This is my understanding of how a Zener diode works and the function it plays in the OVP relay. The OVP relay contains what is known as a Zener diode which has a reverse bias (the diode's anode connects to the negative supply and the cathode connects to positive supply). When the reverse voltage across the diode exceeds the rated voltage, then current starts to flow in order to limit the increase in voltage. What is great about this diode is that it maintains almost a constant voltage over a range of current values as long as the current remains within the diode's breakdown range (between the current min and current max rating for that specific diode). What this does, and why it's important, is that it can maintain a very stable voltage level over a widely varying load. With DC circuits as the load varies, so to does the average output voltage. By adding a Zener diode you can stabilize the output voltage. Why is this important? Well, there are certain components that are very sensitive to voltage changes, and in an incredibly complex set of circuits such as a modern automobile with all sorts of various components that may or may not be operating at any given time, that may be operating at varying load levels, and with just good old physical wear and tear...there is going to be some serious variance in the load. Adding the Zener diode makes sure that this load variance doesn't affect the voltage output, and thus you don't fry your components & fuses or generally experience frequent malfunctions. A zener diode, and the OVP relay for that matter, does not regulate the current to my knowledge. It regulates the voltage as the name implies. Are you thinking about the saturation current?

Oh, and one handy little bit of information to have about the Zener diode in the OVP relay is that it is actually incredibly easy to test. Set your DMM to the diode setting, and connect the positive lead to the pin that corresponds to socket 31 and the negative lead to the pin that corresponds to socket 30. Do this with the relay disconnected from the vehicle. You should receive a result between .4V - 1.5V. There could certainly still be other issues with the relay, but at least from that point on testing is the same as any other relay...which is pretty much what your procedure outlines. Oh, and one other thing...given the nature of this diode, you might be able to guess why it's a really bad thing to reverse the leads when attempting to jump start a vehicle, or why you really don't want to create an electrical arc at the terminals. This sort of thing tends to kill your OVP relay.