Where is the 5A fuse on the 24v transformer in the Trane tem6 air handler?

I checked the continuity of the fuse and it is ok. However, the condensate overflow sensor is getting no power. It has a green led that stays lit but is now off. I have 27v ac at the stat between the C and Rh and between the C and RC. All the LEDs on the zone controller are green except one. But I have not checked what it is supposed to indicate. Am I correct in assuming that the overflow sensor which is in series with the compressor relay is powered separately by the 24v xformer and not through the zone controller board?

Despite power at the wiring plate, my brand new Nest gen 3 stat still says it has no power. Any troubleshooting suggestions?

The Zone controller is a Honeywell unit and can control up to 4 zones, but I am using 3 zones. I will post some pic tomorrow when I get back into the attic. I found the installation manual near the unit which was installed last year. The circuit diagram in which EdTheHeaterMan has asked to check the voltage at the R and B terminals are not easily identifiable on the mother board. But those terminals are where the 24v secondary terminates. Absent any loose connections in the connectors(I will take the connectors out and reseat them) If the secondary has voltage, and it has, since I get it at all three stats, then the only reason the overflow sensor is off is because it is not getting power from the secondary. Not clear why, since the accidental short at one of the stats left the xformer fuse intact.

Here are some pics of the zone controller. These were taken with a flash and do not show the LED status. The last 3 white LEDs are turned on while the red led between the yellow and green is turned on.

I show no voltage on the condensate overflow detector, and it is off-the test button also doesn’t work as a result. I have 27.5v at the thermostats for each of the 3 zones.

Thank you all for your helpful troubleshooting suggestions and questions.

The AC and steam heat were working fine with Nest gen 2 until I shorted some wires while changing the base plate for Nest gen3. The boiler power was turned off but I had forgotten to turn off the power to the zone controller in the attic. The new Nest as well as the old Nest indicated that the units were getting no power even though all the connections were identified by the units. I made no changes to the wiring anywhere else except at this one stat that controls both the boiler and the AC.

I restored heat by replacing the Nest with a Sensi stat. that controlled the AC on my 3rd floor zone. This stat is working normally with its own app. and I can control both the AC and heat. So far so good. But two problems remain:

1.Cannot wire the Nest 3 to work due to the error.

2. The float switch on the air handler has no power.

I rebooted the controller board and the Idle LED flashed red for a few sec and turned off while the Purge LED turned Amber and stays illuminated indicating a ZMS failure. Not sure why a duct mounted temp sensor would fail on the TAM6 after 1 yr of service or what it has to do with a short that did not blow the xformer fuse. Anyhow where is this sensor located and where are its terminals connected on the controller board? Perhaps I can bypass it and see if the Nest error disappears.

I have two switches for cutting off power to the air handler in case the condensate overflows. One is a mechanical float switch and the other is a water sensor that sits in the tray. Both are NC and in series and thus the unit will turn off even though the sensor has no power. I will check if it interrupts the Red xformer wire or the yellow one and rewire for yellow as suggested. The problem right now is that the sensor switch has no power. All the 3 wires are connected tightly. I may have to remove the air handler panel again and trace the wires back to the Rand B wires on the xformer after the fuse.

The Peerless boiler has a 2 wire stat connection, R and W, which come from its own xformer and control unit inside the enclosure. There is no common from the boiler; common for all three stats comes from the TAM xformer. All the stats still have 24v to power them after I rebooted the zone controller. I have not checked if the AC works after the purge LED has come on.

My Nest gen2 has been operating since 2012 on the same zone controller. The TAM6 was installed in August of 2024 and uses the same controller that was installed on my Carrier in 2001. It has worked with the same PTC for at least one year. The only reason I wanted to switch to Nest gen 3 is because Google stopped supporting the app for gen2.

After the reboot, both Nest models give the same error- C wire not connected even though it is there and shows 24v at both Rh and Rc. The Sensi that I have connected has batteries, but since it gets the 24 v it runs without them including its WiFi that allows for remote control. I run the heat at one temp constantly.

If the float switch circuit gets its power through a relay when the AC is switched on then I am not concerned.

I called Nest support since I purchased the gen 3 a few days ago and based on the 24 v supply that I am seeing they concluded that the unit is defective, but I don’t think so. Anyhow, if I can’t get it to work, I need to return it soon and try to use my older unit.

Thanks for your assistance to date. This is a brain teaser.

@109A_5 the Nest as well as the Sensi in its place are connected as you have indicated. I had no C wire at all three stats until last year when the TAM6 was installed. The two other Sensi stats ran on their AA batteries for approx a year while the Nest worked without a C wire by stealing power.

I will try to run the Nest with all except the C and see if it works.

Probably not worth spending more time troubleshooting the Nest when the Sensi works. The only things I don’t like about it are poor WiFi connection compared to the Nest and the to ‘slow’ cycle rate kicks in the boiler with a 0.5 degree temp drop in the room.