The OVP is apparently programmed to self destruct after ownership changes. I can't count the times I've heard "Previous owner said he had NEVER had any problems with the car, but two weeks after I bought it the 'check engine' light came on and it began to [ hard start cold, hard start hot, stall, ABS light on, skip, roll over and play dead, etc, etc, etc. ] :rolleyes: Go figure! Of course replacing the OVP doesn't actually fix any of the problems but it certainly sells a lot of OVP's.

That is my question, originally or somewhere on this thread;) . Obviously something is causing it to blow.... changing ovp's is like a bandaid and not really fixing the problem(s) causing it to blow.:confused:

A car is a hellhole for electronics to live in. I read an interesting Intel article some time ago, and the WORST thing that can happen on the +12 line is a "alternator dump", where all of a sudden load get shed quickly, like, from turning your headlights off or the A/C relay kicking on/off (high-current deltas). This puts a spike on the +12 line. Even the battery (acting like a giant capacitor) can't handle it entirely because it can't handle really short pulses (this is one of the reasons to NEVER run your car with no battery or a dead battery).

I THINK (don't know for sure) that the OVP is like a "crowbar" circuit on a switching power supply - that is, when voltage goes over, say, +16 volts, a big SCR (a cheap, high current semiconductor) latches in and presents a dead short - which will then deliberately blow the fuse (and maybe the semiconductor, which is designed to fail shorted). Depending on how the circuit is made, the circuit might NOT latch so the circuit can survive short overvoltages.

This "cheap" (in the relative term) part sacrifices itself (or its fuse) for the sake of other stuff.

Of all the magic MB modules, this one is the most promising to reverse-engineer!

Does anyone know if the europeans use a different OVP in their W201s and W124's? Or is the same part number?

Here is all you ever wanted to know about how the Zener diode in an OVP relay works:

http://www.allaboutcircuits.com/vol_3/chpt_3/9.html

I just replaced the OVP in my SL. It was expensive - $57. I took the cover off and discovered that the problem was that the internal fuse - a piece of meltable wire - had "blown." At least the newer ones have an external fuse that can be replaced!

I examined my OVP yesterday and found nothing suspect inside, looked as new. Have to find another source to the limp mode in hot weather...

Anyway it had a production date of 25.08.93. There were two numbers; one under a MB-star: 000 54067 45 and on the other side it had: 89 7219 000.
My car is sold new in Germany in 1994.

Thats for a W202 right?

No, it is a W124, or rather C124, the E 36 AMG Coupé. Same engine as the C36 though...

Post some pictures of your W124 E36 (engine, tranny, etc.) and some specifics (calipers, tranny, diff ratio, etc.).

My W124 wagon has the C36 engine in it.

I'd like to see what's different.

:-) neil

Hi, I have just a few pictures of the engine, nothing else, but I can take some more in a couple of days...

That's an excellent description. The OVP's purpose in life is to protect the electronic control units. If the voltage goes too high, the zener conducts causing the fuse to blow.TV electronics use spark gaps around the picture tubes to protect it from voltage spikes.

It's the sacrificial anode of our cars.

Funny stuff, Cap'n. Worth resurrecting an old thread.

My diesel idle speed control (ELR) wasn't working until I changed the OVP relay. The OVP fuse was intact, and there was nothing obviously wrong inside. I do not know how to test the individual components on the circuit board, but nothing was burnt or broken. I would conclude that it is entirely possible for the OVP to fail without any apparent physical indication.

Very easy to diagnose. The actual schematics for the circuitry is right there on the case, it can't get any better than that;)
My first encounter with the OVP (when I had no clue what it was) was when I first got the 560SEL (did someone mention it usually timed to ownership change?:D ) the ABS light was on and the car would idle high.

Intermittent problems
 

On startup, the ABS light is off, and presumably all the circuits downstream of the OVP are happy [e.g. the ABS stuff works].

Hit a rough spot in the road [it doesn't always take a big bump] and the ABS light comes on. I'm interpreting this as a bad connection, either in the socket or in the wires connected to the OVP relay socket. Are any special tools useful to recrimp the connectors in the socket?