How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site. Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Phil Your Own Question
Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8702
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
Type Your HVAC Question Here...
Phil is online now
A new question is answered every 9 seconds

hi I have a byant central ac I changed capacitor and compressor

This answer was rated:

hi I have a byant central ac I changed capacitor and compressor will not come on fan is running on outside unit. contactor plunger does engage. could contactor be bad.
Welcome to Just Answer!.

I will stay with you until the situation resolves.
I hold questions open after positive ratings to allow for unlimited follow up.

It is not likely that the contactor is bad if the fan comes on and runs.

You will need a multi meter ($30 at a hardware store) to check the compressor motor windings.

The compressor has 3 terminals C, S and R

The S to R ohm read should be the total of the C to S and C to R reads

typically 2 Ohms C to R, 3 ohms C to S, and 5 ohms S to R.

Then check each terminal to the copper tubing, you should get no read at all.

Let me know how all that goes, we can proceed from there as long as you keep rating my assistance positively... I hold the question open in that case with no time limit.

Customer: replied 4 years ago.

are you talking about the three prongs on the compressor


Yes.... those 3 prongs, marked C S and R.

If a person is trained, competent and safe working on appliances he might consider doing those tests mentioned, Otherwise a trained HVAC technician must be called in to do the tests.

Be sure the power is OFF... double check that with two voltage meters to be sure you are getting no voltage to the unit, then examine the 3 prongs and their wires and terminals and make sure none show signs of over heating.

Make a good drawing of which wire goes to each prong,

then remove the wires from the prongs and use a multi meter set to OHMS to check those readings as mentioned above.

Customer: replied 4 years ago.

will do that in the morning


my plug on the compressor is a rubber boot type only one way on. Also is there a pressure switch any where on the air handler or furnace unit that could be overriding the compressor. This is a split unit only thing in the condenser unit is the contactor and capacitor. No switch high or low outside are there any inside that may be stopping compressor from engaging.

Hello again, there is no pressure switch on the air handler that overrides the compressor circuit.

If the contactor that puts power to compressor is pulled in, then the controls are functioning to put power to the compressor.

You need a multi meter to check that.. set it on AC volts, range up to 250 volts, and check across the two wires that go from the contactor to the compressor... if it reads the rated voltage, most likely 230 volts or close to it, the contactor is closing its contacts and sending power to the compressor.

Stay in touch, I will keep this question open for you as long as you keep rating my replies positively.

Pull the plug on the compressor and check for OHM readings between all combinations of the 3 prongs, and between each prong and the copper tubing on the unit. Tell me what the reads are.

Customer: replied 4 years ago.

the reading are 3ohms C to R C to S 3ohms S to R 6ohms


did scratch test of each prong and was infinity

Hello again,

The motor windings as most likely OK, but your reading should have a bit lower for C to R, than for C to S. Your results are close enough to the normal error range that I think the windings are OK however.

Next thing to check is the plug itself, it may have burnt out inside... check that by unplugging it from the compressor... and using your volt meter to determine if there is power in each of the plug sockets when the contactor is pulled in.

If the plug is good, and there is power across each socket then the compressor should at least try to start since the there are no open windings.

Let me know what you find, we can go from there.

Customer: replied 4 years ago.

ok question I will have to depress plunger on contact. Do I just unplug fan from capacitor to turn it off The reason is I have to remove fan from top of unit to get to compressor. don't want fan coming on.

Hello again, one of the wires from the fan goes directly to the contactor that has the plunger on it... disconnect that wire and tape it off with electrical tape.

Then unplug the compressor, and check for voltage across all 3 receptacles on the plug. This work is intended only for trained technicians, not most home owners... you are of course assuming all risks.

Good luck, and be safe.

I would set each test up with the power off, then energize the unit to take each read.

You can also use an ohm meter, with the power off, to see if you can get continuity between each plug socket and the other end of the wire that serves that socket... you can do that with no power on at all.

Let me know how it goes.

Customer: replied 4 years ago.

I have 240 at the plug. will the compressor come on if the exhaust fan

is not reconnected? if I depress the contactor


I also tried the "cooling" of the compressor with a bag of ice just in case some internal because of heat .


would a hard start kit be worth looking into???

Hello again,

If the compressor were off on its internal over load you would not have gotten the 3 and 6 ohm reads you got across all 3 combinations. The internal over load breaks those connections... so because there were good ohm readings we know that internal overload was OK. That assumes you took the readings properly.

When I take such readings, I take them 3 times to be sure I have done it correctly and made good contact and was touching the prongs properly.


The fan is wired to come on whenever the contactor is pulled runs at the same time the compressor runs.

Regarding the 240v at the plug... it needs to be tested between the C to R sockets, and at the C to S sockets.

If just one of the socket connections is bad, the compressor will not run... or even hummm like it is trying to start.


Since the windings gave good reads, if there is actual voltage at the 3 prongs the compressor will either humm heavily as it tries to start, or hum and trip the will not just sit there silently in 99% of the cases.

in 1% of the cases, you will get good ohm reads across all combinations of the 3 prongs, but compressor will not run because there is a bad connection inside the compressor.... a connection that will pass enough current for the test meter, but not enough to run the compressor. Thats exceedingly rare.

I strongly suspect that you are not getting 240 across both combinations of the plug sockets for that reason. C to R and C to S

Take a second hard look at all of that.

We can go from there.

Customer: replied 4 years ago.

did that test twice between cr and cs and yes it is 240


If the windings are good, then they will be allowing heavy amperage to flow between C and R, and if the capacitor is good, heavy amperage will flow between C and S as well.

If that were happening however you would here a heavy humm sound... the fact that you are not means there no power getting to the prongs.... or that your ohm readings were no good... however your ohm readings were in right range, indicating that you did the ohm testing properly.

The pins in the plug may not be making proper contact with the prongs... that is a bit hard to test.

If you buy a clamp around type amp meter with a scale that goes to 100 amps or higher you test for that.... however the lack of a heavy hummm indicates there is no voltage getting to the prongs.

Let me know, we can go from there as necessary.

Customer: replied 4 years ago.

could I cut the plug boot and put new prongs on the wires


Customer: replied 4 years ago.

also here is how the wire comes to the plug. yellow from contactor and black from contactor blue from herm on capacitor. Assuming I am correct on volt test. I put prong in connector of plug leading from black and other in blue got somewhere around 240. Next I put the lead in connector on plug with yellow wire going to it and other in blue and got 238-240 . when you line up plug on compressor top two prongs are plugging into black and yellow bottom prong on blue from capacitor.

Hello again, the black wire goes the C terminal as a rule, the Yellow to the Run terminal, and the blue to the start terminal.

The wiring diagram and the markings on the compressor itself if you look closely will show C S and R...but the plug only fits one that the wiring is probably correct...your description of the blue wire going to herm on the run capacitor is correct.

You can buy an after market wire terminal kit for those at Grainger Industrial supply or from Johnstone Supply company or on line.

Tell me if the prongs on your compressor are round with a flat piece welded on one side, or just completely round.

We can go from there.


Customer: replied 4 years ago.

will check but i believe just roind


Customer: replied 4 years ago.

Will check in morning if round I would assume I need one type of kit if flat and round a different type.

If they are just round, go to Grainger industrial supply or Johnstone supply and ask to see their air conditioning compressor terminal kits and look on the package for the brands they fit... then buy those.

Your assumption is correct.

Let me know how that goes.

Customer: replied 4 years ago.

what gauge wire 8/10/12

The kits come with the proper wire gage, 12 gage for 3.5 tons and under already attached to the terminals... 10 gage over 5 tons.

We are looking for brass terminals with a screw in them to clamp them to the posts on the compressor. You may want to take a one inch long sample of the wire with you to the wholesale house so that you can insure the copper wire diameter is the same as what you have.

Its a special finely stranded wire. These are short runs so that the sizing is quite a bit smaller than an electricians wire size chart shows for long runs.... the chart that applies here is the 'electrical panel wire size chart'... not your concern however as what is supplied is generally correct... but do check it against a one inch sample of what is in place now.

You want a kit with the wires already permanently attached to brass screw clamp type terminals... not any that require any soldering.


Customer: replied 4 years ago.

I found the terminal kit today but only in 10gauge wire

Can I splice the 10gauge to the exsiting 12 gauge on the plug

Hello again, yes you can use wire nuts to splice the 12 gage wire the 10 gage wire that comes with the new terminal kit. The right size wire nuts are color coded RED.

Ideally the new terminals with the 10 gage wires will reach clear to the contactor and the run capacitor and you will not need to do any splicing.

Stay in touch as needed, and please do not forget to rate my work for you, its the only way the company pays me from part of your deposit. I will hold the question open without a time limit after you rate my work so far as good or excellent..


Customer: replied 4 years ago.

Hi Phil:


I went to turn on today and could hear the contactor humming so I replaced the contactor.. ... now the thermostat wire do have 24 volts on them they may have grounded out on unit. did that fire transformer of is there a fuse that might be blown?

Hello again.

Contactors very often humm when they are operating... thats normal.

It will be best if you consult with me a bit before you start changing parts... each parts change introduces another opportunity for error.... if you accidentally bumped one of the 24 volt wires against one of the 240 volt wires it would have burned out the control card inside the blower/ furnace unit... thats a complex fix.

The fuse for the 24 volt circuit is inside the furnace/ blower unit that is inside the house...look for an automotive type fuse on the printed circuit card.

Look on the wiring diagram glued to the inside of the furnace for more fuses and locate the transformer... use a multi meter to check across the two 24 volt terminals on the transformer, tell me what voltage reads you get.


Training on how to repair these units generally involves 2 years at a junior college, and a 2 to 4 year apprenticeship... you can cause a lot of damage to your system and even risk fire hazards by working on your own equipment, especially if you begin changing parts at random.

The real problem here, before you shorted out the 24 volt circuit, is getting the dual run capacitor wired correctly... were you able to check that out as I mentioned in my previous posts? if we do first things first, that generally works, if we jump around at random changing other parts it doesn't go as well.

Let me know... I am off to a doctors appointment now and will be back this evening until 11 pm west coast of the USA time.

Let me know if you have identified the terminals on the new run capacitor, and what the old capacitor micro farad readings were.


Customer: replied 4 years ago.

Hi Phil:


Just an update. I ran the terminal kit without a splice. I bought a hard start kit and the compressor is running like a champ.



Good work!

When you do this kind of work yourself, you gain valuable experience that will help you in many other aspects of life.

I will put up the rating box now, and if you will rate my coaching positive the company pays me a percentage of your deposit.



Phil and other HVAC Specialists are ready to help you