Dometic 13.5k a/c Inop

[otrfun]

Cooled perfectly yesterday. Turned it on today--no-go.

Voltage in the TT is 120-122. 1500w microwave works fine with very little voltage drop, so I'm assuming incoming power is fine. For the heck of it I did try the 50a outlet, too---I still get the same symptoms:

A/c unit is non-ducted, with all controls and vents in the ceiling console. Fan works fine on all 3 speeds, but there's no cooling. With the fan on high and the thermostat set for maximum cool, something in the a/c is trying to "cycle" every 30-45 sec. for approx. a second--continuously, every 30-45 sec. When it does, voltage in the TT drops from 120 to about 114 for about a second. You can hear the a/c fan and another fan I have turned on change pitch slightly every times it cycles. Must be quite a bit of momentary current to drop the voltage like that--I'm estimating 20-25a.

I'm camping right now, with plans to depart tomorrow morning, so don't have a ladder or any test equipment except a VM. Bad start cap? Bad compressor? Any recommended tests/checks on the upper unit (or ceiling console) when I get home tomorrow?

Mucho thanks!

P.S. PI EMS has been on 24/7--no errors reported.

[j-d]

For those watching at home...

Some Digital Volt Ohm Meters (DVOM) including an inexpensive Harbor Fright model, can test capacitors using a MicroFarad scale. Might be marked MFD or uF ("u" actually a Greek character with a little tail on the left called "MU"). But most of us don't have a meter that'll do it, so substitution is about the only test.

Cheap clamp-on (see the jaws in OP's pic) can measure AC AMPS in a steady state current flow, like the draw of a water heater or an A/C while cooling. That's what I have from HF. Better meters add DC current capability, and still better ones capture "Peak Inrush" which is what OP grabbed right when his compressor started. His meter also adds "true RMS" frequency measurement, alluding to his checking an inverter in another thread here.

I promise, no attempt to praise or criticize Engineers and Mechanics. Not being an engineer, I don't know what RMS even stands for. As more of a mechanic, what I DO know is that many A/C problems turn out to be a Capacitor.

[otrfun]

j-d wrote:

Ron3rd wrote:
You sound like you know what you're doing. Good job.

I'll say!!! Good Job, OP!!! I hate to think how many A/C's are replaced over a capacitor or less.

And, please note: OP has FAR better test equipment than nearly every other RV'er. Nearly everybody has a DVOM, free Harbor Fright one on up. Most cannot measure capacitance, OP can. I can do that but can measure only Running Amps. OP can measure Startup/Inrush. I'm impressed, so OP...

....What kind of METER do you have?

Glad it worked out.

Thanks!

I used a Mastech MS2108 (not the MS2108A which is not True-RMS or inrush current capable). ~$64 on Amazon. No, not a Fluke, but for 90% of folks out there a pretty good bang for the buck.

[otrfun]

Ivylog wrote:
In another post he "scoped" a MSW wave pattern so he has plenty of test equipment. Surprise he asked about his AC not working.

Being somewhat versed on test equipment says nothing about one's expertise on repairing a given piece of equipment. I know very little about the internal workings of an air conditioner which is why I asked for advice.

Same thing as having a big tool box in your garage. Having tools and knowing how to use them in itself says nothing about your knowledge and/or ability to work on, say, a riding lawn mower . . . or a Ferrari.

[Ivylog]

In another post he "scoped" a MSW wave pattern so he has plenty of test equipment. Surprise he asked about his AC not working.

[j-d]

Ron3rd wrote:
You sound like you know what you're doing. Good job.

I'll say!!! Good Job, OP!!! I hate to think how many A/C's are replaced over a capacitor or less.

And, please note: OP has FAR better test equipment than nearly every other RV'er. Nearly everybody has a DVOM, free Harbor Fright one on up. Most cannot measure capacitance, OP can. I can do that but can measure only Running Amps. OP can measure Startup/Inrush. I'm impressed, so OP...

....What kind of METER do you have?

Glad it worked out.

[Ron3rd]

You sound like you know what you're doing. Good job.

[otrfun]

Old-Biscuit, Golden_HVAC, Ivylog, and j-d, thanks! Appreciate all of you taking the time to reply with all your helpful tips.

Got up this morning and pulled the 5uf/50uf start cap and measured 4.6uf on the 5uf FAN terminal, and 3.9uf on the 50uf HERM terminal. Ok, the compressor side of the cap is bad. One problem identified and hopefully no more to go. Anyhow, like a few suggested I purchased a replacement cap at a local electronics supply house for $18. Not too bad.

Installed the new cap. Switched the fan on high and turned the thermostat to max cool. Heard a click, followed by that faint hum of the compressor. Within 15 seconds I was getting a nice blast of cold air. Works great!

Inrush current was 62.1a on home, commercial power (LRA is 63a). Continuous current was 12.6a (spec'd at 14a). All looks good.

Thanks again!

[j-d]

For a capacitor, I'd suggest finding an appliance parts store unless it's a long trip and an RV parts outlet's nearby. Prices are crazy-different according to where they're sold. One that actually sells to repair places. There are service hour ratings on capacitors. and the label has coding that'll tell that to somebody who knows the game. I'm buying them now at a parts store that charges about half what my previous source did, and for a much higher rated part.

But for now, get whatever you can lay your hands on unless you can do without for a few days. If you can wait a few days, Amazon can send one right to your campground. No truck or toad required...

Years ago (like 40!) a friend and I did a lot of tinkering, and he'd done a lot of work on refrigeration systems. Told me that the act of cutting the tubing to replace a component would release flakes of corrosion that would cause blockages. Yes, there are inline filter kits that should be installed with a new compressor. He still took that position. That said, I have no science on the issue. Fact or lore, thought I'd mention it.

[ernie1]

Golden HVAC:
Fred can you explain the principle wherein a low ohm short to ground causes an acidic condition? I've not ever heard of that and not saying you're wrong, just curious.

[j-d]

Agree with Capacitor First. If the Capacitor is bad, the Compressor will draw LRA (Locked Rotor Amps) like a stuck one.

Your brand MAY use a two-way capacitor. A Fan terminal, a Compressor terminal (likely marked HERM for Hermetically sealed system), and COM, Common, probably between the other two.

IF that's so, and FAN runs, you only need to buy a single capacitor with the higher of the two uF (maybe marked MFD) values, 35 in Fred's example. Just ask for a jumper to connect the COM of the two-way to one terminal of your new one and of course move HERM to the other terminal.

So you may find only one "can" in there. If you find a tall, round, plastic capacitor, a third one, let us know.

[Ivylog]

Start with a new start capacitor... a common problem. It's the bigger of the two you will find. Using a screwdriver short the contacts to ground before removing.

[Golden_HVAC]

Hi,

Yes the compressor might be bad. Or the capacitor. Because you can buy a 35 Mfd capacitor for about $20, you might as well replace it, and see if the compressor starts.

Of course if you know how to use a volt meter, you can ohm the compressor terminals from one terminal to ground (with the RV unplugged) and if you get resistance, then the compressor is bad, and you can not repair it.

Replacing the compressor is 'possible' however a 12,000 Btu compressor will run me around $400, and you are looking at about 6 - 8 hours labor, $40 in freon, $40 for a freon drier, and a 90 day maximum warranty on the replacement compressor. This is why they are called 'non-repairable' because a new unit with a 5 year compressor warranty is around $700 - $800 and sometimes places like Camping World will install them for a low cost (say a $5 installation sale special).

Good luck, and hopefully it will be a bad capacitor!

If you keep turning it on, then it will be a bad compressor very soon! I hope that you did not let it cycle on and off for more than a couple of minutes. It is very bad on the compressor (assuming that the capacitor was the only thing wrong with it to start with).

If the compressor is shorted to ground with say 100 ohms or less to ground, then it is really bad burnt up motor winding, and not worth trying to repair, even if you got a compressor for free. A low ohm short to ground is very acidic, and all that acid in the tubing will quickly start eating away the new compressor motor windings.

Fred.

[Old-Biscuit]

Voltage drops from 120V to 114V because A/C Unit compressor motor is bad

[otrfun]

Oops, double post 😞