Hey Ron,

I'm obviously not the ECU designer, but here's my interpretation. This particular version of the GB ECU has a CPU that's powered by the ECU pack and not the RX pack. The RX lead does connect the receiver's ground and +ve leads, but I don't think they're responsible for much of the functionality.

When switching on the ECU, the bootup screen appears, it tells you what the software versions are, the diagnostic LED on the side of the ECU lights up based on the shaft position, the RPM sensor functions, etc. It will display a no R/C error until the receiver is turned on and connected properly. When uploading ECU software for a particular engine type, only the ECU pack is required to power the CPU; no connection to the R/C system is necessary. On the flip side, when the R/C system is powered up alone, I don't believe anything functions in the ECU. Essentially nothing happens... I can't verify that the RPM sensor doesn't work without an oscilloscope but I'd bet its powered by the ECU pack since a sensitivity adjustment POT is on the 2nd board. The start gas valve is definitely driven by the ECU pack, since its driven by a voltage higher than that of the RC system. My take on the RX connection is that it may or may not share the ground with the ECU pack, but the +ve lead may only be used as a baseline in a comparator circuit where the ECU needs to know a sample voltage with which to compare the pulse width signal maximum. Obviously the bottom of the pulse width curve would be ground, and the upper bound would likely be near that of the +ve lead.

I know for a fact that if the switch is left on and a pack is plugged in, it will drain it. I ran a NiCad dry once or twice before I trained myself to switch it off or unplug it when it's out of action, which is even more important now that I'm running Duralites! Now I make sure to unplug the pack after the day's done, but I just switch it off when its dormant in the pits between flights. I should find some time run a current measurement on the lines in the RX lead, but I doubt it will be much.

The point I was trying make earlier was that a switch does have some value when the ECU is powered primarily by the ECU pack. The earlier FADEC's, for example, are powered by the RX battery but I believe the newer version is ECU pack driven from posts I've read. In the GB Hobby / TEMS case, it makes sense to have a switch, but the downside is adding another component to the system with the potential for failure. They mitigate that concern by designing the system with redundancy.

Kelly