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.

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  • 1
    Do you have the wiring diagrams for the HVAC system, and the control panel? Don't forget, sometimes you can't use the latest and greatest technology with a 20+ year old system. This answer might be helpful.
    – Tester101
    Mar 8 '13 at 14:08
  • No, it appears to date from the 80s, and I can't find anything that quite matches.
    – TomG
    Mar 8 '13 at 16:03
  • Every document I find on Honeywell MABS controllers, shows M1 as C.
    – Tester101
    Mar 8 '13 at 16:10

It appears according to the manual for the Mastertrol Automatic Balancing System (MABS) EZ Zone (EZ-2 and EZ-4) Control Panels (PDF), that M1 is used for C.

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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 M1 being used for C.

enter image description here


After further review, you should be able to connect the C wire directly to the transformer terminals. Measure for Voltage between the T5 terminal of the zone you're working in, and the 1 and 2 terminals 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 as C.

  • @Tester- I've seen other comments pointing to M1, and one other person with the Mark V that couldn't get it to work on M1. Like them, I get 0V across T5 (thermostat's R) and M1, and 24V from T5 to M2. Perhaps there was a change made after the Mark V to the newer panels like you show. +1 for finding and including the diagrams. If a Mark V diagram doesn't surface, I may have to go with an external power supply.
    – TomG
    Mar 8 '13 at 17:16
  • @TomG After working on this question a possible solution to your problem has finally struck me. Please see the Update section of my answer, which hopefully helps.
    – Tester101
    Oct 9 '14 at 13:43
  • @TomG I think you're right. I tried to contact Honeywell with no success, but looking at the wiring of others it appears M2 is more likely to be used as C on the Mark V.
    – Tester101
    Oct 24 '14 at 13:24

I have just successfully installed a Honeywell RTH6500WF Wi-Fi Series thermostat on Zone 1 of my Mastertrol Mark V zone control system. I used a separate 120VAC to 24VAC transformer (AUX) with one lead of the AUX 24VAC hooked to Zone 1 terminal T5 of the Mark V panel (R terminal on the thermostat) and the second lead of the AUX 24VAC hooked to the wire (4th thermostat wire) that goes to C on the thermostat. A separate 24VAC transformer is required for each zone thermostat that needs a C connection.


I used the spare Green and Orange wire from my 3 wire system. On the Master Control unit I connected Green to 2 and Orange to 1.

I then connected Green to C and Orange to RC. I removed the jumper.

So far so good!


Go to the transformer and see what wire is common, it will be opposite of RED going to the thermostat on R or Red. Common is the side of 24 v power that every 24 v circuit terminates upon to complete its path. You will read 24 volts across every wire with Red , only with a call for heat will white not read 24 volts to red from white as the switch between red and white will be closed in the stat. open switch will read 2 sides of power closed switches read only 1 leg of power.

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