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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 7920
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Trane XR90, AC/fan is not turning on, control board flush is

Customer Question

Trane XR90, AC/fan is not turning on, control board flush is off, checked power supply into the control board and looks good, tried a new control board but same result. fan gets turned on if supply power directly
Submitted: 8 months ago.
Category: HVAC
Customer: replied 8 months ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Video Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Expert:  Phil replied 8 months ago.

Hello, use the paper clip icon at the top right of your tool bar, or the attach files link to the left of your send button to send me a close up, well lit, and very well focused photo of the wiring diagram glued to the inside of the blower access panel, I will mark it up with trouble shooting instructions. We can go from there.

Customer: replied 8 months ago.
Control board is CNT03076 replacement cnt05165
Customer: replied 8 months ago.
The Red LED is always off
I took all the measurement you asked and found all are zero, I made sure the control board has the power 120 V, measured input high against metal frame.
Expert:  Phil replied 8 months ago.

sorry for the wait, I got hung up on the phone with another customer.

Expert:  Phil replied 8 months ago.

The problem is no 24volt control power to the thermostat terminals

Expert:  Phil replied 8 months ago.

the 120 volt power to the board is to power the blower

Customer: replied 8 months ago.
what needs to be done?
The transformer need to be tested replaced? C340041P04 has 4 wires black, white, red and blue
Customer: replied 8 months ago.
Ok looks like input is good 120v but output is zero
Expert:  Phil replied 8 months ago.

Thanks, ***** ***** all that, make sure the door switch (blower compartment door) is closed, hold it down with tape or your finger.

If there is still no 24vac on the secondary side of the transformer, look for a tiny float switch fit into the white 1" plastic condensate drain line, with two wires attached, that is an optional device that may or may not be installed... if the float sticks in the up position it will kill power to the 24v controls.

Try checking the 24v secondary of the transformer right *at the transformer... if we are sure the transformer is not putting out 24v it needs to be replaced

all they need is the furnace brand and model number, or you can remove the transformer and take to Grainger industrial supply for a match up.

Expert:  Phil replied 8 months ago.

Once we get this resolved it will be time to locate the "Finish and rate your expert" link and click on it, that is the only way the company pays me for my time and expenses. THanks!

Customer: replied 8 months ago.
There is no condensation switch, disconnected terminal output voltage is zero, I will replace the transformer and see if it work and will let you know.
Expert:  Phil replied 8 months ago.

Thats a good plan, but be sure the transformer is actually getting 120v on the primary and not putting its required 24vac out on the secondary as shown on my mark up.

Once we get this resolved it will be time to locate the "Finish and rate your expert" link and click on it, that is the only way the company pays me for my time and expenses. THanks!

Customer: replied 7 months ago.
I had replaced the low voltage transformer with original part and was working but it is blown again in few weeks. So there must be some other problem. I don't think the transformer should be blown so quickly. Please advice.
Expert:  Phil replied 7 months ago.

Hello again, there is an **intermittent** short in one of the 24 volt devices that plug into to the printed circuit card..or.. the wiring to such a device.

These can be very hard to detect, since even the act of checking them for a short to ground can 'correct' the short... temporarily.

Try wiggling the wires attached to the printed circuit card ONE AT A TIME... very carefully, ONE AT A TIME, from the printed circuit card to where it ends at the 24volt device ... such as the 24volt gas valve, or the 24volt contactor inside of the AC unit that sits outside.

Shorts are very often where one of the 24volt wires passes through an opening in the sheet metal, and the where the sheet metal cuts through the wire insulation.... a good place to look is where the two small 24volt wires from the printed circuit board inside the furnace go outside to the AC unit, the wires pass through a hole in the sheet metal there... those are often the problem.

*Meantime, fit a 3 amp fuse in series with the + terminal (or red wire terminal) on the transformer, so that next time a short develops you will not burn out the transformer, instead you will just burn out the fuse.

You can buy the fuse and fuse holder at any auto supply, use a 3 amp fuse only, not a larger or a smaller fuse.

click here for a picture

Customer: replied 7 months ago.
Does not look like wires are short circuited, I checked wire resistance of all the thermostat wires. the resistance between 2 wires goes to AC is 18 ohm when not connected with the board but when connected with board and thermostat is connected the resistance is almost zero- 1 ohm. same thing happening with thermostat wires - resistance between yellow and red or yellow and green is almost zero, but when thermostat is not connected connected it is infinite. Unit was looking good when a/c was not on, as soon as i switched AC in minute or so fuse burnt. The transformer's primary coil is getting burnt.
Expert:  Phil replied 7 months ago.

Hello again, the zero or 1 ohm read between R, Y and G at the thermostat is normal when the thermostat is set for cooling and below the room temperature.. that is not a short.

Expert:  Phil replied 7 months ago.

Locate the two small 18 gauge wires that run from the thermostat terminal strip on the printed circuit card to the outside unit and disconnect them.

If you no longer blow the fuse then the short is either in that wire rubbing on a raw sharp sheet metal edge, OR the big relay coil is burnt (called a 'contactor')

Expert:  Phil replied 7 months ago.

Look all that over, let me know what you find, we can go from there.

If the fuse still blows when you have the two small 18 gauge 24volt wires that run to the outside unit disconnected, then the short is most likely internal to the printed circuit card.