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
    Commented Mar 8, 2013 at 14:08
  • No, it appears to date from the 80s, and I can't find anything that quite matches.
    – TomG
    Commented Mar 8, 2013 at 16:03
  • Every document I find on Honeywell MABS controllers, shows M1 as C.
    – Tester101
    Commented Mar 8, 2013 at 16:10

6 Answers 6


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
    Commented Mar 8, 2013 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
    Commented Oct 9, 2014 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
    Commented Oct 24, 2014 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!


I have a MasterTrol Mark V Trol-A-Temp zone controller and successfully installed Nest Gen 3 thermostats. After much difficulty getting it to work I discovered the following issues.

  1. The R wire on a conventional A/C system is directly connected to the 24v transformer. On the MasterTrol this is NOT the case, it is connected to the relay. As a result it is NOT possible to use the C wire to provide supplementary power to the Nest thermostat and still support both A/C and Heat. The C wire will just cause the MasterTrol relay to chatter.
  2. The MasterTrol has small relays for each zone that only require 60ma of current to operate. This is a marginal level of current for the Nest power stealing circuitry to stay fully charged.

Solution: Do not use a C wire from the MasterTrol. The only connections to the nest are:

  1. Y1 - A/C
  2. W1 - Heat
  3. RH - Relay

Add a bypass resistor in parallel with each zone relay to raise the current needed to energize the relay. This allows the Nest to steal sufficient power from the MasterTrol to operate normally.

This can be accomplished by adding three 220 ohm 5 watt resistors between each of the three “T5” connections and the “1” (transformer) connection.

Hope this helps someone…


I have a Mastertrol Mark V and successfully attached 2 Nest thermostats (2 zones). In my system the T terminals correspond to current color coded wires as such:

T6 = Y (cool)
T5 = W (heat)
T4 = R (24v power)

The nest works when only attached to RWY, but the Nest kept buzzing its zone relay when using only 3 wires (RWY). It also complained about power availability. Amp meter only showed ~60ma. I believe the buzzing was the Nest intermittently sipping power from the R line. I could get the relay to buzz by manipulating the settings on the Nest and the relay would buzz with each manipulation (even just checking it on the phone).

Connecting a C line to terminal 2 was partially successful, in that it did provide power but also created an alarming continuous clicking in the zone relay. Note, removing the W wire (in heat mode) from T5 eliminated the relay noise, but that makes the system useless. I suspect the clicking was due to the power for the T terminals being supplied by a different transformer than the transformer on connections 1 and 2 (24V 40VA Transformer). It's AC power, and AC cares about phase synchronization. The two transformers will not be in sync.

Anyway, the solution was to detach the R wire for the Nest, in my case that was from T4, and attach the R wire from the Nest to terminal 2 (24V 40VA Transformer) and connect the C wire to terminal 1 (24V 40V Transformer). This means T4 has no wire attached to it. Note, I learned from HVAC research, and as mentioned by a previous post here, that the C wire is meant as the return path for power from R. So on my setup this leaves us with:

T6 = Y (cool)
T5 = W (heat)
T4 = empty
2 = R (power)
1 = C (return)

Note, it's AC, so it really doesn't matter if you attach R to 1 or 2 and the same for C, but the convention seems to be that R should be on whatever the hot leg is.

Now, Reboot the Nest! If you end up updating the wiring info in the Nest, be sure to reboot it again just for good measure.

Everything should work and the Nest should have no complaints on the screen. Because the Nest is attached to the 24V 40VA transformer terminals, it will receive power even when the master mode selector switch next to the Mastertrol is set to OFF. In the 3 wire setup, setting the mode selector to Off removed power from the Nest, which is not what you want.

I hope this helps someone.

The following are photos of my unit as well as the wiring I put in place to supply 24V to my nest thermostats.

Mastertrol Mark V Schematic for Nest install Mastertrol Mark V Control Panel Wiring for Next


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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