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
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...
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
see figure 3-5 of http://www.tpc-nacg.com/servicehandbook.pdf with the
one difference that the start winding has a series resistor...
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...
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.
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. <G>
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.
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.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.