Today I made quite a bit of progress at cleaning up the wiring in the forward fuselage. Then I decided to troubleshoot the MSTR WARN annunciator lamp that is triggered by the EIS 4000 Engine Monitor. This lamp wasn't working correctly, and I didn't understand why. I eventually spied a wiring problem, and "fixed" it, but the light still wasn't illuminating. I studied the Installation Manual more closely, and was horrified to note that I had completely misunderstood how it was supposed to be controlled. I had wired it so that it needed to get power from the Engine Monitor to illuminate. But, the Engine Monitor is designed to provide a ground when the lamp is supposed to be active. :(
This would be an easy fix, if I didn't want to be able to dim this lamp at night. I spend quite a while looking at various possible solutions, and came up with three options.
- Completely rewire the dimmer control, replacing the existing two-pole switch (and one Zener diode) with a three-pole switch, and two Zener diodes. Two of the poles would mirror the existing installation (one pole to control the lamps that need power to light, by putting the Zener in the common ground, and one pole to control trim indicator dimming). The third pole would control the MSTR WARN lamp by putting the Zener in its switched ground. This would be a big mod, and I didn't look forward to trying to solder two Zener diodes to a three-pole mini switch.
- Add a relay with a Zener diode, and control the relay by the trim indicator dim signal. The trim indicator dimming is controlled by the second pole of the dimming switch - the LED trim indicators go into dim mode when power is applied to the dim enable wire. I could tap into this wire to control a relay, and the relay would put a Zener diode in the switched ground that controls the MSTR WARN lamp. This would be fairly easy to add to the existing wiring, but it means I need to make yet another trip to the electronics store, and I need to find a place to mount the relay, it adds extra weight and complication. Plus it adds new failure modes that could keep the MSTR WARN lamp from lighting.
- Rewire the MSTR WARN lamp control to bypass the dimming control. This would be a simple fix, but the MSTR WARN lamp would always be in bright mode. I didn't like this idea at first, but warmed to it after some thought. I don't plan to fly at night too often. Normally, the MSTR WARN lamp should only illuminate before engine start (due to low oil pressure, and low bus voltage), and after engine shutdown. It should never illuminate in flight, unless one of the monitored parametres exceeds the preset limits. If the parametre goes back inside the allowable limits, the lamp will extinguish. If not, and the bright lamp is distracting, I can extinguish it by either pulling it slightly towards me, which will disconnect it from its socket, or I can change the limit on the parametre that is triggering the lamp. There is one bonus to this mod - it removes the risk of inadvertently flying with the BRT/DIM switch in DIM mode, and not seeing a lit MSTR WARN lamp because the sun is so bright.
I'll sleep on it tonight, but I will almost certainly rewire the MSTR WARN lamp so it is always bright.