EIS issue

Ok here's a strange one. I have a Grand Rapids EIS-3 on my 582 powered IV. The other day the EIS would not come on when the Master inst. switch was thrown. After checking all the connections, grounds, ect and finding nothing I happened to push the PTT switch on the stick. Well what do you know the EIS comes on when the PTT is held. I'm really scratching my head on this one ? I know the EIS is hooked to the intercom system so that an audible warning will go off in your headset should you exceed the preset limits of various readings. What I can't figure out is exactly how and why pushing the PTT enables the EIS to power up ? Anyone ever had something like this happen with their EIS ?

I have no personal experience of this phenomena but it sounds to me like a grounding problem. I know you said you'd checked the wires but it might be worth clipping on a temporary ground wire to replace the one you've got, just to check the existing one has not got an internal break etc. The reason I say this is because, to me, it sounds like your intercom switch (that goes to ground when pressed) is being used as the ground return for your EIS, hence it only works when the button is pressed.

That is my first impression too Paul. The EIS is getting it's ground through the push-to-talk circuit.

Definitely a grounding problem. With PTT turning the EIS on, you should probably double check everything...

Thanks for the input guys. Lost ground has to be the problem. Hopefully I haven't lost contact on a pin in the Molex. I'll report what I find when I can dig in and do some tests.