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Hoss7 · 06-06-2020, 05:05 PM

Hi everyone,

The dash A/C quit working in my 2005 Monaco Signature 45' and I've been attempting to repair it. I've been learning a lot about A/C systems on the way but I'm a bit stumped. My coaches' manual calls for 4.0 lbs of R134A (64 oz). I have replaced both the air receiver-dryer and thermal expansion valve, and they're both located up front in the generator bay. I purchased a proper A/C vacuum pump and vacuumed the system for 3-4 hours. It remained in a >30 Hg vacuum for several hours indicating no big leaks.

After adding only 25 oz of R134A, the high side was up to 240 PSI with the low side being only 13 PSI. I tried to leave the system run for a bit, but the pressure only kept rising, and I finally shut it down when the high side creeped up close to 300 PSI. The A/C compressor clutch is correctly turning on and off depending on the mode selected on the A/C switch inside.

There is no sweating on any of the A/C lines up in the front generator bay. The dryer-receiver filter lines are hot to the touch, one of the expansion valve lines feel cool (not cold) while the rest are warm/hot.

I have tried to research the cause of a high pressure side being too high, and I can only find that there could be a blockage, but the expansion valve and receiver-dryers are typically the culprits, but they're both brand new.

Thanks for any insight that you can provide me!

dmtaylor · 06-06-2020, 05:43 PM

I had the same problem with mine. I found the module that controls the hyd. fan was not coming on to high speed to cool the condenser. Unplug the four wires to the module and the fan should go to high speed on default mode.

Hoss7 · 06-06-2020, 06:29 PM

Quote:

Originally Posted by dmtaylor

I had the same problem with mine. I found the module that controls the hyd. fan was not coming on to high speed to cool the condenser. Unplug the four wires to the module and the fan should go to high speed on default mode.

That's interesting! Where is this hydraulic fan controller and what does it look like?

jakestarr · 06-06-2020, 06:45 PM

Hi Hoss,
The condenser and fan (electric) is located right behind the front axle. There are 2 wires to the fan. I had a problem with mine and it was the ground wire not making a good connection. This fan will come on immediately when you turn the control knob to AC.
hope this helps, Jake

Hoss7 · 06-06-2020, 06:50 PM

Quote:

Originally Posted by jakestarr

Hi Hoss,
The condenser and fan (electric) is located right behind the front axle. There are 2 wires to the fan. I had a problem with mine and it was the ground wire not making a good connection. This fan will come on immediately when you turn the control knob to AC.
hope this helps, Jake

Thank you, Jake! I'll look to see if the fan is running or not when Max A/C is selected. If the fan isn't running, would this be causing the high side to have excessively high pressures like I'm experiencing?

jakestarr · 06-06-2020, 06:53 PM

yes I believe so.

Isaac-1 · 06-06-2020, 07:55 PM

You might try spraying the condenser with a water hose to see if the high side pressure comes down, this will give you a clue as to the problem, if the fan is running and you are getting good airflow the condenser may be clogged (either internally or externally)

MarvinG · 06-06-2020, 09:57 PM

Yes, not getting rid of the heat on the high side will absolutely push your head pressure up. You’ve received good advice. Make sure the fan is running to speed, and if necessary trickle water over the condenser to see if head pressure goes down. With no more charge than you have put in, you shouldn’t be seeing much heat on the high side line going to the dryer.

If the fan is up to speed, and trickling or spraying water on the condenser pulls the head pressure down, and you are able to add correct volume of refrigerant, system sweating back etc. then you would be looking for filthy/blocked condenser coils, or any other coil or obstruction in the air flow. If all this checks out and you still have trouble, let us know.

Other issues can cause too much pressure but you have addressed the most probably causes by evacuating the system, and replacing the Expansion valve and dryer.

Cj D · 06-07-2020, 03:47 AM

What was the initial problem with the system that you had to change the expansion valve?

hypoxia · 06-07-2020, 05:09 AM

Quote:

Originally Posted by jakestarr

Hi Hoss,
The condenser and fan (electric) is located right behind the front axle. There are 2 wires to the fan. I had a problem with mine and it was the ground wire not making a good connection. This fan will come on immediately when you turn the control knob to AC.
hope this helps, Jake

I also assume your condenser is up front behind the axle, one speed & it has to run to keep the pressure down. When you turn the A/C on it should turn on as jakestar says.

dmtaylor · 06-07-2020, 10:27 AM

in the engine compartment , close to the fan . follow the wires from the fan to the module

hypoxia · 06-07-2020, 10:40 AM

Quote:

Originally Posted by dmtaylor

in the engine compartment , close to the fan . follow the wires from the fan to the module

Not a factor on his coach since the condenser is not associated with the hydraulic fan and is at the front axle.

Hoss7 · 06-07-2020, 06:03 PM

The amount of knowledgeable people in this forum always completely surprises me! Thank you so much to everyone who helped - it was indeed the condenser fan, located behind the front axle. As soon as I touched the wires, the fan turned on; I unplugged it and thoroughly sprayed the connectors with electronic connector cleaner, firmly plugged them back together and it seems to be working fine no matter which way I wiggle or twist the wires.

I have successfully recharged the system with a hair under 72 oz. I accounted for 2 oz lost in the service lines, and 1 oz lost per can used (purging the service lines after connecting each can, etc), which should have yielded just about 64 oz (rated system capacity). The high side was 175 PSI, with the low side being 13 PSI at idle. Outside temperatures were 74°, and temperatures measured 1" from condenser coils underneath the coach was 90°. The Monaco manual called for 198 PSI on the high side when 1" from the condenser coils were 90°, but I didn't want to overcharge the system as the manual didn't specify whether that is considered at idle, or at a raised idle. Raising the engine speed from idle (600 RPM) to 1000 RPM, the high side increased to 197 PSI and the low side decreased to 10 PSI.

Is it normal for the low side to be so low on these coaches? With the ambient (inside RV) temps of 73°, the vent temps were 50° on Max A/C.

Would I see a noticeable decrease of vent temps if I were to clean and spray down the condenser coils and evaporator coils? It's DIRTY underneath that coach!

Thanks again for everyone's help!

MarvinG · 06-07-2020, 09:36 PM

Rule of thumb: 20 degree drop is generally considered good. Your low side pressure seems a bit low, assuming R134. (I don’t have a refrigerant chart in front of me.) I would not charge more than the manufacturer states is the correct charge however. You can have too low pressure on the low side for the same reason as you had too high on the high side: not enough air going across the coil, if the evap coil is dirty, cleaning it would increase air flow and raise the low side pressure.

Other things causing too low a pressure: obstruction (but you replaced the txv) low charge (but you added the specified amount). I personally wouldn’t get overly worried about low side pressure as long as I have good air flow, and as long as the evaporator doesn’t freeze up with prolonged use.