Help diagnose what's wrong with the refrigerated truck.

I had a belt to the compressor go out the other day, so i emptied the contents, changed the belt, and turned it back on.

The problem is now with the inside of the box, the condenser just ices up, no blowing of cold air, nothing, just ices up.

I called the local refrigeration place, and they said because the door was opened, and because its super humid out (80%) the condenser is trying to get rid of all the moisture.

OK, make sense to me.

Problem is, it has been nearly a full day, ive run the defrost cycle probably a dozen times, and it still ices up.

Is there just that much moisture in the box? Or is something else wrong.


There should be a fan that blows the inside air over the coils that are icing up. This helps the coils from getting so cold they actually freeze the water removed from the air. If that fan is not working, it will ice up.


Compressor clutch not cycling?:smack:

It’s just a dumb guess, I’m not even good with a/c diagnostics.

I may have heard this wrong, but even if I didn’t, I still don’t understand the concept:

Any time an A/C unit has iced-up in a house I’ve lived in, the repair guy says too much refrigerant has escaped. He puts in some more, everything’s copacetic. I don’t get it.

One time, a relay closed and wouldn’t open, so the unit never shut off and icing-up occurred.

Disregard this post as you see fit.

Low on refrigerant.
Evap fan not running.

During the deforst cycle the cal rod that melts the ice may not be working.
Is this a unit that is running off the truck engine or does it have an electric motor.

When there is not enough refrigerant in the system, the pressure in the evaporator is lower than normal. At this lower pressure, the liquid refrigerant boils to vapor (evaporates) at a lower temperature than it would if it has the proper amount of refrigerant. If this temperature drops below the freezing point of water, the water vapor in the air freezes on the evaporator coil, causing the icing up.

I agree it’s not intuitive (less refrigerant should mean less refrigeration, amirite?) but once you understand what is happening in the evaporator coil it makes sense.

To the OP, does the fan inside the box that blows air across the coil run at all?

And as an aside, the coil inside the box is the evaporator, not the condenser. These are named for what the refrigerant is physically doing inside the coil. In the outside coil, the refrigerant gas is condensing to liquid, giving up heat to the outside environment, hence it is the condenser. In the inside coil, the liquid refrigerant is evaporating to gas, absorbing heat from the inside of the box, hence it is the evaporator.

Dag Otto, thanks for the explanation. Kinda makes sense.

I know almost nothing about refrigeration, so fair warning here – this might be a chance to laugh at my ignorance. But it strikes me that maybe a coil could ice up if the fan that should be blowing air over it is not running. And that a fan could be run off a belt from the main motor drive. And that if one belt failed, another belt could easily be messed up, either from the same thing that caused the first belt to fail or from something in the process of replacing it.

This wasn’t on a refrigeration unit, but I’ve seen where the compressor in an old auto a/c system had enough contamination inside that it caused an oscillating load on the belt that in turn broke the belt tensioner. The repair involved the usual replacement of the compressor, dryer, orifice/filter and condenser

Cornflakes posted about the complications of a contaminated A/C , and listed most of the components.
He listed, [orifice/filter] witch in my extremely limited knowledge is the Expansion Valve witch also has a screen (filter),
So could someone explain to me how one would troubleshoot a expansion valve?
Compressor(hydraulic pump) displacement I believe is controlled through a compensator to maintain volume at the proportional (probably the wrong word) adjustable orifice or expansion valve that can be adjusted in some units from cold to colder, how again would one know this9cold to colder) is taking place??


Thanks for all the replies, ended up doing some digging and the throttle for the diesel motor (that powers the A/c) was stuck wide open.

Guess the unit was working too good, and wasn’t able to go into the cycles it was meant to don

When a fridge ices up inside, it means the defrost cycle is not working usually. The evaporator gets really cold, so of course any humidity is going to freeze on the unit as ice. Every so often, the defrost cycle works things the other way, heats up the evaporator coils so the ice melts and drips off. (In you home fridge, out the small pipe in the back to a pan under the fridge where it evaporates…)

Is the defrost not really happening? Does it melt the ice in the evaporator inside the box? Where does that meltwater go?

I had a similar problem where there was dust bunny fluff in the system (or was it onion skin flakes?) so the drain plugged up. yes the water melted, but it could not run out so it simply pooled there and froze again. Eventually the overflow pooled in the bottom inside of the fridge and ran out the bottom of the door.