25 yr old Carrier...Air Handler Runs...compressor doesn't

[bobtx]

I've got a 25 yr old Carrier that's been a real work horse.  The air handler is working fine, but the outside unit is totally dead.  I replaced my other original Carrier 2 years ago and last year I had a similar problem and was told it might be the capacator.  Stole it from my old one and it worked great.  I'm wondering if it's the capacator again or is there another electrical component that would keep it from cranking up.  All breakers on.   Starting to warm up here in south central Texas, so wife starting to complain.  Fortunately it is downstairs unit, so a little cooler.  Thanks for any help.

Bob

 

[clman_old]

See if the contactor realy in the condenser electrical section is drawn in. If it is check for high voltage in and out of the contactor. If it is not, check for the presence of 24v at the coil terminals of the contactor. If no 24v are present check for a damaged t-stat wire from the condenser back to the house. If no damage to t-stat wire the fault is in the wall t-stat or in the furnace / air handler. If the contactor points are drawn in and no high voltage is present you have a bad or tripped breaker / fuse in the disconnect or main panel.

[bobtx]

I really appreciate the reply, clman.  I have to plead ignorance around HVACs.  I have a meter but wouldn't know how to use it around a HVAC and "high voltage" does concern me.   I hadn't thought about the t-stat wire.  It's been exposed to weather for 25 years and wasn't put in a conduit as I've been told it should have been.  Don't understand the "contactor relay" part, but I'll pull the cover and take a look.  Would a pic help?

Thanks for your help.

Bob

[clman_old]

If you can't check the voltage all you can do is guess and change parts till you get it right. - Or someone call for service.

[RegUS_PatOff]

[user=25771]bobtx[/user] wrote:

...  I have a meter but wouldn't know how to use it around a HVAC and "high voltage" does concern me....

HVAC is Heating, Ventilation, & Air Conditioning....

The only "high voltage" is the usual 240v AC

[bobtx]

I realize that and it hurts...I've tried it before.   It put me on my rear!  I just don't want to accidently fry something.  I know how to use the meter around plugs, etc. but not around a compressor.

[dkpd1581]

Look at the bright side, the unit is already broken.  How much more can you reasonable break it by checking things.  Two schools of thought say that you can check a unit energized (set you on your tail if your screw up - The Quick and The Dead style) or de-energized using your Ohm meter.  In your case I would choose option 2.

[Bobice]

Please read "Grasshopper"

Air Conditioning Contactor

 

 

An air conditioning contactor is an electrical switch that delivers line voltage to the compressor.

Sometimes power is supplied to both the compressor and the condenser fan motor by the same contactor.

 

When control power is applied to the contactor holding coil, the contacts should pull in, and the compressor should run.

 

Here are the steps to take when an air conditioning contactor problem exists.

 

If the contactor doesn't pull in when the thermostat is turned on:

First, verify that there is correct control voltage at the coil terminals.

Measure the voltage with a voltage meter, and read the coil voltage rating on the contactor.

 

If you're getting correct control voltage at the contactor and it doesn't pull in, turn off power to the unit, pull the leads off the contactor coil, and check the resistance of the coil.

You should get a readable resistance if the contactor is good.

A reading of infinity means the coil is open, and in that case the coil or contactor must be replaced.

 

If you get a readable resistance and the contactor isn't pulling in, push in on the armature (the set of contacts that move in and out).

If it's stuck and doesn't move in and out, the next step depends on the size of the contactor and the circumstances.

 

If it's a small air conditioning contactor, it's probably more cost effective to replace it on the spot.

If it's a large contactor or motor starter and replacement parts are readily available, then it might make sense to disassemble and repair the contactor or starter.

 

 

If the contactor pulls in and the compressor and condenser fan don't run, the first thing you have to do is determine whether the load device is actually getting correct voltage, and has not failed.

 

After you've verified that the load is not getting correct voltage, and has not failed,

verify that there is correct voltage on both the line side and the load side of the contactor.

 

Turn off power, and remove all leads from the load side of the contactor, then turn on power and energize the unit so the contactor will pull in.

 

Check voltage between terminals, and from each terminal to ground.

If you have a bad set of contacts, this should identify them.

Clean or replace the bad contacts, or replace the contactor.

 

If all readings are good, you may have a set of contacts that are failing when a load is applied across them.

 

To check for this, turn off power, re-connect all leads to the load side of the contactor, turn on power, and energize the system so the contactor will pull in.

 

Check voltage between the load side terminals, check voltage from the terminals to ground, and check voltage from the line side terminals to the load side terminals on each leg.

This will identify a bad set of contacts, or maybe 2 or 3 bad sets.

Clean or replace the bad contacts if possible, or replace the contactor.

 

If you have good voltage on all load side terminals, your air conditioning contactor has not failed, and the problem is in the load side circuit, not the contactor.

 

Is your air conditioning contactor noisy?

I have heard contactors that made loud humming noises, and that the customer didn't want to replace until it failed.

Those contactors worked fine, and I never had to replace them.

 

Chattering is different. It's when the contacts open and close at machine gun speed.

It makes a loud hammering noise, and is harmful to the contacts and the load device.

 

If you're working on an air conditioning contactor that is chattering:

Verify that you have full, steady control voltage at the contactor coil.

24 volt contactors chatter when receiving only 21 volts.

To raise the voltage, change the tap connections on the control transformer or replace it with a new one.

Contactors chatter when they were wired in series with certain types of solid state delay on make timers.

This was in a circuit with several parallel contactor/timer circuits controlled by a multi-stage temperature control, and supplied by one step down transformer.

The solution was to install a step down transformer for each leg, and control each leg with a relay energized by the temperature control.

The temperature control energized the relay, and the relay conducted 24 volts to the timer and contactor.

The chattering stopped.

 

When replacing an air conditioning contactor,

Be sure the control voltage rating is correct,

Be sure the load capacity (amperage) is correct,

Try to get a contactor with the same number of poles,

Try to get a contactor with terminals that match the original,

And always follow safety precautions.

 

[Bobice]

My apologies to the "Grand Sensei" If I am long winded.

[bobtx]

Thanks for all the input!  I took off the cover and saw a bunch of dead ants in the contactor, so it helped to make the problem look obvious.   I replaced contactor with the one from my old unit I replaced a couple years ago.  Sprayed some contact spray and it cranked right up!  Thanks for all the help.