Air handler doesn't come on with compressor?

elfie

Air handler works when independently turned on via thermostat, but when thermostat turns on compressor, the air handler is silent.

Strange. It was working fine yesterday.

Ideas?

mattmia2

y wire broken at the air handler, relay for high blower bad.

pecmsg

Bad Electrical Connection?

mattmia2

or the connection to the high winding on the motor or the high winding on the motor is bad

HVACNUT

If you switch the fan to on (not auto) and cool, does the fan stay on with the condenser, or turn off by itself but the condenser continues to run? 
What make and model thermostat?

elfie

I would imagine the fan would stay on if the compressor cycled off but I haven't tested this. If I switched fan to auto after the compressor came on I don't know what would happen at this point. The compressor turns on and off with the thermostat when I turn it up or down.

Actually it's an old mercury dial thermostat. As of yesterday it did automatically turn on and off the unit.

The AC compressor is such that it shuts down when overheated but this never happens I just know the design allows for this. Hopefully this served to protect system as fan may have turned off or malfunctioned leaving the compressor to continue working. If compressor continues to work without the air handler I guess we're looking at a very frozen system at some point.

It did cycle on and off last night and worked OK, and it was cool outside overnight and it was completely off when I woke up so everything seemed OK. And tried turning it on early this morning and only the compressor came on. It's an older AC system from the early 90s which has been working great up to this point. Maybe replacing the thermostat is a worthwhile first effort.

HVACNUT

Any dust build up in the thermostat?
With fan on its manual R to G switching, in auto it's mercury switching. Check the contacts.

clammy

It s 2023 step into the future and get rid of the t87 and put in a new electronic digital thermostat . Without a time delay on your outdoor unit you are liable to try and start the outdoor unit before the pressure equalize and w a scroll compressor this is not good . If the fan runs w the t stat fan switch on then it’s a relayor wiring if it does not run it s the sub base ,either get a new sub base or a new stat ,a new stat would be the wise thing to do . As for myself when home owner won’t upgrade to a new thermostat my job is done and I move on no sense dealing w old crap that’s 40 years old only the cheapest contractor and home owner still use them weather old mercury or new crappy digital t 87 they suck usually there seen on the same job that have stamped faced prison made w lic plate returns grills . Personally if it’s not the t stat I would chk the wiring and In door fan motor and capacitor and fan relay in the air handler could always be dead spot on the motor winding ,while up there chk your indoor coil and drain and condensate lines also check for air leaks and loose duct connection even though not related it always good to chk things while up there
Peace and good luck clammy

mattmia2

Or you could add a delay to the condenser if it doesn't already have one.