Anyone have an example car side wiring diagram / pin out for the OVP?

This isn't a complicated system so a test bench could be built to mimic the car.

Swapping in a known good unit is always easier but at some point known good units will be getting scarce.

So once again, with feeling. There is no valid test for an OVP. You would need to isolate the semiconductor and apply a variable voltage source, ranging the voltage from zero to the specified breakdown voltage. For what it would take in time and test instrumentation, you could replace it three times.

Gee, any decent electronics guy has a bench top variable DC power supply. While mine is only 2.5 Amp capacity , it ranges from 0 to 30 VDC , plenty enough to test the voltage section of an over voltage unit. If this unit does look at current, that is easy to solve too

I'm taking issue with your statement " There is no valid test for an OVP. " not that trying a known good unit isn't faster.

If one can mimic that goes on when the OVP is in the car, one can test it out of car. This isn't like we are trying to test a house numbered IC with an unknown pin out in a ECU.

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...