I don't usually post comments about aftermarket parts, but this is a bit over the top. A while back, I bought a spare OVP on e-bay for cheap. It's unbranded, not a KAE or even a URO, and certainly not OE. I had installed it over the winter to see if an ABS problem I was having was OVP related. Last week while I was driving on a washed out road, the ABS light came on, idle became rough, and and the tach went to zero. It only took a minute to verify that the a/c and transmission kickdown were also gone. Classic failed OVP relay syndrome. Since it happened suddenly and the part was relatively new, I decided to open the can and see what happened.

The first photo shows the insides. The long strip is the "30" contact, which supplies current to the fuses. What happened was that the bumpy ride knocked it aside and opened the circuit. But there's another problem here...

Both fuses are supplied by the 30 terminal, and the outputs are bridged together to feed both relay contacts (next two photos). Thus the fuses are in parallel, rather than supplying the the two relays independently. Parallel fuses supplying the same circuit isn't a great idea. Two 10 amp fuses in parallel supply a bit less than 20 amps, due to heating effects. That's wrong no matter what, because the individual circuits should be separately fused for 10 amps. More importantly, the amount of energy they deliver to the protected circuits prior to opening (called the fusing integral) increases exponentially over individual fuses. The designers were thoughtful: parallel fuses need to be wired in perfect symmetry, and the two fuses need to be positioned close together to prevent temperature gradients. That much, they accomplished. But the fuses have to be secured well, otherwise all sorts of odd failures can occur. In this case, the metal strip is too weak to stand up to normal road shocks, which means that at any point in time one, both, or neither fuse is working. It's a mess. Not sure why it was done this way, but I'm sure it wasn't done well. Aside from the damage this could do to your electronics, when it fails you lose the kick down function. It's a safety issue. (Some of the mess is mitigated because my 190DT doesn't use the second relay circuit, but the assembly still fails on rough roads.)

As can be seen (last two photos) , no manufacturer, no country of origin, and the diagram doesn't match the actual circuit. The diagram also doesn't match the schematic in the shop manual. The only thing that's correct is the part number.

Just as an aside, the fuses aren't core to the OVP's function. They are there because the OVP happens to be close to the battery, so it's powered directly without being fused in the electrical center. That's the only reason why it has it's own fuses. Actual OVP protection is provided by a zener or MOV, which shunts overvoltage spikes to ground. The zener can fail regardless of the condition of the fuses, and the relay can work without the zener. As a result, there is no valid test for OVP functionality. So the only way to be sure the overvoltage function is working is to install a new (not used) relay, but not this one.

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Its there for over voltage from alternator. Or a fail with zenner diodes

If you're referring to the fuses, no. The fuses are there to prevent against a short circuit, not overvoltage. They aren't directly relevant to the purpose of the OVP. The OVP protects against voltage spikes, mostly generated by relay coils and motors, and the critical operating component is the zener.

I've had to buy/return two KAE OVP's to get one that seems to work.

First two were bad out of the box. Had to fight a bit with the supplier who did not want to take back an electrical device. Which I knew the drill...just held my ground. No pun intended

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1986 300SDL 440,xxx