In my third month of chasing the elusive cause of my E300 turbodiesel's miss
at idle, TMAlison suggested the K-40 relay as a potential cause. Is there a way to test the relay before buying one?.
My strange idle issue began when I replaced my original alternator that was delivering 12.8v with a Bosch reman that puts out just under 14v.
I'm suspecting overvoltage as a possibility because when I switch on the headlights or heated seats or climate control, the car idles normally. Turn everything off and the rough idle returns. The shaking on decelleration that some E300 owners report is one that I have too, but again is not present when there is significant electrical load.
Any electronics whizzes taken that relay apart?

Steve

Shaking on deceleration is a known software fault that was fixed in '99 or so. That's most likely the problem. The bad news is that the only fix is a new IFI injection computer.

About the OVP and K40: The OVP monitors the system voltage and cuts current to the sensitive parts if the voltage is too high. The K40 is a mere relay module. The IFI injection computer monitors the voltage and keeps the circuit on, and if it goes too high, shuts the coil current down which makes the contacts release at the K40. This won't result in a miss; it will result in your engine shutting down.

I've taken mine apart twice, since I didn't solder it well enough the first time. I didn't also resolder the coil connections. I should have known better; resoldering the high-current connections probably moved the relays on the board just a bit, causing the low-current connections to fail.

If you do attempt to solder it, make sure that you have a high-wattage iron or gun. The metal bars inside soak up the heat and make soldering difficult.