I have a Coleman roof unit that runs for a few minutes, then the compressor quits running and trips the breaker. Fan works because I replaced it several years back. I have had the capacitors tested and it turns out they will need to be replaced. I would like to test the compressor with my amp meter before I actually replace the capacitors. Any of you kind folks out there know how I would go about doing this?
Thank you in advance for any replies.

Capacitors are fairly cheap, real cheap when compared to a new compressor.

There are 3 leads going to the compressor, black is common, yellow from the run capacitor, and red is more or less the common on a 120v system. This is spelled out on the wiring diagram.

Easiest way is to hook the amp meter to the black lead and have someone inside the coach turn it on. There will be a spike when the compressor (also called a can) comes on. The spike, the start up current, should only last no more than 2 seconds. Depending on the manufacturer, the run load amps may be on the compressor but certainly be on the rating plate for the unit, hopefully legible.

I honestly don't suggest you grab the suction line (the larger copper line) with your hand, but you need to check and see if it gets cooler as the compressor runs. One possibility is that the system is out of refrigerant and the can eventually overheats, causing the breaker to trip.

On a camper unit, I would not suggest replacing the compressor. It would be cheaper, in my opinion, to replace the entire unit. You can replace the entire unit with a new one that has a warranty for close the same money just replacing the compressor.

Hope this helps.

You'll not get an accurate diagnosis if you check the current before replacing the capacitors; one or both may be the source of the excessive current draw. And you could fry the compressor if you keep cycling it with bad caps.

If the compressor is drawing ~1-2 amps more than normal operating amps, this is a sign that the compressor is damaged or worn out.

What is probably happening is when the unit is first tuned on, there is no pressure in the system so the compressor is starting up without much of a load. The compressor motor needs the capacitor to start and as report, they need changing.

So, after the compressor has been running for a bit, the load increases as it builds system pressure, the motor is working harder and because of the faulty capacitors the compressor stalls and the input current goes way up, blowing the breaker.

As Rusty says, you will not be able to measure the current with any meaning unless the capacitors are replaced with new ones, also be aware that as the unit runs, the load current will change depending on where the compressor (pressure) is in the cooling cycle.

The compressor amperage will stabilize after initial start up. Put an inductive amp probe around one of the compressor wires to test. If the amperage rises to the locked rotor amps--LRA, (this will be stamped on a plate on the compressor), then the compressor is toast.

Many thanks for all of the helpful replies JL, Rusty, Mr Wrench, and Ed Chevalier. It would seem that consensus would have it that I need to replace these capacitors first and foremost. I believe i remember the person who tested the capacitors bad saying that they no longer sell these types of caps and that I would be purchasing a single capacitor to do the job of the two. Does that sound correct?

@ Rusty, Reading the amps WITH the existing capacitors is diagnosing the problem. Continued use with weak capacitors can damage the can, but that takes a long time.

@ MWrench, there is ALWAYS pressure in the system. Properly charged, it may have 120# on it before the compressor starts. Suction pressure of a properly charged system will be between 60 and 80 pounds when the compressor runs, depending on ambient and inside temps. Discharge pressure can be higher than 275 pounds, again depending on ambient temps. If the ambient or indoor temp is extremely high, you can see much higher pressures.

@ Ed Chevalier, you need to put the amp clamp around the common wire. If the amp clamp is put around the wire that leads to the start winding, you'll get a lower reading once the compressor has started.

@ Brandman, you can get single or dual capacitors. You need to ensure the proper mfd and voltage. When capacitors are replaced on central systems, you can go up on voltage (from 370 VAC to 440 VAC) but you can't go down without damaging the equipment. Your problem is where to get them locally but you should be able to get what you need off the 'net. I always test the new capacitor before I install them.

Good luck.

quote:

@ Rusty, Reading the amps WITH the existing capacitors is diagnosing the problem. Continued use with weak capacitors can damage the can, but that takes a long time.

It may be a matter of semantics, but testing with the old caps just confirms there is a problem; it doesn't necessarily isolate it. (It won't likely determine whether there's a leaky cap or whether the compressor motor itself is drawing too much current.)

I agree that bad caps will take a long time to damage the motor, but how much of that time has been used already? The issue is already severe if the unit is popping the breaker.

quote:

Originally posted by JL:


@ MWrench, there is ALWAYS pressure in the system. Properly charged, it may have 120# on it before the compressor starts. Suction pressure of a properly charged system will be between 60 and 80 pounds when the compressor runs, depending on ambient and inside temps. Discharge pressure can be higher than 275 pounds, again depending on ambient temps. If the ambient or indoor temp is extremely high, you can see much higher pressures.

Again a matter of semantics! I fully understand there is pressure in the system BUT, the differential pressure across the compressor will become ZERO if off for awhile. I was stating the conditions BEFORE the compressor starts after a rest period.

What this means is the compressor motor will not see a FULL load when it first starts up, yes there will be a starting surge but running current will settle to its lowest level until it runs for a bit and the differential pressure will go to the numbers you stated and then the load on the compressor motor (and the running current) will be at its highest.

This is why you never shut a A/C system down and immediately restart it, unless the system has a built in delay to restart the compressor, most roof A/C systems of modern design has the feature.

a cap tester makes it easy to diagnose...caps cost $12. put a hard start kit on it too while your in there.