Dash A/C stopped working....Should I be surprised?

[Scoundrel]

Should I be surprised? Not with my coach.

OK here it goes, I went out on a short drive to a local beach last weekend and ran the dash air with no problems. However when I took the RV out yesterday for a day jaunt to Palm Springs the dash air didn?t work I did a physical check of the pump and it click on when the button is pushed and the top hose gets very cold and the bottom hose is very hot. I believe this is what it supposed to do. Nevertheless the air temp that comes out of the dash vents is warm regardless of the temperature level knob position. Where should I start looking for the problem?

Oh and the dash A/C was serviced 3 months ago.

 

[Jim Godward]

Start looking for either a non working vacuum pump or a loose hose under the dash.  sounds like the shutter doos are not changing position from warm to cold settings.  Just a guess!

 

[Karl]

Don,

the top hose gets very cold and the bottom hose is very hot.

The input to the compressor (the large, insulated hose) should be cool or warm, and the output (the smaller metal pipe going to the condenser in front of the radiator) should be quite warm; even hot. Based on your description of the large hose being very cold, it appears that the compressor, condenser, evaporator, and expansion valve are operating properly, but the blend air doors are not - just what James said.   

You have a gasser right? If so, there probably won't be a vacuum pump, but it will get vacuum from a fitting on the intake manifold. It looks like a large bolt screwed into the manifiold and has a 1/4 to 1/2" diameter sealed pipe coming out of the top with several smaller side branches where the small plastic or rubber tubes connect to it. Usually towards the rear of the engine. You can usually tell if one or more of the tubes is disconnected or leaking without climbing under the dash. With the engine running at idle, remove each of the tubes one at a time. If the engine changes speed or starts to run rough and then smoothes out again when you reconnect it, that line is almost surely good. Repeat for the other lines. If one of them makes no difference whether it's connected or not, chances are pretty good you do have a disconnection or leak somewhere. Be careful when you do this because tubes are likely to be quite brittle from age and heat. If they check out o.k., then suspect a damaged or binding vacuum actuator on one of the blend air doors (it's a round, flat unit that has a small hose (rubber or plastic) going in one side, and has a metal link rod coming out the other side, connected to one of the blend air doors. Also check for obstructions that may prevent the doors from operating. If all that fails to turn up the problem, there should be a cable or vacuum operated valve on the heater hose which shuts off hot water flow to the heater coil when in a/c mode. If this is disconnected or not operating, hot water is constantly being circulated thru the system and will seriously degrade any cooling effect of the a/c. This valve will be in one of the heater hose lines, and may be mounted quite near the water pump or near the firewall bulkhead where the heater hoses go into the interior of the vehicle. This is most likely NOT the problem; if it were, the large hose going into the compressor would be quite warm; not very cold as you said.

 

[John From Detroit]

I hate it when I type a long, detailed post, and it does not show up.

Short (for me) description of vehicle dash ac starting with the compressor (pump)

Gas (vapor) is compressed, causing it to 1: Get very hot and 2: Turn into a liquid, this is then pumped, through lines to a CONDENSER which is a high pressure radiator usually located in front of the vehicle's raidator.  Here it sheds much of the heat and is pumped through lines, still as a liquid, to the EVAPORATOR, which looks like a high pressure heater core, only it's a cooler core, and is often next to the heater core.  Somewhere in this line there is a can, called a dryer or accumulator, it may have a glass eye called a sight glass.  During normal operation this sight glass should be about 1/2 full of liqud and the inlet and outlet fittings on the "can" should be about the same temperture (NOTE: test with caution HOT is one of the tempertures)

In the Evaporator the liquid turns back to a gas, which makes it very cold, it picks up heat from the air passing over the evaporator's outside, it it then returned back to the pump for re-compression and the cycle starts again.

Possible problems.  If there is not the right amount of coolant in the system, then the liquid will turn to gas either before entering the evaporator or after leaving, thus not cooling the right things.  If you have no expierence with fixing this professional help is recommended.  I have fixed it many times and have the tools, but I don't do it professionally.

Next we have the air handeling system  This may be simple or complex.  In the simplist system air enters your intake grill (on cars this is normally at the bottom of the windshield, a very stupid place to put it, motor homes it can be in many places) and then passes first over the Evaporator and then over the heater core and then to a diverter assembly that sends it to either the Dash vents, floor vents or defroster vents (windshield) or some mixture thereof.

This is the simple one,,, A much more complex system uses multiple diverters

First there is the "Source" divereter (Recirculate air from inside the car, or take in fresh) normally a vehicle with AC has this option somewhere (MAX ac is all recycled air, normal is mixed, and heat is all fresh air, unless you have a manual source selector like my wife does)

Then we decide if we are going to heat, cool or bypass

Then we decide where to deliver (Dash, floor or windshield)

This system may be controlled by cables off sliders or knobs on your dash, or it may be controlled by vaccume solonoids which are in turned operated by either the dash controls or thermostats.

My guess is that the problem is either in the AC charge (amount of coolant) or in the vaccume controls for the air flow. 

That said, I have symptoms simular to yours, I can clearly hear the pump cycle on and off when SWMBO changes the switch position, however the dealer told me it was a low pressure sensor....

In my case, warranty applies

 

[Tom]

I hate it when I type a long, detailed post, and it does not show up.

John, assuming that you clicked the Post button and that you waited, you should be able to click the Back button in your browser and your typing will be intact. However, the system probably won't let you re-post, so you'd need to copy/paste the text into a new post.

 

[John From Detroit]

John, assuming that you clicked the Post button and that you waited, you should be able to click the Back button in your browser and your typing will be intact. However, the system probably won't let you re-post, so you'd need to copy/paste the text into a new post.

Tom, it's not that, It is I thought I clicked post and wated (must not have waited long enough) I don't know it did not post till my next online session, way too late to use the "Back" button

It is most very likely my fault, not the softwares. 

 

[Tom]

Thanks for clarification John.

Meanwhile, don't be afraid to let me know if the software misbehaves. If I appear defensive, just whack me with a 2x4.

 

[Karl]

John,

Just to clear up a few points:

My guess is that the problem is either in the AC charge (amount of coolant)

Don stated that the return hose to the compressor gets very cold. If the refrigerant charge were low, it wouldn't be cold at all.

During normal operation this sight glass should be about 1/2 full of liqud

Assuming a full refrigerant charge, during 'normal' heat loads, the sight glass will be completelty covered with liquid. It will have some bubbles in it during operation at high heat loads.

Short (for me) description of vehicle dash ac starting with the compressor (pump)

Gas (vapor) is compressed, causing it to 1: Get very hot and 2: Turn into a liquid

The compressor merely compresses the gas which results in a hot gas. It is then sent through the condenser which cools it below its' boiling point, condensing it back into a liquid. That's why it's called a condenser. 

In the Evaporator the liquid turns back to a gas

The expansion valve preceeds the evaporator. It is there where the liquid under high pressure is passed thru a small opening into a low pressure area, causing the liquid to again change into a cold gas that can pick up heat from the air being passed through the evaporator.  Works just like when you depress the Schrader valve on a hot tire - the air comes out much cooler than the air inside the tire.

 

[Gary RV_Wizard]

One very common point of failure is the mixing valve, which controls the temperature of the incoming air in response to the setting of the temperature control on the dash. Many RVs use an HVAC system made by Evans that has a mixing valve notorious for failure and when it fails it usually goes to the heat mode.  This valve is actually a water control valve, which allows hot water from the radiator/heater system to flow through the a/c air path, warming the air as it passes.  Your a/c may be pumping out plenty of cold, but the mixer reheats it.

If you can find this valve, you can pinch off the hot water hose with vise grips (carefully, of course) and see if the dash air gets cold again.  If the hose going into the valve is hot (as opposed to merely warm) its a good bet that hot water is moving thorugh the valve. A replacement valve is $100+ plus labor. Or you can put a manual valve in the water line and open or close it as needed.

There may also be a vacuum or electrically controlled air door that routes the air through the system. On some systems this door can fail or get stuck in a position that directs warm air through the vents rather than the chilled air. Its usually under the dash somewhere. Sometimes its just a wire or air hose that has come loose and the door doesn't move.

 

[John From Detroit]

John,

Just to clear up a few points:
Don stated that the return hose to the compressor gets very cold. If the refrigerant charge were low, it wouldn't be cold at all.

I mentioned  both over and under charge, an over charge the return hose would be hot, I did forget the expansion valve, though. thanks for pointing that out

The bottom line is unless you are comfortable working on AC systems. professional help is suggested... I learned by watching professionals

 

[Scoundrel]

One very common point of failure is the mixing valve, which controls the temperature of the incoming air in response to the setting of the temperature control on the dash. Many RVs use an HVAC system made by Evans that has a mixing valve notorious for failure and when it fails it usually goes to the heat mode.? This valve is actually a water control valve, which allows hot water from the radiator/heater system to flow through the a/c air path, warming the air as it passes.? Your a/c may be pumping out plenty of cold, but the mixer reheats it.

If you can find this valve, you can pinch off the hot water hose with vise grips (carefully, of course) and see if the dash air gets cold again.? If the hose going into the valve is hot (as opposed to merely warm) its a good bet that hot water is moving thorugh the valve. A replacement valve is $100+ plus labor. Or you can put a manual valve in the water line and open or close it as needed.

There may also be a vacuum or electrically controlled air door that routes the air through the system. On some systems this door can fail or get stuck in a position that directs warm air through the vents rather than the chilled air. Its usually under the dash somewhere. Sometimes its just a wire or air hose that has come loose and the door doesn't move.

I went on the assumption it was the heater valve and replaced it.......Didn't help. Now what?

 

[Gary RV_Wizard]

Well, if the chiller line from the a/c compressor & condensor is cold and the heater line is not hot, then perhaps air is not being routed properly through the system so that it gets chilled.  John described the basic mechanics of a dash a/c system to you in an earlier post, so its a matter of working through the various components of your particular system to see what gives.  It's hard to be more specific without seeing your actual set-up, since there can be quite a variety at the detailed level.

 

[Scoundrel]

It embarrasses me to reply what my finding was but I feel I owe it to those who responded with suggestions.

As I indicated in previous posts I had replaced the heater control valve and it didn't appear to help matters. Well, it helps when you install it correctly  : No I didn't put it in backwards. Everything appeared to be installed correctly however when I was just about to start tearing things apart I grabbed hold of the valve and gave the cable/butterfly mount a twist. I noticed that it turned about a 1/4 turn closed. I didn't think it would make much difference but it did.

Apparently the original valve had a spring that closed it internally although I don't know that for sure. The new one seems to depend on the cable to push the valve closed. The new valve is slightly different then the OEM one and the cable doesn't appear to push the valve back all the way. What I did was install a light throttle return spring on the heater valve cable and attach it to the heater hose. Now the valve closed completely but still allows the control cable to move freely when I want to open the heat.

Maybe someone else has run across the problem and has a better fix. But in the mean time it works great....

 

[Jeff]

Scoundrel:

I am sorry my mind has gone, someone did that to our 83 Pace Arrow with the same problem but I had forgotton the fix.

 

[Karl]

Don,

Thanks for being so candid with us. We're just glad that you've found a solution

 

[Gary RV_Wizard]

Glad to hear the replacement valve fixed it after all.  It's quite often the culprit.  I'll have to remember the trick with the spring,  though.  Thanks for the report.