Dash AC

Topic: Dash AC (Read 804 times) previous topic - next topic

Dash AC

August 05, 2023, 08:17:40 pm

Going to Whistler the dash AC quit and luckily that trip I could get away with out it. Trouble shooting
the problem was the pressure sensor isn't working and it is only a couple of years old as I replaced
both the pipes off the compressor. It is the one on the left and the one on the right had been removed
by a previous owner. I am not sure why they had 2 pressure sensors as any ACs that I have worked on
only has one. If I could find one that fits on the left maybe I could get away with that.

Re: Dash AC

Reply #1 – August 05, 2023, 08:28:14 pm

Mine quit yesterday and the compressor is froze up. Have to wait until we get home to repair.

Re: Dash AC

Reply #2 – August 05, 2023, 09:11:00 pm

On our coach that high pressure cut off switch is A22-45194-001 but yours could be different. Found the last one on the net for about $30.

Mike

Re: Dash AC

Reply #3 – August 05, 2023, 10:38:25 pm

Thanks Mike so that's for the high limit so that won't help if the system runs out of Freon to
to protect the pipe. I wish the low limit switch could be screwed in and was easy to change.

Re: Dash AC

Reply #4 – August 05, 2023, 11:07:07 pm

Thanks Chuck I was wondering about putting it somewhere else. I think there is a plug in my
drier. I will check it out tomorrow.

Re: Dash AC

Reply #5 – August 06, 2023, 08:45:07 am

Quote from: oldguy – August 05, 2023, 08:17:40 pm

... I am not sure why they had 2 pressure sensors as any ACs that I have worked on
only has one....

Note that the following is from observing the operation, not from tracing wires:

There appears to be two low pressure switches. The first will not allow power to initially reach the clutch unless the pressure is above a certain point (60 psi?). This one's operation appears to be like a latching relay in that it only affects initial operation and stays closed after that.

The second one is a low pressure cutoff that kills the power to the clutch when the pressure drops below 30 psi. Whenever this one cuts power, or the system is turned off, the first one appears to reset.

Re: Dash AC

Reply #6 – August 06, 2023, 10:01:33 am

As others have said, the one in the pic is a high pressure limit switch. It resets when the pressure goes down.

TOM

Re: Dash AC

Reply #7 – August 06, 2023, 05:13:48 pm

So it is the low pressure switch that is removed and bypassed and it is the high pressure switch
that is still there and is not letting current threw. I guess the low pressure switch is to protect
the compressor and what does the high pressure switch protect.

Re: Dash AC

Reply #8 – August 06, 2023, 06:03:07 pm

Quote from: oldguy – August 06, 2023, 05:13:48 pm

what does the high pressure switch protect.

If the expansion valve, in line filter, or dryer gets stopped up the pressure switch should keep the compressor seal and high side lines from rupturing.

Mike

Re: Dash AC

Reply #9 – August 06, 2023, 06:16:00 pm

Thanks Mike. I just googled the part # for the high side that you gave me and sensors showed
up. I need to see if it will fit on the drier as it has a spare fitting with Shrader Valve in it the same
as the low side. The high side one I have is silver soldered into the pipe coming out of the
compressor. The low side on never should of been bypassed and removed. I don't supposed
you know the low sensor part number.

Re: Dash AC

Reply #10 – August 06, 2023, 06:38:18 pm

The high side can't go on the drier filter. Have you verified there is no continuity across the high side sensor? I have never had one of them fail.

Re: Dash AC

Reply #11 – August 06, 2023, 06:50:01 pm

Yes I've checked for continuity and why can't it go on the drier as that is still high pressure.

Re: Dash AC

Reply #12 – August 06, 2023, 07:03:03 pm

Sorry about that you are right it is high side pressure. I never checked the sensor there and assumed it was for low side.

Re: Dash AC

Reply #13 – August 06, 2023, 07:49:11 pm

That high pressure switch should be screwed on to the high side fitting at the compressor. Check it and it should be able to unscrew it.

Re: Dash AC

Reply #14 – August 07, 2023, 08:37:29 am

Back when I was designing and selling industrial systems, best practices dictated that both high and low pressure switches be located as near to the compressor as reasonably possible. The thinking was (and is) that the compressor is where the pressure is mechanically changed, so any pressure related problems will manifest there first, stopping operation before (hopefully) more damage occurs. That said, most automotive systems have the low pressure switch installed on or near the drier.

TOM

Re: Dash AC

Reply #15 – August 07, 2023, 05:24:05 pm

Thanks Ron because of your post I went out and looked at the sensor again. It looked like one
piece but you said it should come apart so I put a wrench on it and you were right off it come.
Took it to the bench and tested it and it tested good. Put it back in and it wouldn't work so I
removed it again and scraped the connections and bingo it works. I will phone Foretravel
tomorrow and order the low pressure sensor as someone removed it and when I put the new
connectors in I should have order the low pressure sensor then but I didn't know that it should
be in then. Thanks everyone for your input, we have a lot of great mines on this Forum.

Re: Dash AC

Reply #16 – August 07, 2023, 06:54:08 pm

Peter,

That low pressure cut out pressure switch ain't anything special to Foretravel. You should be able to find one at most any auto supply store or a HVAC supply company or even the net.

Mike

Re: Dash AC

Reply #17 – August 09, 2023, 01:35:34 pm

Last night I got thinking after I got the coach the compressor wouldn't run as I was out of Freon
so this morning got out the Tone Tracer and went to check this wiring that made no sense and
for some reason someone moved the low pressure sensor to the back of the drier which I hadn't
notice before. Chuck said his sensor was a was at the drier. I wonder if there was a problem and
they moved the sensor. It isn't OEM as the two wires go to the sensor and back to where the
senor should be. At least now I know everything is protected.

Re: Dash AC

Reply #18 – August 10, 2023, 09:24:37 pm

My 1999 is the same sensor its on the dryer next to the condenser strap to the frame on the driver

Re: Dash AC

Reply #19 – August 10, 2023, 10:38:42 pm

This schematic shows the sensor at the drier is actually a binary switch. Anyone know what it does?

Re: Dash AC

Reply #20 – August 10, 2023, 11:18:43 pm

I just googled binary switch and it is a high or low pressure switch and you would think the
switch at the drier would be a high pressure switch as it is on the high pressure side of the
system. As I have a high pressure switch at the compressor I imagine that the one at the
drier would be a low pressure switch.

Re: Dash AC

Reply #21 – August 11, 2023, 11:01:10 am

Quote from: oldguy – August 10, 2023, 11:18:43 pm

As I have a high pressure switch at the compressor I imagine that the one at the drier would be a low pressure switch.

This is a long answer to a short question. First a general disclaimer: There are thousands of different configurations for AC/refrigeration systems. Without knowing exactly components and system design(s) were used, it is impossible to definitively answer questions. That said, I will abandon all caution and dive right in to trouble....

NOTE: all terms (low,high, hot, cold, etc.) are relative, I.E. the "low pressure" side may be 20PSI to 300PSI depending on the system design.

A basic AC/refrigeration system works as follows:

The COMPRESSOR sucks in Low Pressure, Warm Gas, and compresses it to a High Pressure, Hot Gas.

The HPHG is pumped through the CONDENSER (usually mounted in front of the radiator in automotive applications). A fan moves ambient air through the condenser making the HPHG give up heat, which "condenses" the HPHG into High Pressure Hot Liquid.

The HPHL goes through a METERING DEVICE (either fixed [orifice tube] or variable [TXV- Thermostatic Expansion Valve]) where it changes to a Low Pressure Cold Flashing Gas. There is more to this stage but I'm keeping things simple.

The LPCFG passes through the EVAPORATOR where it completely evaporates and picks up heat turning into Low Pressure Warm Gas.

The LPWG returns to the COMPRESSOR and the cycle repeats.

There are two major kinds of "driers".

A filter-drier, which is usually aluminum and is inserted between the EVAPORATOR and the COMPRESSOR. It will usually be used in a system that utilizes a fixed METERING DEVICE. It has desiccant to remove moisture and a filter to remove foreign particles. The capacity to remove either is finite and the filter-drier is only meant to remove the final smallest amount of each after the system has been thoroughly cleaned and dried. Any switches attached on this component will be open on low pressure.

A RECEIVER-DRIER is usually steel, smaller, and is inserted between the CONDENSER and the EVAPORATOR. It is usually used in a system that utilizes a variable METERING DEVICE (TXV). It has a filter, desiccant, and storage capacity for liquid refrigerant. It will normally have a small sight glass in the top and should be mounted close to upright. This type of system is usually filled until the sight glass is clear of bubbles, UNLESS the system is being converted from one type of refrigerant to another. Again, the filter and desiccant are only meant to remove the final amounts of foreign material and moisture and should be installed only after the system is thoroughly cleaned and dried. Any switches attached to this component will be open on high pressure.

There are a lot of other considerations in making the AC system operate at peak efficiency. One of the main ones is moisture content. Water is a deadly enemy of refrigeration systems and the only accurate indicator of moisture content in a system while evacuating is a micron gauge. Without that, we are just guessing.

The amount of refrigerant required can be accurately determined if the system was designed using a TXV and a receiver drier with a sight glass, assuming no components have been changed to non-OEM specs. A system that has a fixed metering device needs to be filled using the temp/pressure chart that the engineers compiled when the system was designed.... if you can find it! Or, the OEM's stated capacity, again, assuming nothing has been changed.

Many of the systems on our coaches require specialized knowledge to to function at peak efficiency. My old pickup truck may be fine for trips to the parts store. But, if I want to drive it to Central America with no problems, It needs to be in top condition; and I may not have all the knowledge I need to make it so.

A lot of people work on AC systems with little knowledge, and have some amount of success doing it; however, if you want it to work right, educate yourself or find someone who actually knows what they are doing.

TOM

Re: Dash AC

Reply #22 – August 11, 2023, 02:36:12 pm

That is an excellent explanation.

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