AC Tech help needed
 

AC quit working last night. 5 ton Lennox HS26 unit about 23 years old.

Moved in 4 years ago, AC wasn't working, Called service, found a bad Dual Run Capacitor, replaced it, no issues since.

Suspect it has failed again but haven't been able to check it. Rated 70/5.

Discovered that the manual calls for an 80/10. I assume the tech didn't have that one so installed the 70/5 that he had.

I have a 3 Zone system controller, each has its own thermostat. Had 2 of them set to cool last evening when I saw the temps going up, not down. Checked and discovered the outside unit not running. Slept with the windows open, luckily it was coolish overnight and today.

I have 240V at the Line side of the contactor. With the main floor thermostat (a Nest) calling for AC, the blower comes on but the outside unit does nothing. I have no idea if it should, but I noted that the contactor did not pull in.

Checked for 24V from the thermostat. I get nominally 26.5V both at the stat wiring and at the terminals of the contactor.

I pulled the disconnect and checked resistance across the coil at the terminals. From what I understand, I should get 12 to 27 ohms across the coil.

I'm only getting 10.8 ohms. Could this be my issue?

Before pulling the disconnect, I manually pushed in the contactor. The condenser fan started running. I admit, I did not listen to see if the compressor kicked on. I only ran it briefly. As soon as I stopped pushing on the 2 pole contactor, the fan cut off.

I don't know what that tells me about the contactor or the capacitor.

There is also one more component, a Supco Super Boost "relay and hard start capacitor" installed. Supposed to boost compressor starting torque. It has been there all along, no idea when or why it was installed but from what I can tell, even if it was dead, it would not prevent the dual run capacitor from starting the fan and compressor if it was in good shape.

I may be able to check the capacitor tonight with a friend's multimeter.

But wondering if the contactor resistance is telling me it's a bad contactor?

Thanks for any advice/help.

Nevermind. The capacitor tested right on the money (69/5).

Thought about all the reasons the contactor might not pull in and realized could be a Low Pressure Switch or the High Pressure Switch which has a reset button. Bent down and pushed in the reset and it clicked!

Put the capacitor back in, installed the disconnect, turned on the thermostat and it is running/cooling normally.

Obviously it tripped for a reason but I suspect the condenser coil needs a good cleaning and the dirty coil likely caused the trip.

Until further notice I'm sticking with that assumption. I'll give it a good bath in the near future, hopefully it will be fine until I can get back to it.

Supco Super Boost is a band aid for hard start low voltage issue compressors , 23 years old may be time