I have spent hours searching the web for information on this 20+ year old zone control panel, which supports 3 zones of Heat and A/C.
The thermostat I'm replacing (on Zone 3) connects to T4/T5/T6 as W, R, Y respectively (the wire colors do not match because it's sharing a cable with Zone 2, which go the matching colors). I need a C (Common) wire to supply power to the thermostat, and found 24VAC across M2 and T5 (M1/M2 are the power leads to the zone damper, and seem to be connected internally to "1" and "2", the 24VAC supply.
However, when I connect the C terminal on the thermostat to the M2 connection, there is a click from some of the zone relays and the transformer hums as if it's overloaded. I removed power after a few seconds and disconnected the wire (now hanging off to the right of the picture).
I've disconnected the wire from the C terminal at the thermostat, and measure 24VAC across it and the Rh terminal on the 'stat, so I don't believe the issue is in the wiring.
The thermostat is a Filtrete 3M50; I've read that it draws less than .1 Amp, so I don't think this should be enough to put the system over some maximum current draw.
I'm looking for ideas on how to troubleshoot this.
Best Answer
It appears according to the manual for the Mastertrol Automatic Balancing System (MABS) EZ Zone (EZ-2 and EZ-4) Control Panels (PDF), that
M1is used forC.Click for larger view
This does not appear to be the manual for your device, so this information may not be correct.
The manual for the MABS 2 Zone Panel (PDF), also shows
M1being used forC.UPDATE:
After further review, you should be able to connect the
Cwire directly to the transformer terminals. Measure for Voltage between theT5terminal of the zone you're working in, and the1and2terminals in the 24V 40VA Transformer section. One should measure 24 Volts, while the other measures 0 Volts. The one that gives you 24 Volts, should be able to be used asC.