OVP. The KE controller is not noticed much when warm. It is necessary for electronic fuel correction when cold. To verify operation of either the idle or the fuel control (EHA electro hydraulic actuator) one should use an ammeter reading ma (milliamps). The idle will range about 600-900ma. The EHA will have readings from -60 to +60ma or more. (See the DIY article on evaluating engine controls). A reading of zero should be followed by a new OVP (overvoltage protection relay).
The biggest problem with OVPs is the intermittant problems. I always replace an OVP with original date code if I am chasing an intermittant in any of those systems. Every original one, through three different versions in many models, has an updated version. Most have gone to two fuses and thus separate control for separate systems.
[Edited by stevebfl on 05-05-2001 at 01:01 PM]
Bosch Master, ASE Master, L1
33 years MB technician