AC starts, then immediately shuts off

Hello,

Could someone please help me with the problem with my 12 yrs old Trane AC.
Air handler model: Trane TWE031E13FB1 2.5 ton
Problem symptoms:
air handler makes a sound like fan or motor starts, tries to speed up for 1-2 sec, then shuts off (or stays on low RPM, not sure), then waits few seconds and tries again, and so on.
This allows it to blow some cool air periodically, but not enough.
The fan on the compressor unit (outside) turns on from time to time.
First I checked start capacitor - it's fine.
Then replaced control board - no change.
Moisture sensor works fine.

Are there any other common problems with such symptoms?

Thank you.

 

Welcome to the forums.

You have an ECM motor. It electronically varies the speed.
Nothing outside will cause a blower issue so don't replace any more outside parts.

These electronic motors are not easy to repair. They cost over $750 to replace.

You'd need to check the blower to see if the motor is binding. You may need to just replaced the end control unit.

Midwest HVAC Parts. Variable Speed Motor Module - Trane MOD00816

 

Thank you for the response!
I did not replace any parts in the compressor unit outside. I just checked the capacitor, it appeared to be fine, so I put it back.
I did replace the control board in the air handler (part CNT03600), but it did not change anything - still same symptoms. Is that what you meant by replacing end control unit?
I am not sure how to check the motor. Can I just remove the air handler cover, then turn the AC on and see if motor rotates?
Again, thank you for your help!

 

The ECM motors have 2 main parts: the motor itself, and the control electronics. What PJmax was referring to is the control electronics (module) that bolts to the back of the motor. With the power turned off, you can check to see if the motor shaft & squirrel cage move freely (no binding or rubbing). If the blower motor isn't turning on (or shuts off quickly), there are 3 things it could be: the air handler circuit board (you said you replaced it), the ECM motor itself (possible, but unlikely), or the motor control module (most likely cause).

 

Ok, got it.
Motor shaft & squirrel cage move without a problem.
So I wanted to check the motor control module, but it's not easy to access.
Do I have to unmount the whole blower (not sure if this is the correct name for that big cylindrical part with the squirrel cage and the motor inside) in order to access ECM motor control module? Or maybe there is an easier way? Sorry, could not find any useful video.

 

To access the motor, you'll most likely have to remove the blower assembly. Usually, the blower assembly slides out (at least in a typical furnace).

 

Still trying to figure out how to get to the motor. That blower assembly is bigger then the opening so it just slides out a little, and then gets stuck. Not a DIY-friendly design

 

Finally figured it out, removed the motor control module, replaced thermistor (which seems to be a common issue) - still no change. So I tried all chip ways to fix it
Or maybe there is some more?
Replacing control module is another $340 - not sure if I want to risk it.
I am going to replace the AC, since it's 13 yrs old and I already spent $350 for repairs last year.
Thank you everybody! Got some DIY experience