Air conditioner STILL won't work in an outlet that checks out okay. Help?

Question

This is part two to the question that I was helped with earlier today (thanks to all who answered). Part one was: I had power in an outlet but my 240V Air conditioner would not work when plugged in. I called Con Ed who had botched up some work a few months ago, they came, they blamed it on the last crew and they fixed it. My 2nd floor A/C works but my 1st floor still does not.

I ran the A/C by extension cord to the upstairs outlet. Plugged it on, it works. So I know the A/C is not faulty.

Next I bought a multimeter and tested the outlet.
Voltage is 240 which is where it should be.
Resistance is between 4.4 - 4.3 which the manual says is ok.
Continuity is OK according to the multimeter, in other words the multimeter buzzed.
I also measured diodes not knowing what that means but it came up OK. The manual says it should be 0.5 - 0.8, mine was 0.5.

So I'm stumped! I have the right voltage and everything else looks good. And I know my A/C is ok. Why won't it work?

Answer 1

**Note: a 240 V plug does not have "Straight" prongs on it, if your AC plug does have straight prongs it is "NOT" a 240V AC - in there lies the problem.

If it does have a 240 V plug determine that the outlet and the plug is a match. Today, the outlet is a four-prong outlet ( two grounds), which replaced the three prong. If the outlet is 4-prong you need a 4-prong plug or you will have to get a 4-prong plug installed on the unit.
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NOTE2: I have not seen any extention cord to operate off of a 240V outlet, or higher. Because of the high voltage there are strict limits on this, and how far a device plugged into these outlets can be located from the outlet itself.
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I believe, the extention cord was plugged into a 120 V outlet. Have you tested both outlets in the exact same manner.
a. From pos to ground ( +voltage)
b. From neg to ground (-voltage)
then its a regular outlet, this can be seen again, look at the orientation of the prongs on the outlet, and how many prongs total there are. (regular outlet:straight prongs, 3 Pos, Neg, Grnd).

I believe " Continuity Test" are to be done on an outlet with the power removed. You may have damaged your meter if you did run this test directly on the outlets.
Does your meter have a "hold" feature, and you are seeing the last value rather than seeing the current value of this outlet.
Does the AC have a fuse / breaker that needs to be reset.
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Once all of this is eliminated and the main breaker is on, and is rated higher amperage than the unit will require then we can move on.
Just so you know, a Diode has two leads to it. Upon reading it you will get an extremely high resistance in one direction, and an extremely low reading in the other. This means, it will allow current to flow in one direction only. If it test low or high in both direction then it is faulty and needs to be replaced. when i say high I mean in terms of " Infinite Resistance" also remember, Resistance Test are performed with the power removed to the device, otherwise, damage to the meter can occur.
If testing the voltage of a Diode (power on), it should give a reading of .6 or .7 volts, you may want to allow it to fall within the readings your manual gave you, it should not matter that much. But this test is done on the voltage not the diode test of your meter.
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Let's test the accuracy of your meter first, test it in a 120V outlet and be sure the results are (115V - 120V) this is acceptable.
Go to the manual for the meter and be sure it does not have any memory function, or hold function that needs to be turned off.

1.Now test the upstairs outlet - write down the results.
2. Plug the AC in to this outlet- Note how many prongs are on both Cord and outlet.
3. Unplug and test outlet voltage again, and write down results in the next column.
4. Bring cord with you to the non-working outlet.
5. Test this outlet voltage (ground to pos, ground to neg)
6. Write down the results next line
7 plug in the cord to this outlet, then the AC.
8. Does it work? If yes, you are done, if no continue.
9. Unplug AC and test outlet voltage again.
10 Write that down.
11. Look at the end cord of the AC, look at the end plug-in of the extention cord - are they the same? (either 3-prong, or 4 prong, but not one of each).
12. Look at plug-in end of cord and outlet. Are they the same? (either 3-prong, or 4-prong, but not one of each).

If all prongs are the same, and outlet still has power of 240V continue.

Check AC to see if it has a breaker / fuse that could have been blown / kicked-out because of a surge or fluctuation in the elec. coming into the unit. Reset breaker if needed, on AC.

The problems left are:
1. Your prongs are not making contact in the outlet. So the outlet could have been cracked and shifted the contacts inside the outlet. Your outlet may be designed to "Twist" the cord once it is inserted to deliver power. Your problem "Is" in the outlet, or a mismatch between plug of AC and the outlet.

A. If both outlets have constant same voltage.
B. Both outlets are of the same style(3/4 prong) and match to the AC.
C. If both outlets deliver power to the extention cord.
D. No breaker had be kicked on the AC
Then the AC should work in both outlets, if it fails to work in one then power is not being delivered from the outlet to the AC. Since the AC does work in the one outlet this implies damage in the non-functional outlet, even though the probes can get power despite the damage, the AC is not. Which further implies the contacts in the outlet have be reoriented 'cause of the damage, which I would suspect to be a cracked outlet caused by over tightening terminal screws.

Answer 2

Using the AC Volt setting on your meter, check and record voltage readings on all the pin combinations for the working 240V outlet. compare this to the non working outlet. If there is a difference (more than a few volts) then pursue that difference either with Con Ed or an electrician.. If there are no differences, I am stumped. A missing/swapped ground or neutral is highly suspect here, but kind of sophisticated troubleshooting. they could read correct on a meter, but newer AC/s might not power up.

Answer 3

Since you now have a tester test the outlet that worked on your a/c unit and then test the outlet you are trying to use and find out what the difference is.

Obviously there is some kind of difference. When you find it, make the change and then you should be finished.

Answer 4

Hi,

Are you sure your AC is rated for 240 volts? Not everyone has an extension cord that is set up for 240 volts with plugs etc. so they can test other 240 volt appliances elsewhere.
From what you are describing it seems as if everything is ok in theroy, but we are missing something somewhere.
Goodluck..

Answer 5

hmmm. Are the places you read the voltage the proper ones? is therer voltage on just the neutral? Something is still not right in that regard.

Answer 6

The circuit probably does not have enough amperage.