[TJ Clark]

So the front AC is cooling only when the original T-Stat is set to On and Cool. I can hear the compressor com on after a short delay 1 minute or less. The compressor will run for approximately 10 minutes then it stops. As the T-Stat is set to on the fan is continuous.  


When the T-Stat is set to auto and cool nothing happens. 

This is new as of Friday 4/14/23. 


Thoughts? What additional information do you need? 


Thanks.

[Richard]

Based upon your description, this could be a whole host of different causes, so thanks for being open to collecting data to narrow the diagnosis.

I am pretty sure you have basement SCS units. Would you confirm?
Are you comfortable working in a control box with live electrical circuits?
Do you have a clamp on ammeter?

First let’s talk about what the original tstat is supposed to do. On cool, there is an approximate 3 second delay before the compressors will kick on. If the setpoint is 2 degrees or less than the actual, then only Compressor 1 will activate. If the setpoint is 2 degrees less than actual then both compressors will activate.

If low cool or low fan is selected ONLY one compressor will activate. The primary purpose being to allow you to eek by on a 30 amp service.

Now to collect some data. Turn off the rear unit, no need to confuse the amperage readings with the rear cycling on and off. Turn the front unit on low cool, not low auto and set the target temperature 10 degrees below the actual. Does the inside fan come on ? Look at the ammeters above the driver cockpit and record amperage readings on both before you start the experiment. Now watch the ammeters when the tstat tells the unit to start. Record what happens and the reading. You should see a spike on L1 and then it should settle into the 15 amp range (12 to 15). Let the unit run for 15 minutes in this condition if it will and monitor the amperage reading periodically. Does it remain steady? Decrease? Or Increase? This tells us if it is a stopped up expansion valve or potentially low refrigerant. 

When that is complete after 15 minutes of run time, without turning the unit off with the selector switch, turn the selector to HI cool , not HI auto. And repeat all the observations including 15 minute run time for a second time. You should see similar behavior for L2 on the ammeters.

This info is going to tell us what is going on with the compressors. The data on air discharge temp is very helpful. Please continue to provide. 

Now pull the cover off the circuit board panel on the front of the AC unit. One screw at the bottom is all it takes, and a hook pick to pull the bottom straight out. Take some pics of the board please. I ask for two reasons. One, it will tell us which board you have, for there a a few generations of them. And if the board is “smoked”, we may be able to see an issue.

We can go further when you report this info.

If you want to guess that it’s a thermostat issue, you can simply swap the front and rear tstats. The wiring is simple, but take a pic just for grins.

But you know me, In God we trust, everybody else bring data.

[TJ Clark]

Based upon your description, this could be a whole host of different causes, so thanks for being open to collecting data to narrow the diagnosis.

I am pretty sure you have basement SCS units. Would you confirm?
Are you comfortable working in a control box with live electrical circuits?
Do you have a clamp on ammeter?



But you know me, In God we trust, everybody else bring data.


Yes, yes and yes. 
I'll do the testing later today. Thanks. 

[Richard]

If you pull the circuit board cover you should be able to find the wires leading to the compressors. The cover usually has a schematic on the inside. If you can hang an ammeter on the compressors to see what amperage they are pulling, that would be more better than just looking at the ammeters in the coach.

[TJ Clark]

I have a limited access to the basement compartment at the moment. 

Nothing much happens with the t-stat set to auto. 

With the t-stat set to on, low and cool and the set temp set 10+ below ambiant temp the compressor cam on for a few minutes and the Newell amp gage was reading 17 amps total with the compressor running. Thats a net increase of 15 amps. The compressor rean for just a few minutes. Unfortunately I wasn't able to detect when the compressor turn off. But the amps returned to 2 when the compressor turn off. 

After the compressor ran for that few minutes (less than 7) it didn't cycle back on. 

I'll let it all rest for 15 minutes and retry the test.

[TJ Clark]

With the set point at 54f and the actual temp reading 77f. Setting were Cool Auto Low and the compressor ran for 7 minutes 36 seconds. It then turned off the. The temp reading at the low duct next to the door was 27f using an infrared thermometer the tep across the coach at the slide room trim was 69f the t-stat said it was still 77f. I let it sit for a minute then switched it to cool on low. 5:30 later and the compressor has not come on again.


I am beginning to think is might just be low on refrigerant.

[Richard]

I tend to agree with you. The amperage is a wee bit high. The air temp is way too low indicating a low charge. A low charge will cause the compressor to overheat tripping the thermal breaker on top of the compressor. The fact that the compressor will come back on the next day leads to the thermal tripping.

So what happened when you repeated the trial with the thermostat set on Cool Hi ? This should allow comp 2 to come on. If it will run, I can walk you through a hack to get you by until you get get comp 1 addressed.

[Jack Houpe]

TJ the evaporator temp should be above 32F or the evaporator will freeze over and block air flow. And just maybe it has a temp switch for low evaporator temp?

[Richard]

Good Morning TJ.

I think that you may be at Leo's getting the radiator situation addressed. And if so, I can guess two things. The coach is landlocked, and it's HOT cause it's already summer in AZ. So it may be difficult to run the diagnostics right now.

However, the data you reported is somewhat in conflict with what I would expect. Primarily high amperage and low discharge air temperatures. Those don't normally go hand in hand. Keep in mind that the remote armchair quarterback is not able to observe the problem, and can only react to what is reported. I asked some very specific questions in my first reply for very specific reasons. I probably should have taken more time to explain why the answer factored into the diagnosis.

I am going to edit the first reply and put the questions in red, so that if you still want help, you can run the diagnostic when the circumstances permit. There are a number of scenarios that fit what you are describing. One, is low refrigerant. Two, is a partially obscured or failing expansion valve. That produces high compressor current and low pressues (low air discharge temps). This is why we need to see what the current draw is over time. If it starts low and steadily climbs until the thermal protection kicks in on the compressor then that points to a failing expansion valve.

If you are cooking and want to try a hail Mary, then access the circuit board and swap the connectors on the compressor leads so that Comp 1 is now plugged into the Comp 2 plug, and Comp 2 is plugged into the Comp 1 plug. That might get you some AC.

Another question when you can access the unit. Have ports been added for gauges? The units did not come from the factory with ports. If you see them, then the unit has been serviced previously. But ports would allow us to take some very valuable readings.

Jack is correct, any discharge temp lower than 32 is bad news. Ice will form on the evaporator coils and block the air flow, once that happens the pressures will spike, compressor overheats and trips the thermals. However, given you are in dry AZ and this happens within ten minutes, we need the other data to try to see what is going on.

[TJ Clark]

Good Morning TJ.

I think that you may be at Leo's getting the radiator situation addressed. And if so, I can guess two things. The coach is landlocked, and it's HOT cause it's already summer in AZ. So it may be difficult to run the diagnostics right now. Yes I am. 



I am going to edit the first reply and put the questions in red, so that if you still want help, you can run the diagnostic when the circumstances permit. There are a number of scenarios that fit what you are describing. One, is low refrigerant. Two, is a partially obscured or failing expansion valve. That produces high compressor current and low pressures (low air discharge temps). This is why we need to see what the current draw is over time. If it starts low and steadily climbs until the thermal protection kicks in on the compressor then that points to a failing expansion valve.
I'll run those tests after I leave the shop. 

What amperage do you expect to see with one compressor going?