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NYCGeordie 05-14-2008 09:51 AM

York AC Unit help needed

Just closed on a house (it was a foreclosure so no guarantees or warranties on anything) and I'm working through all the things that need fixed. This house has a York AC unit outside with an air handler in the crawlspace. Airhandler seems to be working fine from the thermostat in both emergency heat mode and when set to Cool however the AC unit outside will not start for regular heat or for cooling. I took a look "under the hood" and can't find any burned out wires or short circuits, the 240vAC supply is also good and it appears to be getting power from the thermostat as well. Here is what I did find, the solenoid energizes when the thermostat operates but doesn't start the motor, sounds like the compressor is operating (although very quiet), if I manually operate the solenoid by pushing on the plastic tab in the center the fan and compressor definitely start up. Strange thing is that the Fan and compressor start when I push the solenoid tab in but when the thermostat operates the solenoid it is forcing the tab out. Anyone got any suggestions before I start replacing parts blindly?

Thanks Dave

glennjanie 05-14-2008 10:46 AM

Welcome Dave:
You have used good logic in your testing, checking for power and for 24v power. And you know the components will run if you force the contactor. I don't understand the lo-voltage solenoid in the contactor working backward. It is still AC voltage so it shouldn't help to reverse the wires on the solenoid, but it may be worth a try.
There also may be a small circut board in the condenser unit that has a transistor or resistor burned out on it; look for one that is black and crispy. If that's the case, a new circutboard is the answer.
Could someone have replaced the solenoid in the contactor and got it upside down? You might try taking it apart and see if it can be turned around so it will pull in rather than push out.
BTW I admire you for looking for a burned off wire first; that has been the problem on 2 out of 3 service calls I have done. Please let us know how it turns out.

NYCGeordie 05-14-2008 10:58 AM


Thanks for your quick reply and suggestions. I'll be having another look after work tonight (and I'll remember to take my glasses with me this time). Used to work on electric typewriters and photocopiers so the burned wire thing is pretty well burned into my brain.
The ides that someone might have switched wires or moved stuff around also crossed my mind, the house used to be a rental and the last tenants trashed the place pretty well, have a feeling they didn't get on too well with their landlord!

kok328 05-14-2008 04:33 PM

If the contactor coil is AC, switch wires won't help. I don't believe the solenoid is pushing out, it's just not pulling in. This could be a bad coil on the contactor solenoid or the compressor is being held out for other conditions.
Other conditions could be; bad T-stat calling for cool and heat at the same time however, one is suppose to take precedence; low ambient temperature or it could be locked out on low pressure. Check the freon pressures (hi/lo) on the unit and maybe use a leak detector to check for leaks. If this has service ports on the fridge lines, those are highly suspect as well as where the fridge lines enter and leave the condenser coils, same may apply to the evaporator lines/coils but, the compressor coil lines are more typical. You could temporarily bypass the pressure sensor to test it but, don't run it for long (a few seconds tops). However, this will only isolate the problem to pressure if the compressor comes on w/the sensor bypassed. You'll have to determine the system pressure to know if the pressure sensor is bad or whether the system is low on charge.

NYCGeordie 05-15-2008 07:39 AM

OK, good news on this problem at least - checked it out again last night and found someone had switched the 24v wires to the solenoid (solenoid actuation can only be by DC current as AC would produce a magnetic field that switched poles at the same frequency as the AC input). They are connected to the board with screw terminals where the input from the thermostat connects and when I took a closer look (wearing my glasses) could see they weren't exactly 'tidy'.
I suppose it's an easy mistake to make but only if someone was trying to fix another problem, otherwise no need to even touch the wires.
Don't know if there is any connection here, but the home inspector insisted that if I bought AHS through him they would "come out and fix the air no problem", seemed suspicious to me so I didn't take him up on his offer!
Thanks for your help

kok328 05-15-2008 04:03 PM

Yea, thinking about this, a DC transformer would explain the weird behavior of the contactor coil. Also, good call on the AHS, most come with a deductible = to the cost of DIY repairs.

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