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Coach A/C compressor won't run


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My 2005 HR Scepter Air Conditioning doesn't work.  The compressor won't run.  With the A/C off, the system low pressure port measures about 75 psi, the same as a fresh can of R134a, so there is Freon in the system.  Then I started the engine, turned on the A/C and the compressor did not turn at all.  (I can turn the compressor shaft manually, so it does not appear frozen.) The 20 amp dash air fuse looks good.  It could be the low pressure switch is keeping the compressor off, but at 75 psi that doesn’t seem likely unless the switch is bad.  I can’t even get more Freon into the system if the compressor won’t turn.  I have cycled the temperature rheostat and on/off switch repeatedly but the compressor hasn't budged.

Where would the low pressure cut off switch be located?  The expansion valve and service ports are on the right front inside the generator slide out compartment.  Any advice?

Dave Jones

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Check your compressor clutch with an ohm meter. It should be between 2 and 5 ohms. If it's good try jumping it with 12v. Then you'll know if it's the clutch or an electrical problem. Next check your low pressure switch with the ohm meter if you find it.

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1 hour ago, daveyjo said:

Where would the low pressure cut off switch be located?  The expansion valve and service ports are on the right front inside the generator slide out compartment.  Any advice?

The low pressure cutoff switch should be on the low pressure side (larger diameter tube) up there near the service ports.  Look for some wires attached to a module on the low pressure side.  You can check the sensor with an ohmmeter (at least one wire disconnected) or short the wires to get the compressor going with freon ready to go in. 

Did you compare that 75psi with a temp vs psi chart for R-134?

- bob

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Follow the blue wires from the low pressure up to the evaporator temp control switch (likely screwed to the evap box). 

Unplug them from this switch and put a fuse in place of the switch. 

If this switch fails, there's no juice going to your compressor clutch!

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1 hour ago, 96 EVO said:

Follow the blue wires from the low pressure up to the evaporator temp control switch (likely screwed to the evap box). 

Unplug them from this switch and put a fuse in place of the switch. 

If this switch fails, there's no juice going to your compressor clutch!

Forgive my “non technical” terms.  But there is a wire (blue on the Scepter/Camelot) as Ben says.  It comes out right behind the HVAC box.  Then it goes to one “switch” and then to another…thus we have two switches, presumably pressure and they are series strung.  Either one open, then the output wire, which runs to the rear, has no voltage.  Can’t remember without the print’s…but I THINK, there is an AC Compressor Relay there.  That up front “blue” wire energizes it.  I’d start with the wire as it comes out…unplug and measure….then check each switch.  12 VDC gotta go back to the relay in the rear (assuming memory if OK)….then verify power to that relay.  That energizes the clutch on the compressor….FWIW.  On our MH and much newer, you can turn DOWN or OFF the TEMP control….and even with system on MAX CONTROL….you get NO Signal from the HVAC system.  Many like me use it for heat….the Max Air shuts off the incoming air.  That REALLY HELPS on super cold days…

Let us know….

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OK. Here is the latest:  I found a switch on the low pressure line near the service ports.  I measured  it with one lead disconnected and it had continuity (a few ohms across it).  I measured the compressor clutch resistance and it had some, so I applied 12vdc to it and the compressor clutch worked.  Then I started the coach, externally energized the clutch and the compressor ran.  With the compressor running, I was measuring around 20 psi suction side pressure.  Then I added 12 oz of Freon, and things got colder.  Added another 12 oz. Freon and low pressure was still about 20 psi.   Didn't seem to be going up much.  Shut it down, disconnected 12vdc to compressor clutch and reconnected it to coach wiring.  Started coach, a/c compressor still won't run so something is denying power to the compressor.  Guess it was and still is low on Freon.  At 75 degrees, Freon chart suggest about 35 psi, so I thought I'd add more Freon and see what happens.  Maybe when it gets charged enough it will run.  Ben's? advice about checking switches at the evaporator is for consideration after nothing else works as it looks like a one or two day job just to see the evaporator.

I also am considering using leak sealer with the Freon, but I am nervous about using a sealer for fear it will plug things up.  Amazon has a couple of 3 oz cans and both are 4 star rated, and then they have some 10-12 oz cans with sealer.  Anybody had experience with the sealers?

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I have found that with these big rigs and long lines going from the front to back the low pressure side is much lower than what the chart reads. I just go by the high side. When it’s in the range of the high side that’s enough Freon. My low side is consistently 10 psi lower than what it’s supposed to be, but is ice cold.

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25 minutes ago, daveyjo said:

At 75 degrees, Freon chart suggest about 35 psi, so I thought I'd add more Freon and see what happens. 

The R134A chart I looked at shows 78 psi at 75d.  That's static, not running.  Allow plenty of time to let pressure and suction sides equalize. 

I remember trying to get my suction pressure "up to" 30-35 psi . . . . it only cost me $2600 for a new compressor plus all the trimmings.  At $150 / hr labor added up real fast.  They pulled out 72oz of freon, almost double the recommended amount. 

So be careful . . . .

- bob

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Dave, you don't need to get into the evaporator, this switch is likely screwed to the outside of the evaporator box.

For testing, just pull the two spade connectors off the switch, and replace with a good fuse.

No guarantee's, but I just had to replace this temperature switch this year!

I had the same symptoms as yourself. The spade connectors on this switch were corroded badly.

Your call, but since I just went through this, it would be one of the first things I would test if I knew I had some pressure in the system!

Edited by 96 EVO
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What kind of gauges are you using.  Do you have a set that shows high and low pressures. 

When I first bought my rig in 2008 the AC wouldn't run, I got by with adding one can of freon and that lasted fro ~12 years, not bad.

The rig sat for ~4 years while I built a house and then waited for Covid to pass.  In 2021 I was getting ready to start traveling and the AC wouldn't work so I bought a good set of gauges.   I did check the compressor by jumping the low pressure switch and it worked.   My neighbor does HVAC and sold me a large bottle of the 134 and came over and helped me charge the system.   Been good ever since.  My wife was on a trip and actually got by with just running the dash air for most of the trip and this was with a bunch of dogs crated. 

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Well, with gauges on both the high and low side, the Freon pressure was about 26 on the low side and about 140 psi on the high side (with the compressor clutch jump- wired).  Outside temp currently about 80 degrees.  Looks like it still needs Freon??  More coming in the mail, along with sealer.  I'll post results.  Also, found the stuff mounted to evaporator housing but haven't tackled it yet.

Found these guidelines:

For vehicles currently running with freon R-134a only
Outside-----low side-------High side------Center vent temp
60 F -----28-38 psi----130-190 psi ----44-46 F
70 F ------30-40 psi ---190-220 psi ----44-48 F
80 F ------30-40 psi ---190-220 psi ----43-48 F
90 F ------35-40 psi ---190-225 psi ----44-50 F
100 F -----40-50 psi ---200-250 psi ----52-60 F
110 F -----50-60 psi ---250-300 psi ----68-74 F
120 F -----55-65 psi ---320-350 psi ----70-75 F

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OK, here is the latest on my search for why my coach a/c compressor won’t work. I assumed the system was low on Freon and that a switch detecting that was preventing the compressor from running.  I checked that switch but it tested good.  Since the compressor wouldn’t run, I couldn’t add Freon to fill the system, so I “jumpered” the compressor clutch with 12vdc from another source and was able to charge the system.  But the compressor still wouldn’t run when unjumpered.  Ben (96 EVO) suggested testing the evaporator temperature control switch, but I put that off as I misunderstood where it was and thought it would be difficult to reach.  Well, it turns out it wasn’t that hard and bypassing it make the compressor run without jumpering!  Eureka, maybe that’s the problem!  So, I have one on order ($16, marked down from $60!)  Thanks, everyone, for all the advice!  I’ll post the results.

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20 minutes ago, daveyjo said:

OK, here is the latest on my search for why my coach a/c compressor won’t work. I assumed the system was low on Freon and that a switch detecting that was preventing the compressor from running.  I checked that switch but it tested good.  Since the compressor wouldn’t run, I couldn’t add Freon to fill the system, so I “jumpered” the compressor clutch with 12vdc from another source and was able to charge the system.  But the compressor still wouldn’t run when unjumpered.  Ben (96 EVO) suggested testing the evaporator temperature control switch, but I put that off as I misunderstood where it was and thought it would be difficult to reach.  Well, it turns out it wasn’t that hard and bypassing it make the compressor run without jumpering!  Eureka, maybe that’s the problem!  So, I have one on order ($16, marked down from $60!)  Thanks, everyone, for all the advice!  I’ll post the results.

GREAT.  To make this simple.  There are (TYPICALLY) two inline switches (my terms) and they are, on mine, FIRST, Thermostat and then TRINARY. FORTUNATELY...both are exposed and OUTSIDE (memory) the air box in the Genny area...they are  INLINE and in SERIES.  If you want to test...individually jumper and test...for a quick test. BUT, you can also VOM test them.  Use a paper clip or a needle or straight pin.  Put one on each "terminal" so they are in the circuit.  If you do NOT READ ZERO VOLTS or there abouts, then the switch is OPEN and the Compressor relay ain't gonna close.  Look at your INDIVIDUAL prints...but that is how MOST of the Monaco systems works.

MEMORY fails me... but PRINTS do not...here is mine. Follow the LIGHT BLUE wire from the VENT CONTROL Module on the upper LEFT SIE.  That is the SELECTOR SWITCH. 

It then goes (to the RIGHT) o the Thermostat.  Then to the Trinary Switch.  THEN to the Compressor Relay... 

 

Chassis Heating & AC.pdf

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