I'm trying to work out a problem I have on my W140. I want certain things to happen when I arm/disarm the alarm. This is an add-on I've got in mind, nothing to do with normal systems operation.

There are two wires on the ATA connector that, according to the wiring diagram I found on AllDataDIY, see temporary chassis ground during the arm/disarm events.

I verified this with my digital VOM, using the continuity sensor setting (beeps when the circuit is complete).

I want to use those signals to trigger a relay - positive side of the coil is to 12v+, negative is provided from the wires referenced above.

The problem is, at rest, these wires still show a small amount of conductance to ground - if that's the right phrase. Very high resistance, but just enough that the relay coil activates when I don't want it to.

For reliability, I really need infinite conductance to ground for the coil, except when the arm/disarm event is occurring.

Is there a way I can "filter" this circuit so that only "true" ground connections get through? Can I put a resistor of sufficient size on the line to push the resistance over the threshold necessary to prevent the relay from triggering except for when I want it to?

I understand electronics only at the basest level, so go easy...

Thanks!