No start on old 2 ton compressor - troubleshooting

I have a small cabin with a 2 ton heat pump circa 1992. It is very old, but, has not been used a lot in it's life. It may have died today. It refuses to startup the compressor completely. It has power, I can hear a transformer buzzing inside the outdoor unit, and the blower system still works fine. Just no compressor.

Can anybody tell me what the next trouble shooting steps should be? Anything else that would prevent the compressor from starting, besides a bad compressor? (It makes no attempt, no noises)

If it needs repair we will likely replace it with a mini split, I'd just like to figure out if it is dead or not before I call somebody out.

 

It sounds the compressor has likely seized up. Thirty years, regardless of how often it was used, is a long lifetime for a compressor to last. It might be best to save the expense of a service call and put that money toward a new system and retire the old one.

 

Is the condenser (outdoor unit) fan running when the thermostat calls for cooling? If so, then it means that the contactor is working. If not, then it might be something simple such as a break in the cable bringing the control voltage (24VAC) from the house to the outside unit. The 24VAC signal energizes the contactor (relay) and comes from the air handler/furnace via a multiconductor cable.

 

Welcome to the forums.

It sounds like you are discussing a 2 ton packaged heat pump.
A model number would be very helpful.

Most systems have at least low pressure and some have high pressure refrigerant monitor switches.
If the system is low on charge the condenser.... condenser fan and compressor... won't run.

 

Thanks for the info! I'll get the numbers and take off the cover when I'm back out there tomorrow. The fan never starts either, it is like something is just saying "stay off". No grunting or anything like it wants to start.

Are the various sensors normally open or normally closed? Can I just short across them and see what they do? Or just unhook them? I just want to figure out if it's a simple sensor or totally dead before I call somebody. Not really afraid of damaging it at this point...

 

No, do not short anything. Use your meter to figure out what is wrong. Don’t make your problems worse by damaging the unit even more.
Is the contactor coil getting voltage?
If so is the contactor closing?
Is there power on the load side of the contactor?
Are there any pressure switches? Are they open or closed?

 

The inline sensors (low pressure, high pressure) are normally closed. If the sensor detects a problem it opens, thus preventing the 24VAC control voltage from reaching the contactor. If you have a voltmeter you can check for 24VAC at the contactor when the thermostat is calling for cooling. You might want to turn off the 240VAC going to the outside unit (there should be a quick disconnect nearby) to prevent the possibility of getting a lethal shock. The 24VAC is connected to the contactor coil and the terminals are usually on the sides of the contactor and is independent of the 240VAC.

If you don't have 24VAC at the contactor, check where the cable from the house comes into the outside unit. The 24VAC cable usually connects to the internal wiring using wire nuts. If you have 24VAC where it comes into the condenser unit, then likely one of the pressure switches has opened.

If you don't have 24VAC at the input to the condenser unit, check for the voltage where the cable connects to the air handler/furnace. If you have voltage there, there is a break in the cable going out to the outside unit. If no voltage there, you need to troubleshoot the inside system ( thermostat & air handler/furnace).

 

Please read first for A/C repair