A/C fan runs, no compressor

[gado]

The outside A/C condensor unit (Trane TTJ type) of my home has a CSR-type compressor. It has two phase 120VAC fed to it. There is no (apparant to me) low-pressure cutoff switch on the unit. When the thermostat calls for cooling, the fan runs but not the compressor, even after it has rested for 24 hours. The A/C ran last summer. I just bought the house last year so I don't know the history of the unit or its maintenance. I took the cap off the freon return line and pressed the valve, there is at least some freon in the system but I don't have gauges to see how much pressure.

There is a run capacitor across the S and R terminals of the compressor. The electrical schematic diagram of the compressor shows a resistor (heater?) between the electrical coils of the S and R terminals.

The resistance between the C and R terminals is 2 ohms. The resistance across the S - R terminals (and the run cap) is between 1-4 megohms. I take it this means my run cap is not shorted. Based on the high resistance, is the S terminal open or the S coil burned up?

I have not measure the voltage to the compressor while the relay is engaged, so one of the relay contacts may be bad...but I thought I would try some meter probing first...

Gado

[gado]

Author adds:

This is a permanent split capacitor type hookup where both run capacitor and start winding stay in the circuit during start and after the motor is up to speed...the C terminal has an internal thermal overload protector...

see figure 3-5 of

with the one difference that the start winding has a series resistor...

Gado

[gado]

Author adds (I keep reading...)

I think the resistor of my unit that I was confused about is a start winding thermal protector like in figure 4-13 of that same service manual...maybe if the run capacitor shorts it would cause the thermal protector to trip...

Anyway, looks more and more like my start winding is bad...just my uneducated grasp of this problem...

Gado

[HeatMan]

The resistor is not a heater, but a bleed down resistor. It's there to hopefully keep people from getting hurt.

[Dr. Hardcrab]

"gado" wrote

Do you mean single phase 208 or 240?

[PrecisionMachinisT]

Maybe bugs or other crud in the compressor contactor, since its been sitting for so long.

Pull the your mains disconnect at the condensor unit and jumper "Y" to "R" a few times in order to shuttle the magnetics...your relay solenoids will still work work because the 24 vac control voltage comes from your air handler transformer.....after that, suggest maybe blow any debris out of the contactor with compressed air, if available.

If the capacitor is suspect, usually best to just replace it with a new one.

[Jeff Wisnia]

I took the cap off the freon return line and pressed

Tsk, tsk, now that you've told the world what you did...Expect a visit from the global warming nazis swithin a couple of days.

It sure looks that way. The other end of the start winding ought to be connected to the C terminal, so you should have seen a low resistance between S and R, (the sum of the start and run windings) regardless of what else may be tied in parallel.

Go ahead and try that. If it's a dirt simple unit the fan motor is being switched on by the same relay as the compressor, so the relay's probably doing it's thing and I bet you'll find voltage going the the compressor too, stymied by an open start winding.

Let us know what it turns out to be.

Jeff

[TURTLE]

This is Turtle.

If you have 1-4 megohms between S and R of the compressor terminals, It is looking like a burnt start winding in the compressor. now to be sure remove wires from the compressor and read across C and S on compressor terminal and if you don't get a normal OHM reading like less than 100 and get Megohms, You compressor start winding is burn if you get the megohms reading or none.

TURTLE

[Stormin Mormon]

Large numbers of global warming nazis, probably driving black Hummers which get 2 MPG and don't have catalytic converters?