The diagram is a bit fuzzy, I got it from here
https://www.benzworld.org/forums/r-c107-sl-slc-class/1612048-part-time-ovp-relay-2.html
Pin 1 / 30 connect to variable DC voltage
Pin 2 / 87 Connect to a load ( under 10 A so the fuse does not clear )
Pin 3 / Blank
Pin 4 / 87 not used
Pin 5 / 31 Ground
Pin 6 / 15 connect to variable DC voltage through a switch
Make the connections as noted, turn power ( set to 11 V ) and 6 / 15 switch , increase power to 14.8 ish, _lightly_ tap the relay. Start turning up the power farther, note at what voltage the fuse clears. Hopefully there is a spec somewhere, if not , cracking the relay open and looking for markings would be helpful. If it the fuse does not clear, leave things on at 14.8 V for a while to see if heating causes a failure.
An advanced test set design would include a power supply with a bit of electrical noise to mimic an alternator.
On the car side, I'd be looking for a relay coil that has a bad spike snubber. It may be possible for a voltage spike ( when power is removed from a coil ) to trip a OVP relay that is almost ready to trip because charging voltage is already a bit too high.
As a late entry, I found some better pics here
http://www.peachparts.com/shopforum/diesel-discussion/305766-ovp-woes-dissection-pics.html
All of this is barely middle school science project stuff...