Jon,
You most likely have a loose wire somewhere. When you do your mag check, if the RPM reading on your EIS goes to zero when selecting the P-mag, then you are getting RPM information from your mag. On the EIS, this is a wire running from your P-lead, through a 27K ohm resistor to a pin on the EIS. If you get RPM indications during a mag check no matter what mag is selected, then you are getting your RPM information from the P-mag. This information is supplied from Pin 6 on the P-mag to the EIS or EFIS, depending on GRT EFIS generation. There may also be an additional ground wire running form the P-mag ground pin (pin #1) to the EIS. I suspect your RPM input is coming from the P-mag. Check pin #6 on the P-mag to make sure it is tight. Also check pin #1 the same way. To check these, just grab the wire and give it a little tug to see if it?s tight. Don?t try to rip it out by the roots - just a tug will do. You should check all these connections whenever you have the cowl off. These connections are simply a wire inserted in the connector and then tightened in place with a very small flat screw driver. These connectors work great on a computer, not so great on a Lycoming that shakes like a wet dog sometimes. The wires in this case, and all cases, are stranded, and when you clamp them in this type connector it tends to squish the wire strands and put undo stress on the strands. Wires used in this type connector should be ?tinned? prior to inserting them in the connector. This means applying a small amount of solder on the wire strands to solidify them and give the clamping mechanism in the connector something to grab on to. Checking the security of these wires will take less than 1 minute when you have your top cowl off. If there are loose wires, loosen the screw, pull the wire out and see if it has been tinned. If not, heat up your soldering tip and get to work - it only takes a few minutes, and you shouldn?t have to worry about it again.