I did a search and found some valuable info but still have some questions. The car is a 1980 450SL, completely stock and unmodified. The sticker under the hood says that it is federal and CA. certified, (maybe there was only one model?), we bought the car new in the midwest.

The lambda system is completely inoperable with no signal to the frequency valve next to fuel distributor. I've just replaced the O2 sensor with no change. The OVP relay does not seem to be blown, (wire still intact in hole), but not 100% sure that it's good. Also not sure if I tested it correctly.

In the plug for the OVP relay, #30 has 12 volts, #31 is a good ground and the hole for #31a has no metal contact in it that I can see. I assume that #30a is supposed to be the receiving end of the 12 volts.

You will all have to forgive my stupidity when it comes to electricity, but this "relay" is really just a circuit breaker, correct? I've been told that it will blow to save valuable components when a voltage spike occurs due to jump-starting and the like. On the newer cars this means the ABS and SRS, on this 1980 car it is just the ECU/lambda control box.

I had someone put a tester on the relay and it had continuity but "a ton of resistance" between 30 and 30a. I have no idea what it's supposed to read.

Is 31a supposed to be a *dead hole* on the plug? Does anyone have access to a diagram for this plug on this car? Any other suggestions for trouble-shooting it? It seems like a fairly simple system with only the O2 sensor, the OVP relay and the ECU. The frequency valve is not buzzing at all no matter how much I lean or enrich the FD. It does not seem to be energizing at all at the plug, (tested with test light).

Thanks a million for any help!