HVAC question, not RV related

[timjet]

My home Air Conditioner compressor fan is not turning on. I can hear the compressor running and it gets warm around the AC housing but the fan is not running. I had the start capacitors checked and they are good. I pushed the fan with the compressor running and it did not start turning however it is free to turn. I checked for voltage at the fan motor and got none.

This is a Rheem unit with a control board. I traced the fan motor wires back to the control board with the compressor running and again got 0 voltage. I don't want to just start replacing parts so is there any way to test the control board? Any other suggestions?

[MrMark52]

I used to install a lot of Rheem’s back in the 80’s/90’s. Rheem is very good about providing a wiring diagram inside their unit whether it’s the furnace or the condenser.

Condenser motors are known to fail open.

Did you feel the body of the motor to see if hot or warm?

Follow the wiring diagram to troubleshoot. Back in the day, motors were powered direct from the contactor that also powers the compressor. Yours may be variable speed which likely requires a control board.
You would need to confirm if the motor was operable on 240vac (or 120vac but most likely 240vac) and if so, connect directly to 240vac (or 120vac) bypassing the control board.

[timjet]

Thanks Mark, yes the compressor motor is 240v via a yellow and blue wire each 120 volts. There is also the yellow/green ground wire and a brown and black wire which I think are signal wires. I'm not sure what the signal wires are for.

The motor did not feel warm to the touch when it should be operating. But again when testing I did not see any voltage even though I could hear the compressor running.

However I will power the compressor motor directly with 240 volts via the blue and yellow wires and report back.

[DFord]

Don't try to run the compressor with just two wires! It needs 3. Common, Start and Run. A capacitor has to be between the run and start wires. Common being one side of the 240v and run being the other side and start coming from the capacitor.



The fan probably has a capacitor too. The capacitor may be dual purpose with one side for the fan maybe 5 to 7 mfd and the other side for the compressor with a rating 35 to 55 mfd. Refer to the wiring diagram and the figures on the side of the capacitor. Make sure you've got them wired properly. If the top of the capacitor is puffy instead of flat, it's bad.

[MrMark52]

DFord’s right - it takes 3 wires to run the compressor - C (common), R (run winding), and S (start winding). There will be a big metal canned capacitor connected between the R and C terminals and depending, there will be a soft or hard start device in parallel with that capacitor (and if a hard start type system, there will be a current relay across the start winding).
But your issue isn’t the compressor right now, it’s the condenser fan motor.

Again, DFord is right on the condenser motor - it will have 3 or 4 wires - 2 for 240vac (color TBD). If a 3 wire motor, the 3rd lead goes to a run capacitor and one of the 2 leads is also tied to the capacitor. If 4 wire, 2 of the leads will be brown, and they are both hooked across the capacitor.

A green or green/yellow wire is generally always ground.

My only comment about the temperature of the motor was if hot it tells you it was getting power but may have tripped out the internal thermal overload (which likely means either it’s capacitor has failed or the motor is shot).

Failed capacitors don’t always bulge, but a bulged capacitor is a good sign that it has or is failing.

[timjet]

I know the compressor is running because I can hear it and feel the vibration. I feel pretty sure the AC is not cooling because the compressor fan is not running. I took both the capacitors to a HVAC supply house and they both checked good. The fan motor has 5 wires running to it. Blue-120v Yellow-120v Green/Yellow-gnd, Black-sensing, Brown-sensing.


I removed about 1/2 in of wire insulation from the yellow and blue wires on the wire harness near the fan motor. With the compressor running I checked voltage at the points I removed the wire insulation and got no voltage on any wires. However when I put one probe of a multi tester on the yellow wire and the other probe on the blue wire I got 28 volts.


I cut the yellow and blue wire from the compressor motor (leaving enough to be able to reattach later) and made up 2 jumper wires. I then disconnected the two 120 volt wires that come from the CB box to the computer board on the AC unit and connected the jumper wires from the motor to these wires from the CB box. I left the yellow/green wire attached to the motor. I did nothing to the brown or black wires. I reapplied power and the fan motor did not move, did not try to move and when I spun it it did not continue to move. The motor did not get warm. The motor was supplied voltage directly from the CB box without the aid of the capacitors but I would think I would get some movement from the motor. Again I did not.



So it would appear the fan motor is at fault but before I cut those wires leading to the fan motor I tested voltage going to the motor and got nothing except for the 28 volts as I described above. This would seem the control board is at fault.

Originally Posted by DFord

Don't try to run the compressor with just two wires! It needs 3. Common, Start and Run. A capacitor has to be between the run and start wires. Common being one side of the 240v and run being the other side and start coming from the capacitor. Never did mess with the compressor other than to just turn on the AC when testing voltage at the compressor fan motor.




The fan probably has a capacitor too. The capacitor may be dual purpose with one side for the fan maybe 5 to 7 mfd and the other side for the compressor with a rating 35 to 55 mfd. Refer to the wiring diagram and the figures on the side of the capacitor. Make sure you've got them wired properly. If the top of the capacitor is puffy instead of flat, it's bad.

There are 2 capacators, one is

Pictures of the capacitors is attached.

Originally Posted by MrMark52

DFord’s right - it takes 3 wires to run the compressor - C (common), R (run winding), and S (start winding). There will be a big metal canned capacitor connected between the R and C terminals and depending, there will be a soft or hard start device in parallel with that capacitor (and if a hard start type system, there will be a current relay across the start winding).
But your issue isn’t the compressor right now, it’s the condenser fan motor. Correct.

Again, DFord is right on the condenser motor - it will have 3 or 4 wires - 2 for 240vac (color TBD). If a 3 wire motor, the 3rd lead goes to a run capacitor and one of the 2 leads is also tied to the capacitor. If 4 wire, 2 of the leads will be brown, and they are both hooked across the capacitor. 5 wires as described above.

A green or green/yellow wire is generally always ground. I traced that wire and it goes to ground.

My only comment about the temperature of the motor was if hot it tells you it was getting power but may have tripped out the internal thermal overload (which likely means either it’s capacitor has failed or the motor is shot). Motor did not get hot or move when powered directly from the CB panel.

Failed capacitors don’t always bulge, but a bulged capacitor is a good sign that it has or is failing.

They checked good if the HVAC checked them properly.

[MrMark52]

You need to track down the wiring diagram before cutting any more wires! I don’t understand why you would cut wires to the compressor if the compressor is running.

I failed to mention previously - quite often compressors have heaters in the base, so there will be 2 more wires. So C, S, and R going to the covered terminal area of the compressor, and 2 wires going to the heater for a total of 5.

Unless the HVAC shop load tested the capacitors, just measuring with a meter isn’t a good test - although in your case, I don’t think you have a capacitor problem.

The first capacitor picture (metal cased) is probably the compressor run cap based on its MFD rating although I’m surprised it’s 370VAC rated (usually they are 440VAC).
It’s also a single purpose capacitor (compressor), not dual purpose (compressor and condenser fan) based on the indicated ratings.

The 2nd picture of the black plastic cased capacitor is a start capacitor. It’s ratings and construction are typical of one for a compressor. Not needed for a condenser fan.

[timjet]

Thanks Mark, I did not cut the compressor wires. I cut the wires going to the condenser fan, I probably miss led you on that. I fed voltage directly to the fan blue and yellow wires and the fan did not spin. I gave up and called a HVAC guy and he confirmed this morning that the fan motor is at fault. I ordered a new one, arriving Friday. I will install it.

[MrMark52]

Originally Posted by timjet

Thanks Mark, I did not cut the compressor wires. I cut the wires going to the condenser fan, I probably miss led you on that. I fed voltage directly to the fan blue and yellow wires and the fan did not spin. I gave up and called a HVAC guy and he confirmed this morning that the fan motor is at fault. I ordered a new one, arriving Friday. I will install it.

Thanks for the reply timjet! I was banking on the problem being the condenser fan motor.