I've been having an unusual problem with my Grand Rapids EIS 4000. Whenever I key my push-to-talk button, the EIS display "goes haywire". All the readings start to climb upwards, with Voltage and CHT's climbing the highest/fastest. Of course, this is very disconcerting when you're not expecting it, as it appears the engine is getting ready to self destruct!
After some initial troubleshooting, I determined that it wasn't due to a wiring problem or ground loop, but was most likely due to RF energy from the comm radio reflected back into the EIS. Part of the problem might have been caused by the routing of the antenna wire behind the panel. It's a tight fit back there, and you need to pay attention to routing the antenna wire away from other wires. In the end, to make sure any stray RF energy stays out of the EIS, I researched using ferrite toroid beads as RF filters. There's not a lot of information out there, but I did find some good stuff.
The concept is to run the wires through the beads a few times, then any stray RF following the wires is trapped in the beads and dissipated as heat. If the wires are loose at one end, and don't have a bulky connector already attached, the wires can be looped through the center of the bead a couple times and the job is done. Wires already attached require split beads that merely clamp over the wire. Pretty cool, actually, although they are a few bucks more.
The toroids are constructed of different alloys, each alloy corresponding to a specific RF range. You first identify the frequency range that is generating the interferance, then select the best mix for the toroid. The effectiveness is determined in part by the size of the toroid, the linear length of the toroid, and the number of turns the wire wraps through the toroid (effectiveness goes up by the square of the number turns).
In my case, I was pretty sure the comm radio was the culprit, and that puts the frequency range in the VHF spectrum. According to the chart, Mix 43 (NiZn) is the overall best frequency for VHF. I ordered seven Mix 43 split beads from Polomar Engineers (3 x 1/2" diameter, and 4 x 1/4" diameter). The two data cables going into the EIS each got a large bead, the wiring harness coming out of the back of my comm radio got a large bead, and the antenna cable coming out of the comm radio got the 4 small beads. I also re-routed the antenna wire so that it wasn't sitting right against the EIS cables. The combination of re-routing the antenna cable and the beads eliminated any further EIS problems! Now I can transmit cleanly without screwing up the rest of my instruments.