Coleman Mach 8 Plus Compressor Short Cycling

[ncitro]

Woke up this morning to the front air not cooling correctly. Its not super warm here, but pretty humid so we are running the air. Watched it closer and found that the fan is running (per thermostat setting for fan on low) but the compressor is short cycling. I pulled out my meter and checked the thermostat and confirmed its calling for AC when the compressor is cycling off. If I let it sit for a while, then turn it on the compressor will run for 2-3 minutes and blow cold air, then after that the compressor turns off, and 30-60 seconds later turns on again. This repeats with the compressor cycle time getting shorter and shorter until it only runs for a short while (15-30) seconds before turning off. Power draw seems normal, the unit draws 10-11 amps when the compressor is on and does not spike when it shuts off. I pulled the inside filters and opened the dump valves with change. Next step is to try pulling out the freeze sensor (I am sure its not frozen up) then head to the roof when it cools down outside and see if anything is amiss, but was curious if anyone had any thoughts while I wait.

My thoughts are (in order of expense)

Freeze sensor
Run Capacitor
Board
Low refrigerant (possible but I am hoping unlikely since it does cool when it runs)

My money is on the cap causing the compressor to overheat and shut down, but not sure I will be that lucky. Any way to test the cap? I have always just replaced them.

[Scott'n'Wendy]

Well for a cap, using a multimeter you are really just checking to make sure it doesn't show open or shorted. So if you put your leads on the cap while on resistance mode you are looking for a constantly increasing resistance reading to indicate a good cap. It's the best a multimeter can do for you.
There is a control box in the plenum of your a/c unit. At least there is in my Coleman Mach 10. Maybe a poor connection up there? I had my A/C act poorly and it turned out to be a bad connection in that box. Spade connectors.

On Edit...make sure the cap is discharged before testing

[ncitro]

Yeah I am going to check those connections when I pull the freeze sensor. I did this install myself probably three or four years ago, so anything is possible, but given how it starts consistently when I let it sit I suspect something is overheating.

[howson]

ncitro

Are you sure the evaporator and condensor coils are clean (free of debris)? That's where I'd start. The only way of evaluating the evaporator coil's condition is pulling the cover and then the internal cover over it. (The evaporator coil, obviously, is the one at the front of the unit.) Forgive the obvious statement (more for anyone else reading this thread)--make sure power is off before pulling the cover. Also, as Scott mentioned, ensure the caps are discharged prior to checking (if you decide to do that while you're on the roof).

So could it be the compressor?

The expected amperage draw under "Standard Conditions" (80 F. DB 67 F. WB Indoor, 95 F. DB Outdoor) for the compressor for a Mach 15 (which I assume you have) is 12.7A. With the fan on low add another 2.7A. On high add 3.4A.

But...based on OUTDOOR temp (at the a/c unit) add or subtract from the compressor RLA as follows:



I used the Orlando, Florida temp as of 4:45PM (as I'm writing this response) and it shows 79 degrees, so I'd use the RLA - 3 adjustment. That means the expected draw should be 9.7A + fan. That's a long way of saying I agree the issue doesn't appear to be compressor related (which is good news)!

I'm surprised your meter doesn't have a capacitor setting. Fluke's website is a great resource for how to discharge and check a capacitor. See https://www.fluke.com/en-us/learn/bl...re-capacitance

Howard

[ncitro]

howson the cleanliness of the coils is first on the list to check in the morning when we go up.

It's a Mach 8,the ultra low profile one with two blowers so a bit different than yours.(I bought and installed it myself so it's not a standard OEM unit). That said the power draw seems "close" to what I remember it being.

[howson]

ncitro - doh! -- missed you'd put the model in the thread title.

Mach 8's specs:

[Jlawles2]

For checking capacitors, some multi meters will test giving you the value of the capacitor being tested:






On this one its the continuity (default), then you cycle through Ohms, Diodes and Capacitors by pressing the orange button.

If you choose to get on the roof and test, check that the board is sending power to the compressor even when off. From there it may have an external or internal thermal sensor. If the sensor leads are accessible, then test the output to compressor side. And of course the obvious is the good old fashioned hand to compressor test to see if it's getting internally hot (would not explain the short cycle completely) tripping the thermal.

If the thermal has a high internal resistance it's possible that it's overheating itself and tripping then cooling off and resetting after it has a few seconds to dissipate the heat into the rest of the compressor.

Not an AC expert but these are my highly uneducated assumptions and guesses... I may or may not have slept in a Holiday Inn on occasion and I may or may not have experience these personally.

[ncitro]

I suspect something along those lines as well Joe. I plan to check the freeze sensor first (its inside lol) then open up the roof unit and check for dirt as suggested and then start poking with my meter. I suspect that when the compressor stops the board will not be sending power anymore, indicating an overtemp sensor somewhere, but thats just a guess.

[cnote72]

ncitro You check a capacitor for microfarads with a meter that will read them. Usually a label on a capacitor that will say what it’s rated at, and will say +/- 5% or 6%, whatever the value. Like was said, make sure it’s discharged before testing, I use a screwdriver to short across the terminals, disconnect wires and check. When replacing, make sure you also check the voltage rating, you can replace a 370 volt with a 440 volt but not vice versa, or make sure to get one rated for 370/440 volts and you’ll be good.

[ncitro]

Well in todays episode of prepare for the worst and hope for the best....

I pulled down the inside covers and found the air dam dividing the supply from the return had bent back and was blocking the intake about 90%. Saw that and figured I would need to fix it, but was probably not the issue, it had to be a sensor. Started to go after the freeze sensor and said... you know... Reseated the divider and ran it with the covers off and ran just fine. Taped it back into place and put the covers on and it is still going strong. Seems like it blocked off the intake enough to starve the unit for airflow shutting down the compressor. Will keep an eye on it, but hopefully fine!

[Jlawles2]

Never look the easy fix in the face and say not possible to be the problem. Do keep an eye on it though.

I foam and taped up mine to prevent cross flow and keep the baffle from moving since it's just held in by friction from what I remember.

[ncitro]

Yeah I had taped mine but it still shifted. Going to retape it next time it's back home. Definitely going to watch it.

[Jlawles2]

I taped it from both sides with the aluminum vent tape. Bend the tape before you try to stick it and work from the corner out. I think there is almost as much foil tape in there as foam.