Fixing My Reversed Zone Switch LEDs on a Smart Temp SZ-SW4

About three years ago, I had a Mitsubishi Heavy Industries ducted A/C system installed with a Smart Temp SZ-SW4 zone switch. From day one, something never felt quite right:

• The LEDs on the zone switches were backwards.
• When a zone was on (airflow open), the light was off.
• When a zone was off (closed), the light was on.

I asked the original installers at the time and the answer was, “That’s just how it is.” It seemed odd — lights usually indicate something is on.

Second opinion… same story

Fast forward to a routine service last week. A different A/C company came out. I asked them to check the LEDs. They went up into the roof to find the zone controller but couldn’t locate one. Their verdict? “That’s just how it’s wired.”

Digging deeper with ChatGPT

Not convinced, I did some homework and spoke to ChatGPT. I learned it could be a jumper or DIP switch setting on a controller, or a wiring polarity issue (e.g., Normally Open vs Normally Closed).

The breakthrough: identifying the controller

I confirmed the unit was a Smart Temp SZ-SW4 Easy Zone Touch Pad. I emailed Smart Temp for advice and John from Smart Temp replied on a Sunday (legend!). He explained the problem wasn’t a jumper at all — it was that RJ12 straight-through cables had been used instead of RJ12 crossover.

Key point: The SZ-SW series expects RJ12 crossover wiring. With straight-through leads, the logic gets flipped and the LEDs appear inverted.

The fix (without new plugs)

John provided the pinout for a crossover. I own a crimper, but didn’t have spare RJ12 plugs. I did have gel-filled IDC connectors from a previous project, so I used those:

1. Stripped back the outer insulation on the RJ12 cable.
2. Left the two middle wires (COM) untouched — in my cable these are Green (Pin 3) and Red (Pin 4).
3. Cut only the four outer conductors: Blue (Pin 1), Yellow (Pin 2), Black (Pin 5), White (Pin 6).
4. Used gel IDC joiners to cross them:
   Blue ↔ White and Yellow ↔ Black.
5. Snapped the IDC joiners closed and insulated the splice.

RJ12 colour order (my cable)

Pin 1 = Blue
Pin 2 = Yellow
Pin 3 = Green  (COM - unchanged)
Pin 4 = Red    (COM - unchanged)
Pin 5 = Black
Pin 6 = White

Crossover that fixed it

Leave:  Green (Pin 3) → Green (Pin 3)
        Red   (Pin 4) → Red   (Pin 4)

Swap:   Blue  (Pin 1) ↔ White (Pin 6)
        Yellow(Pin 2) ↔ Black (Pin 5)

The result

Voila! My zone switch LEDs now work the way they should:

• LED ON = Zone ON (airflow open)
• LED OFF = Zone OFF (closed)

Takeaways

• If your zone LEDs seem backwards, don’t settle for “that’s how it is.”
• For Smart Temp SZ-SW pads, check the leads — they likely need RJ12 crossover, not straight-through.
• Sometimes the fix is a simple cabling change, not new hardware.
• Huge thanks to John at Smart Temp for replying on a Sunday with the exact solution.

Bonus tip: If you prefer a cleaner finish than IDC joiners, recrimp new RJ12 crossover plugs or use an RJ12 crossover inline coupler.