Coleman mach 3 ac freezing up

"Most RV shop techs will not know how to do that or will not have the equipment and that's why they always try to sell a new unit".


Partly true. I do have the knowledge and equipment, but at $130 per hour, it would cost with parts a minimum of $300 for me to check find and solder the leak and evacuate and recharge. IF the leak is easily found and does NOT include a Coil which will increase the cost. THAT is the reason we replace, NOT because we do not know how to do it and only want to sell new units. It is NOT cost feasible at most RV shops to repair RV sealed systems. THAT is why Dometic and RVP replace the upper unit under warranty. Also, as you stated about the wives tale of low charge. I cannot remember when I had a unit that had a low charge (no leak just a low charge) and I am going back over 25 years. Doug

SC camper wrote:
We are having a freeze up problem with the ducted Coleman mach 3 ac unit. The evaporator is clean and the filter is new.

We run it on high cold and turn the thermostat down from max cold.

Do you mean..."turn the thermostat down FOR max cold"..??

Sentence doesn't make sense otherwise.

This past weekend was in the mid 80s and humid but still had a freeze up over night, low 70s. My question is about the copper temp sensor wire.

not copper wire, gas filled capillary tube.

It is in front of the evaporator but not touching it. Should it be touching? I dont hear the compressor cycling. Should I turn the thermostat warmer to make it cycle? It blows good cold air until it freezes up. What else can I look at before taking it to the shop?

At 70F OAT the refrigerant condensing capacity will be beyond the design specifications for the A/C. So yes, raise the target cooling temperature and run the blower at MAX.

dougrainer wrote:
"Most RV shop techs will not know how to do that or will not have the equipment and that's why they always try to sell a new unit".


Partly true. I do have the knowledge and equipment, but at $130 per hour, it would cost with parts a minimum of $300 for me to check find and solder the leak and evacuate and recharge. IF the leak is easily found and does NOT include a Coil which will increase the cost. THAT is the reason we replace, NOT because we do not know how to do it and only want to sell new units. It is NOT cost feasible at most RV shops to repair RV sealed systems. THAT is why Dometic and RVP replace the upper unit under warranty. Also, as you stated about the wives tale of low charge. I cannot remember when I had a unit that had a low charge (no leak just a low charge) and I am going back over 25 years. Doug

Same here. In 15 years, I think I have had one marine unit that slipped out the door with a low charge. But, the common response for low cooling problems on this forum is to add freon, when most of the armchair technicians have never troubleshot a piece of AC equipment.
I still stick with what I said about most RV techs not knowing how to properly diagnose and repair an AC. I know of your credentials and years of experience doing this and I can appreciate that, but based on some of the things forum members say were said to them by other RV techs, you just really have to wonder what is going on in some of these shops.

The cost fact or your repair vs replacement makes sense since a shop repair can easily reach half or more than the price of a new one. My environment is a little different. The replacement cost of one of my marine units can easily reach $3000 so my customers are willing to pay for on-site service regardless of what it costs. The only ones I replace are those that have just been to far past end of life and are too old to justify a repair bill. For me to replace a compressor is around $1300.

Quite some years ago my company often used a split unit A/C, compressor, condenser, condenser cooling fan out in the weather, to cool computers in adverse environments, saw mills, etc. Early on we began encountering evaporator freeze up. The solution was to disable the condenser cooling fan when the OAT was low.

wwest wrote:

SC camper wrote:

We run it on high cold and turn the thermostat down from max cold.

Do you mean..."turn the thermostat down FOR max cold"..??

Sentence doesn't make sense otherwise.

What I was saying is we turn the tstat temp UP from max cold. Dont leave on max cold but a little bit warmer.

Thanks for all the input and help. I need to check into the thermostat next. This is likely a dumb question and I probably know the answer but; can I plug into my outside home power outlet(15 amp) long enough to check for tstat operation? I dont have access to 30amp until we go camping again. Is there a way to check the tstat operation with an dvom without shore power plugged in? I guess that would be ok for checking the temp switch but dont know about tstat.

SC camper wrote:
Thanks for all the input and help. I need to check into the thermostat next. This is likely a dumb question and I probably know the answer but; can I plug into my outside home power outlet(15 amp) long enough to check for tstat operation? I dont have access to 30amp until we go camping again. Is there a way to check the tstat operation with an dvom without shore power plugged in? I guess that would be ok for checking the temp switch but dont know about tstat.

Yes, NO power you still turn it max and minimum. You should hear the contac point click open and closed. Doug

The only way to get your A/C to work at 70F OAT is to HEAT the condenser.

Disabling the condenser cooling fan would also help but very few of these have separate drive motors for the condenser and evaporator.

Many home A/C systems incorporate an "economy" mode wherein they automatically switch to bringing in cool night air rather than wasting energy driving the compressor.

You might think of doing that manually.

Disabling the condenser fan if it were possible, would do nothing but cause the unit to kick off on high head pressure. Not knowing what refrigerant your company's split systems used, I can't comment on the effect of turning off the condenser fan, and that may have worked on those but this thread is about RV rooftop units. The condenser is where the heat transfer takes place so why would you want it to run hot enough to shut the AC down. There are no RV rooftop units that have a separate motor for the evaporator and condenser.

Bob Landry wrote:
Disabling the condenser fan if it were possible, would do nothing but cause the unit to kick off on high head pressure. Not knowing what refrigerant your company's split systems used, I can't comment on the effect of turning off the condenser fan, and that may have worked on those but this thread is about RV rooftop units. The condenser is where the heat transfer takes place so why would you want it to run hot enough to shut the AC down. There are no RV rooftop units that have a separate motor for the evaporator and condenser.

Has anyone else used a RV rooftop A/C for cooling the interior when the OAT is that low? Doesn't that low OAT have the effect of dramatically increasing the condenser section's ability to condense the refrigerant gas to liquid form?