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!

It should be possible . . . probably a 'solid-state' device would make a better interface.

Do you have the schematic so I can see it?? And the VOM readings at 'rest' and during activation are needed.

We'll see what we can make . . . .

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Regards . . . . JimF
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'94 S500 Cpe

Visit my Mercedes Web Page

thanks - I'll get the information together and let you know what I find.

What is the resistance to ground in the normal (off) state?

What relay PN are you using?

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra