Friday, September 17, 2021
Friday, September 17, 2021

Ask Dave: Two-stage AC not working, and it’s not the thermostat! What will make it work?

Answers to questions about RV Repair and Maintenance from RV expert Dave Solberg, author of the RV Handbook and the managing editor of the RV Repair Club. This column appears Monday through Saturday in the RV Travel and RV Daily Tips newsletters. (Sign up for an email reminder for each new issue if you do not already receive one.) Today Dave discusses a two-stage AC.

Dear Dave,
I have a 2007 Winnebago Journey that has a two-stage basement air conditioner. It has always worked well until now. The second stage is not kicking on. Was told at the service center that 95 percent of the time it is that the thermostat is bad and not calling for the second stage. After spending $181 on a new thermostat, the second stage of the two-stage AC is still not activating. What else can I check? The first stage is cold but not enough for the summer heat. Thank you in advance. —David

Dear David,
What you have is a Coleman unit that Winnebago called “TrueAir” and it is actually two compressors in one compartment. These are exactly like the units on the roof, just more efficient in the lower compartment and ducted to the ceiling.

The first unit was introduced in a Vectra and was located on the driver’s side in the middle of the coach. It was a great concept, but was discontinued as it eliminated a storage bay, which was a selling issue.

The number one compressor is the main or default compressor of the two-stage AC and runs all the time. The number two compressor turns on when the temperature raises 4-5 degrees above the thermostat setting and number one cannot keep up. Thermostats have been an issue with almost all types of AC units. However, it’s not a 95 percent issue as your dealer indicated. Just swapping out the thermostat is not the answer, as you have experienced.

How to diagnose the thermostat

The thermostat can be diagnosed by using a multimeter and verifying 12-volt power going out – which should have been done before. What needs to be done is to verify 120-volt power is getting to the second compressor. Winnebago has wiring diagrams available online for your unit. My tech source indicated the orange wire going to the second compressor is the 120-volt power line and should be checked.

To test, set the t/stat at a lower setting than ambient temperature inside the rig, let’s say 75 degrees with an ambient temp of 90 degrees. When the first compressor starts up, you know the temperature sensor is working and you should have 12-volt power on the yellow wire coming off the t/stat which is for the main compressor. After one minute, the second compressor should start up which would mean the orange wire off the t/stat would have 12-volts. For a troubleshooting path, download the pdf file here.

Also, I would suggest contacting Winnebago to get the correct wiring diagram for your unit, then verifying 12-volt power from the thermostat to the second compressor, 120-volt to the second compressor, and then you will find the problem.

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Dave Solberg worked at Winnebago for 15 years developing the dealer training program, as marketing manager, and conducting shows. As the owner of Passport Media Creations, Dave has developed several RV dealer training programs, the RV Safety Training program for The Recreation Vehicle Safety and Education Foundation, and the accredited RV Driving Safety program being conducted at rallies and shows around the country. Dave is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club, a one-stop go-to online resource for RV enthusiasts. 

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RV Staff (@rvstaff)
1 month ago
Reply to  Dave Solberg

Thanks, Dave! That paragraph has been reinserted in your post. Sorry, readers, that it was somehow left out when first published. –Diane

Drew
1 month ago

Dave,

Thanks for this. I’ll bet it will be helpful for people with these units.

TIM MCRAE
1 month ago

Although I agree Dave’s answer was a little brief, I would not be rude about it.

I have a lot of experience t/s circuits so his answer made sense to me.

I would go with does the owner feel comfortable tracing/testing 12volt and 120 volt circuits? If not use a serviceman.

There are several pretty easy & cheap fixes that won’t cost a lot and if it is bad (compressor) then he can’t fix it himself but it still won’t cost a lot to have a tech tell him that.

TIM MCRAE
1 month ago
Reply to  TIM MCRAE

Ok DIY? This is very brief but I am not teaching you how to perform these tests. You really need to know that already.

There should be a circuit diagram inside the unit near the control box. You simply trace the low voltage from the thermostat to the contactor (main relay) in the control box and trace the 120 volt output from the contactor to the compressor.

If thats all good then check any capacitors visually and electrically. Finally once you have identified the compressor and capacitor wires you can ‘ring’ out the compressor motor.

Last edited 1 month ago by TIM MCRAE
TIM MCRAE
1 month ago
Reply to  TIM MCRAE

My guess would be the wiring from the tstat to the contactor or the contactor is bad.

REMEMBER THERE IS VERY DANGEROUS VOLTAGE INSIDE THE CONTROL BOX. If you touch the wrong thing you can easily fry stuff INCLUDING YOURSELF!

Life is short enough DON’T GO EARLY

Dr. Michael
1 month ago

Gee Dave, that answer was a tad short.
What else could it be besides the orange wire? A list of items to check would have been nice.

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