How to test an RV air conditioner's starting capacitor

[Eric Hubert]

How does one test the starting capacitor in an RV's AC unit?  I am able to get to the capacitor but didn't touch the thing out of fear.  These things are as big as my fist and rated for 10mf.  I have no idea if that is a large amount of farads.  The multimeter I have came from harbor freight so I doubt it will test large capacitors like this.  It does do the small through hole ones though.  Where should I start?

[aaron carlson]

What's not running the compressor or fan? Does the capacitor have 2 or 3 sets of leads. If 3 are there they will be labeled common (c ) , fan, and herm( h for compressor) you can check resistance in the wire leads going to either the fan or compressor but that harbor freight multimeter will not check the capacitance.

You can check in the non recommended unsafe way but it's going to pack quite a punch and create a big melt down. Take the capacitor out and lay it on the ground and take a throw away screwdriver and jump across the terminals. If it melts down the screwdriver it's most likely not the capacitor. Fair warning don't expect it to be usable after but if it's bad it won't do anything and you have your answer.

FYI if a fan or compressor are replaced you always replace the run or start capacitor with it.

Fan has a few wires. Black for high speed if it has more than one red and blue are usually the other speeds and white is common. If you see 2 brown wires that's what go to the run capacitor.

If the compressor isn't running there should be 3 leads common, run and start coming directly from the compressor. Check between the leads for continuity. Common to run, run to start, and start to common. If the resistance of 2 equals the total of the highest ohm reading between the leads the compressor is good and the problem is something other than the compressor such as low freon overheating it. If you get continuity or 0 ohm across any of the lead sets than the compressors internal windings are bad and can only bring fixed by a compressor replacement.

On May 9, 2014 12:52 AM, "Eric Hubert" <eric....@gmail.com> wrote:

How does one test the starting capacitor in an RV's AC unit?  I am able to get to the capacitor but didn't touch the thing out of fear.  These things are as big as my fist and rated for 10mf.  I have no idea if that is a large amount of farads.  The multimeter I have came from harbor freight so I doubt it will test large capacitors like this.  It does do the small through hole ones though.  Where should I start?

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[Jasper Nance]

"You can check in the non recommended unsafe way but it's going to pack quite a punch and create a big melt down. Take the capacitor out and lay it on the ground and take a throw away screwdriver and jump across the terminals. If it melts down the screwdriver it's most likely not the capacitor. Fair warning don't expect it to be usable after but if it's bad it won't do anything and you have your answer."

 

I have no idea what you are saying. Are you saying take the cap out but leave it connected? Why would you ever suggest this -- ever. Please don't do this.  If you mean disconenct the cap, a 10mfd cap in no way has the energy at 220V to melt a screwdriver....

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[Robert Douglass]

Hello Eric,

First off, please don't short the leads.

Before testing, you will want to inspect/feel the case for *any* bulging or fluid on all sides. If you have either or both- don't bother testing it, its toast. Unfortunately, the only truly accurate way is to use a meter with a capacitance setting on it. Home Depot sells a Klein unit (Clamp style multimeter with leads) that I have found very dependable for approx $60. If this is outside your budget, the grey area use would be to use it and return it. As long as you are within 10% of the rating on the capacitor or in the wiring spec- it should work fine. 

Please note- If you are on a tight budget you can use a larger used cap that has degraded a bit (Rated at 10 uf but the meter says its providing 5uf, etc...) <completely valid solution, may not last you for years. You **Cannot** take 2 smaller ones and connect them in either series or parallel to increase capacitance without doing a lot of math where the capacitance and voltage rating comes into play. (If you get it wrong- literally exploding capacitors/fried circuits or still not working)

I hope this helps!!

Disclaimer: I am not an HVAC professional, this is simply based on my experience troubleshooting my AC.

[Jasper Nance]

"without doing a lot of math"  Well that's not entirely true.

 

Cseries = 1 / (1/Ca + 1/Cb) => assuming that Ca=Cb=C : Cseries = C / 2

Vseries = Vc1 + Vc2

 

Cpar = Ca + Cb

Vpar = Vsupply

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[Aaron Hicks]

On Fri, May 9, 2014 at 10:29 AM, Robert Douglass <rob...@stagenerds.com> wrote:

Home Depot sells a Klein unit (Clamp style multimeter with leads) that I have found very dependable for approx $60. If this is outside your budget, the grey area use would be to use it and return it. As long as you are within 10% of the rating on the capacitor or in the wiring spec- it should work fine. 

Perhaps an HVAC store would do it for free.

Discount A/C Outlet in Mesa has horrible online ratings, but they saved my bacon (and my dogs) one year by being the only place that would sell me the blower motor I needed. (Seriously- 9 places, none of which would sell it to me direct. What a rip.)

3643 E Main St., Mesa, AZ 85205

They wired it up, slapped on a new cap, showed me how to wire it in, and sent me on my way. But I figure any HVAC place would test a capacitor, no charge.

-AJ

[Jasper Nance]

Grainger's Industrial Supply will sell you things directly and cheaply too but they probably won't tell you much about how to wire it up. Still, I bought a motor and cap from them and they were prepared to sell me each separately if I needed them. You can just walk into their stores.

 

Plus you can ogle their neato welding stuff while you're there at the store  =3

[Robert Douglass]

You are assuming Ca=Cb, in the case of used ones- they are not. From experience, you will do quite a bit of searching to find them close or even complimentary capacitance... especially at half/complimentary voltage ratings. If they identical are because you purchased them new, why not just buy the correct one? Your point is made though that it is possible to Frankenstein a system together, I apologize for assuming that you were unaware of the math since you were asking about measuring for it. The original question I was attempting to answer, was the only way to get a quantified value for a capacitor is using a meter or making the dedicated circuit contained inside the meter. I would not be able to assist with the latter. I hope this helps.

[fixitquickservice]

The point I made earlier was that you have to replace the capacitor no matter what. Installing a new fan or compressor should always go in with a new cap. 

Jumping it with a screwdriver was a quick dirty way to see if it has any juice real fast. 

If you have time on your hands to pull apart air conditioners for purely the idea of testing a capacitor more power to you but I assumed you wanted to fix a legitimate problem fast.

Going to the store for a new test meter is an option but I can tell you if you have to replace it any way and wanted to verify you can have your answer very quickly.

Jumper plus  a few seconds to safely set up and you have your answer. 

Good luck.

BTW I've repaired hundreds of Air conditioners in my time. Not just a few.

Ohm out your fan or compressor. If all is well replace the capacitor.

Sent from my T-Mobile 4G LTE Device

[aaron carlson]

Yes remove the capacitor completely.

It won't melt the whole screwdriver but it will definitely melt the spot where it first touched both terminals. 

[Jasper Nance]

Too bad capacitors won't stay charged in an A/C system

[aaron carlson]

They use them for assisted push on start up and in running to allow for cheaper less powerful parts to be used. YAY FOR COST SAVINGS.

Hard starts are used to multiply starting power on old compressors to get more life when they are getting worn.

They work though

[Larry Campbell]

Considering that a worn or bad cao will be a fast way to destroy a fan or a compressor it is a very good idea to replace ALL caps and the contactor when doing a preventative maintenance or any level of repair...A few bucks per cap and 10-20 for a contactor is cheap in comparison to the cost of most blowers or compressors

I have dealt with a LOT of large industrial ACs as well as several smaller home units, (we have 500 tons of AC in my datacenter, comprised mostly of 30 ton 480v dual compressor units) caps are cheap, when in doubt replace it out, by the time you think you should text it its just better to put in another,

And I can attest that even a 10mf cap charged with 277v is not gonna kill you but you should be cautious, they have a nasty bite. they wont blow screwdrivers to bits...I discharge them like that so I can change them when I do motor changes. 

On Friday, May 9, 2014 11:52:55 AM UTC-7, aaron carlson wrote:

They use them for assisted push on start up and in running to allow for cheaper less powerful parts to be used. YAY FOR COST SAVINGS.

Hard starts are used to multiply starting power on old compressors to get more life when they are getting worn.

They work though

On May 9, 2014 11:46 AM, "Jasper Nance" <

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[Eric Hubert]

I spoke to a few more people today.  The high amperage I am getting may be due to the date of manufacture and it's age.  The AC works fine up to about 95 degrees or so.  I put a clamping amp meter on the hot wire in the breaker box.  It will read about 10 amps when it is running and first turned on.  Since the AC I am working on (I have two units) is mounted in the back of the coach I can not see if there is a spike when the AC is first turned on.  After running for a while the amperage actually goes up by 2 amps.  I would have assumed it would go down.  The temperature outside was actually falling too.  This has me baffled as to what to do to make this AC unit run better and not trip breakers.  I'm not sure if I stated this but the ac works fine up to 95 (other than it seems to vibrate a lot louder than the forward AC).  It is also only at 95 that I actually feel the need to run an AC:(  The unit can not be recharged.  That would have been one of the first things I would have tried but it is not to be.  I cleaned both coils with a brush and some AC cleaner in a spray can I got at Home Depot.  What other things can I do to get this thing to not trip breakers and use less power?

(on another note I broke down and bought a new forward AC today so the pressing need to get this fixed is no longer there.  The AC is more energy efficient and more likely to work compared to a craigslist one.  It weighs 100 pounds.  I have no idea how I am going to safely get it on top of my coach.  And wow, I didn't expect so many people to have replied to my post.  Thanks all)

[aaron carlson]

A HARD START helps old compressors run easier but will it be enough I don't know. 

I take it brazing in Schrader valves to check or adjust the charge is out of the question but it is truly the only other option.

Refrigerators are the same way. No ports until something goes wrong and you add them.

If you can find them they have saddle valves that will tap into the side of the copper tube without any brazing or draining of the freon.

On May 9, 2014 8:00 PM, "Eric Hubert" <eric....@gmail.com> wrote:
>
> I spoke to a few more people today.  The high amperage I am getting may be due to the date of manufacture and it's age.  The AC works fine up to about 95 degrees or so.  I put a clamping amp meter on the hot wire in the breaker box.  It will read about 10 amps when it is running and first turned on.  Since the AC I am working on (I have two units) is mounted in the back of the coach I can not see if there is a spike when the AC is first turned on.  After running for a while the amperage actually goes up by 2 amps.  I would have assumed it would go down.  The temperature outside was actually falling too.  This has me baffled as to what to do to make this AC unit run better and not trip breakers.  I'm not sure if I stated this but the ac works fine up to 95 (other than it seems to vibrate a lot louder than the forward AC).  It is also only at 95 that I actually feel the need to run an AC:(  The unit can not be recharged.  That would have been one of the first things I would have tried but it is not to be.  I cleaned both coils with a brush and some AC cleaner in a spray can I got at Home Depot.  What other things can I do to get this thing to not trip breakers and use less power?
>
> (on another note I broke down and bought a new forward AC today so the pressing need to get this fixed is no longer there.  The AC is more energy efficient and more likely to work compared to a craigslist one.  It weighs 100 pounds.  I have no idea how I am going to safely get it on top of my coach.  And wow, I didn't expect so many people to have replied to my post.  Thanks all)
>>
>>

[Larry Campbell]

You should be able to get saddle valves from ARS down on chambers....south of 24th and roeser....thats where I got mine.... otherwise try webb/aircold down at 33rd and roeser...

The prob is that you are supposed to have an EPA cert (type1 608 cert for small package units under 5lb) if you are going to be messing with refrigerants. You can get a type 1 online (I recommend mainstream engineering for that... http://www.epatest.com/...)
and youll need gauges...its not so simple....

anyways overamping issues like yours that are not cleared up with coil cleaning are not likely going to be easily fixed. sorry

On Friday, May 9, 2014 9:22:58 PM UTC-7, aaron carlson wrote:

A HARD START helps old compressors run easier but will it be enough I don't know. 

I take it brazing in Schrader valves to check or adjust the charge is out of the question but it is truly the only other option.

Refrigerators are the same way. No ports until something goes wrong and you add them.

If you can find them they have saddle valves that will tap into the side of the copper tube without any brazing or draining of the freon.

On May 9, 2014 8:00 PM, "Eric Hubert" <> wrote:
>
> I spoke to a few more people today.  The high amperage I am getting may be due to the date of manufacture and it's age.  The AC works fine up to about 95 degrees or so.  I put a clamping amp meter on the hot wire in the breaker box.  It will read about 10 amps when it is running and first turned on.  Since the AC I am working on (I have two units) is mounted in the back of the coach I can not see if there is a spike when the AC is first turned on.  After running for a while the amperage actually goes up by 2 amps.  I would have assumed it would go down.  The temperature outside was actually falling too.  This has me baffled as to what to do to make this AC unit run better and not trip breakers.  I'm not sure if I stated this but the ac works fine up to 95 (other than it seems to vibrate a lot louder than the forward AC).  It is also only at 95 that I actually feel the need to run an AC:(  The unit can not be recharged.  That would have been one of the first things I would have tried but it is not to be.  I cleaned both coils with a brush and some AC cleaner in a spray can I got at Home Depot.  What other things can I do to get this thing to not trip breakers and use less power?
>
> (on another note I broke down and bought a new forward AC today so the pressing need to get this fixed is no longer there.  The AC is more energy efficient and more likely to work compared to a craigslist one.  It weighs 100 pounds.  I have no idea how I am going to safely get it on top of my coach.  And wow, I didn't expect so many people to have replied to my post.  Thanks all)
>>
>>
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[Eric Hubert]

I have more time than money or common sense to give up.  Would these gauges of which you speak be used also for automotive air conditioners?  If so, I may want to make the investment.  The chassis ac in this thing doesn't work anymore either.  Do I need such gauges to recharge that AC?

[Larry Campbell]

The gauges are similar, (might find a set that shows both r22 and 134a on the dials or you could do the calculations for super and sub) you probably would need to purchase a set of automotive adapting couplers for the hoses

 mainstream does 608 and 609 certs online....

On Saturday, May 10, 2014 11:09:03 AM UTC-7, Eric Hubert wrote:

I have more time than money or common sense to give up.  Would these gauges of which you speak be used also for automotive air conditioners?  If so, I may want to make the investment.  The chassis ac in this thing doesn't work anymore either.  Do I need such gauges to recharge that AC?

[Mike Bushroe]

Eric,

ABSTRACT: Use 100K, 1W resistors to safely discharge the cap. Using DC power source and same resistor to measure time constant can measure true capacitance with simple voltmeter. Small increase in current as coach cools off is normal, watch for frost buildup inside.



LONG DETAILED VERSION:

  If you are worried that the giant, 10mF capacitor is still charged up and dangerous when you remove it from the AC unit, the safe way to discharge it is to place a high value resistor across the leads. If it has 3 terminals (start - run capacitor) put 2 resistors on it, from the common "C" to the other two. If in doubt, put a resistor across all three pairings. If you use a 100K ohm, 1 Watt resistor, even a 277V there will  be less than 3mA of current. 3/4 of a watt. Of course, with that high a resistance, it will take awhile to fully discharge. T = R*C gives 500 seconds, and you want at least 5T before you handle it, so about 3/4 of an hour to make absolutely sure that it is safe. This time is regardless of what the starting voltage is, it all goes in proportion to the starting voltage. I would normally use clip leads or solder the resistor onto the same style spade connectors that the capacitor uses to safely connect the resistor. There should also not be any visible spark when to connect the second side.

   If you want a faster discharge, solder 10 of the 100K, 1W in parallel to get an effective 10K, 10W resistor. That will decrease the decay time by a factor of 10, so 4 minutes to safely discharge what ever voltage was left on the capacitor.

   You can also use the discharge time to measure the current capacitance with just a volt meter. Use a DC power source to charge the capacitor (it is an AC cap, so polarity does not matter). Even several batteries or a wall-wart battery charger although the closer to working voltage that you can safely work with, the more accurate the final result. Once charged, put the volt meter across it (on a high enough scale!), measure the voltage and watch to see if it drops all by itself. If it visibly drops, you already know it is going bad and should be replaced. Also remember that the volt meter itself has some internal resistance and will slowly discharge the cap. To avoid this measure, disconnect, wait a couple of minutes, measure again. Assuming the capacitor holds a charge and that you have charged it up to around 15V, put a 10K resistor across the capacitor. If it is still good at 5mF, it should take about 50 seconds to drop to roughly 1/3 of the original voltage. If it takes less than 25 seconds it is also time to trade it in.

   As the interior gets colder and the outside coils get warmer it is only normal for the compressor load to increase a little and draw more power. Think of it as having to 'push' the BTUs further 'up hill' to get rid of the heat. What you need to watch for is frosting on the inside coil. That will suddenly make it MUCH harder for the compressor to work.

Mike

[Jasper Nance]

Stop calling a 10uF capacitor huge. 1/2 * C * V^2 shows that it only has 0.242 joules in it. That's less than a joule of energy. It might bite you but it won't kill you.

 

They look huge because they are rated for massive amount of current that are being passed through them (and phase adjusted as that happens).

 

A 10uF cap is not a huge capacitor and in an AC system will discharge when the power is turned off because there are no diodes to prevent that. 

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[Mike Bushroe]

Sorry Jasper. I was picking my words for my intended audience, who seems to be worried about the amount of power held in a 5 to 10 cubic inch capacitor. Yes I know that the total power is not huge and that they are designed for high current and prolonged high temperature operation, as well as higher voltages than we normally work with and AC to boot.

Discharge in place would also depend on switches or contactors isolating the cap from the motor windings. Yes, if left in circuit, the much lower impedance of the compressor or fan motor windings will discharge it very rapidly, with plenty of heat dissipation capacity. But when one is sitting on the table in front of you with nothing connected to short out any potential charge, you feel a little better with a bleed off resistor in place.

Also, I see that my numbers were off. I had assumed that the first post using 'mF' meant milli-Farads, where as he was using the 'm' for micro instead of the more common short cut of using 'u'. So the discharge times will be 1000 times faster. I had remembered that compressor capacitors were fairly hefty, so change in range did not immediately jump out and tell me I had read it wrong.

Yes, you are correct. Even fully charged, the capacitor would only be exciting, not deadly or explosive.

Mike

[Jasper Nance]

The motor industry's quirky typeset limited choice to label µF as mF is almost as fun as old radio stuff that called pico farads (pF) mmF (micro micro) :P

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[Bill Kaszeta]

This is an AC capacitor and is used for starting the motor.  By the time you get it out of the unit, the voltage has likely decay to close to zero.

You can do several tests with your Harbor Freight meter.  First, set it for DC volts and make sure the voltage is close to zero, <0.5V.  Then set the meter to read resistance, use to 20K setting on the low cost Harbor Freight meter.  Then place the leads across two of the terminals.  You should get a reading that starts out low and eventually gets to open circuit ( infinite resistance that shows as 1 .    on the one I have).  This means that the capacitor is charging up to the voltage used in the meter, only about 3 volts.  Reverse the leads and test again, it should taker about twice as long to reach the open circuit.  A slightly leakly capacitor will not get to open circuit, but may still be good. If there are 3 terminals, repeat on each pair.

This testing will show that the capacitor is not shorted.  The next step is more dangerous.  You need a 100-watt light bulb and a power cord such that you can connect the capacitor in series with the bulb, and then connect the cord such that the power will flow from one wire from the receptacle, to the capacitor, out the other terminal of the capacitor, to the bulb, and then back to the plug by the other wire.  Make sure that the connections are safe and secure, taped if possible, then plug the cord into a 110V outlet.  The lamp should light, maybe to a lower level.  This shows that the capacitor can pass AC current. Unplug and wait a minute or so before touching the capacitor, check for voltage. If there are 3 terminals, repeat on each pair. If no light, replace the capacitor.

The other check you need to do is to measure the actual value of the AC voltage at the compressor while it is running.  It should drop less than 5 volts from the outlet voltage.  AC electric motors are such that as the available voltage drops, the current will increase to compensate.  If you find that the voltage at the compressor is dropping as it runs, you need to check the wiring.  The wire size may be inadequate for the load (at least a #14 wire, #12 better), or there may be a poor connection.  A poor connection could even be in the thermostat.

On Friday, May 9, 2014 12:52:14 AM UTC-7, Eric Hubert wrote: