I am the local cheap mechanic for most of my friends, until it came to AC work. I told them to go to a professional. I do not like AC systems, you need special tools and if you ever need to open the system, you're required to capture all the freon, blah, blah, blah. Well now my girlfriends dad's 2000 Chevrolet Silverado 1500 truck has been working intermittently for about the past year. It was never a DD during it's problems until now. My girlfriend is driving it to save money as her car was totaled.

Enough about that crap. I replaced the cycling switch about 3 weeks and really haven't had time to mess with it since, and that did nothing. It intermittently works. From my research, I have deduced it to the thermal expansion valve on the evaporator, or the high pressure cut-off switch on the compressor.

The cycling switch on the evaporator has 12V across it when disconnected, which when connected, powers the AC relay in the main engine buss. I have tested the relay, it is good. I thought it might be overheating and causing failure, but, the system stopped working while I had it hooked up with alligator clips and short wires. It was flowing 3A on the load side and the control side was receiving 12V from, I assume, the cycling switch. When I would unplug the cycling switch, the relay no longer had 12V on the control side. I could bypass the relay when the system was not working, and still the clutch would not engage. When I say the system is not working, the clutch is not engaged. The low pressure switch, located in between the compressor and the condenser, always showed 12V. The pressure readings from my gauge seemed on par, with 40-45 when system is operating and escalating to 75 ish or higher when the compressor is not operating. This is on the low side. I would assume the orifice is working, as usually when the orifice gets clogged, the system does not work, and it works intermittently. I did not have time to test to see if the clutch was getting power or not, that will come tomorrow. I am also going to test voltage across the high pressure switch on the back on the compressor. Until i purchase a Chilton's or Hayne's manual, I am going to assume 12V when the switch reads too high of pressure. As far as the thermal expansion valve, that sucker is way in the dash, if everything else checks out good, I am going to default that it is the source of the problem.

Thoughts on my reasoning? I am far from an AC expert and have only done online research. Sorry for the long post...

 

Can you explain what you mean by the "cycling switch"? A/C systems work off of the low and high pressure switches for normal operation. As load on the evaporator decreases (less heat to be removed), the pressure on the suction side of the system (low side) will drop lower and lower until it reaches the point that the low pressure switch either opens and disengages the clutch (older cars/trucks) or it will in conjunction with the computer do the same thing.

What you're seeing can be caused by a couple of things:
1. Low refrigerant
2. Bad TXV, which will simulate low refrigerant
3. Plugged condenser, which will cause the hi pressure switch to shut it down.
4. Evaportaor box drain plugged.

#1 & #4 are the most likely. If the evaporator box plugs and stays that way for awhile, then the condensate builds up, and the evaporator becomes an ice ball. This shows up as A/C that works, but then quits for no apparent reason, and then will work again when some of that ice melts.

I'd start by looking at that drain.

 

The cycling switch attaches at the accumulator. From the research I have done, this is what most call it. If it is called something else, please enlighten me, as I literally only started to learn about AC stuff a few weeks ago.

By TXV you mean the sensor by the evaporator that will shut the system down to prevent the evaporator from freezing, correct?

I do not think it is low on refrigerant as the pressure gauge has been reading a constant 40-45 with the clutch engaged. This was verified today, as I recorded the same readings 3-4 weeks ago.

I do not think the condenser is clogged either, at least I hope not. A little more than 2 years ago, the compressor was replaced and everything was flushed, and new refrigerant was used. So I would hope it is not, and will keep it on the list as a last resort.

I am thinking that maybe the evaporator drain could be plugged. Although I do it drip water, if what I am thinking of, is the same thing you are referring to. I am guessing this could be the ice melting. This would be located under the bottom of the passenger dash, correct? I will check that tomorrow and the voltage across the high pressure cut-off switch. This is the one mounted to the back of the compressor.

Of all the sensors located in and around the AC system, there is the cycling switch, which may have another name, located on the side of the accumulator, the TXV(?) which I think is located near the evaporator to keep it from freezing, the low pressure cut-off switch, which is in between the compressor and condenser, I think that is what it is, not quite sure. That would make logical sense to me anyway, because if the system was low on refrigerant, that sensor would read it coming out of the compressor, and shut it down to prevent damage to the compressor. Then there is the orifice which is located after the condenser and before the evaporator. I have located that quite easily, as the before the junction, the line is hot, and after that, it is ice cold.

Now, just to clarify the plugged evaporator condition, where are talking about the hose the comes from inside the truck, presumably connects to box containing the evaporator, and drips water, correct?

 

For clarification, item 32 is the "cycling switch" and item 12 is what I called the low pressure cut off switch, but I do not think that is correct. High pressure cutoff switch is not shown, as it located on the back of the compressor and in another drawing.

 

TXV is thermoexpansion valve. In my car, this function is done by the orifice tube ('96 T/A). What you're thinking of is a low temperature switch. The expansion device, be it an actual valve, orifice tube, capillary tube etc. does a couple of things. (1) it serves to divide the high and low pressure sides of the system at the evaporator, high on the inlet and low on the outlet going into the evaporator. (2) Causes the refrigerant to start boiling and removing heat.

You're probably right.

I'm not talking internal clogging. I'm talking about an accumulation of bugs, rocks etc. in the fins.

This drain comes out under the car, and with the A/C on you should see a fairly steady drip, drip coming out of it.

The low pressure switch is always either in the compressor or in the line between the evaporator and the compressor. The compressor is the other dividing line between hi and low, low on the inlet side and high on the outlet (that whole compressor thing ). What you see in the drawing as item 12 is on the high pressure side, and the same with item 32. Going into a really low pressure won't hurt the compressor at all, but a really high pressure can.
The orifice is located in the INLET to the condensor.

The basic flow of refrigerant is as a high pressure gas leaving the compressor, becoming a high pressure liquid in the condensor and remaining thus in the receiver/dryer. Leaving there, it remains a high pressure liquid to the orifice tube. As it exits the orifice tube, the pressure drops dramatically (look at the diameter difference between the line coming into the evap and the evap tubing itself - that difference causes the pressure drop) and this drop causes the refrigerant to boil, and in the process remove heat. Now the refrigerant is a low pressure gas, ideally all liquid having boiled off just prior to the outlet of the evaporator. The lp gas goes back to the compressor, where everything starts all over again.

That would be the one, as long as it's dripping on the ground. If it's dripping, it's probably ok. I'd probably stick a coat hanger up through just for good measure.

 

Yea this vehicle has an orifice tube, it is #26, but as you said, and from other drawings, there is a low temperature switch on the evaporator. I am just unsure as to the specifics of it, as there are several drawings, but I do know there is a device near the evaporator, which if faulty, would cause the system to shut down.


Although, I never really see it dip below the 40-45# range. I would imagine, isn't it supposed to drop down to around 20-25# and kick the compressor off, or vice versa?


Thanks, I'll check that one out.


There is a drip, but something could still be blocking it, so I will still check it out, considering this thing used to see dirt and gravel roads, so something could definitely be choking the system.


I think the "cycling switch" AKA item 32, is the low pressure switch. It is located on the side of the accumulator, which is the low pressure side. Item 12 is on the high pressure side, right before entering the condenser, BUT item 32 is a low pressure device. You can see that the orifice, item 26, is located after the condenser, and right before the line enters the firewall into the evaporator. From there is comes into the accumulator, then exits back to the compressor.


I am definitely going to check for reduced drain flow. Thanks for all of your help.

What baffles me the most is, with the connector disconnected from the "cycling switch" or low pressure switch, item 32 on the side of the accumulator, it has 12V! I do not see how that is possible unless inside the actual connector, there is a crossover with a small resistor or something. It friggin reads 12V with it disconnected! Once connected, it reads 0V across the terminals.

 

I went and looked at the diagram again, now that I'm not beat. Item 32 will be the lp switch as you thought.

 

I think I have it figured out. I think it was a combination of clogged evaporator and condenser. Clogged evaporator due to NASTY cabin air filters. It was like one of the Windows 7 commercials, high voices were humming and a light went off in my head. If the system cycles off, and the airflow across the evaporator is reduced(read as clogged cabin air filters), the refrigerant is not going to absorb as much heat, therefor the pressure is not going to rise up to 45# to trip the compressor back on.

I removed them completely, for testing purposes, put the AC on low blower speed, because of relatively low ambient temp outside(cloudy Houston around 80F). The AC was running around 45#, dropped to around 25#(minimal heat for the refrigerant to absorb), and the low pressure cycling switch on the accumulator, item 32, read 12V and turns of the compressor clutch. The pressure immediately began to rise and within 3-5 minutes, it reached 45#, and the low pressure cycling switch read 0V, and system began engaging again. So I think the cycling sensors are working properly, BUT as I am typing this, something else has failed. I do believe the entire refrigerant system has overheat as I am idling in the driveway.

The low pressure switch is reading 0V, so the relay is using power, tested and confirmed, but the high pressure cutoff switch has restricted the compressor use, due to the inefficiency of the condenser, not being able to remove the absorbed heat from inside the cabin. This thing is nasty. I am going to re-assemble the weather pack connectors, remove the front grille and go to the DIY car wash and spray it out. It is heavily clogged. Now, since this system is not running, there is still HOT high pressure in the condenser. This should come to an equilibrium and kick it back in action, just not as soon as if the system was operating properly. I will test this with a water hose. Once the pressure and temperature is reduced in the condenser(read intermittent failure), everything should return back to the cycling system. I believe the inefficient condenser, not being able to remove the heat absorbed by the refrigerant in the cabin and compressor, was killing the high pressure cut off switch and causing everything to work intermittently. The clogged evaporator, or dirty cabin air filters, was just hindering the cycle when it was working properly, making the refrigerant not absorb as much heat as possible, exaggerating the situation.

 

You're on the right track. When you wash out the condenser, make sure you wash from back to front. Also, are you sure the radiator fans are working? With the A/C on, they should be on all the time.

 

I am on the right track, but out of sunlight for the day, with too many other priorities for the night, and still a truck with NO AC! I washed the condenser, but it was half *** at best. I didn't have time to take it out from the mounting point to actually get back to front, but I know I cleaned it out some. I do fear I pushed some of the junk into the radiator though, haha.

As far as the fans go, its mechanical. The fan clutch could be going out, I do not hear it speed up, which was going to be my next check. I think they run at 33% in idle non load conditions, and 75ish% when ac is engaged. Although, shouldn't highway speed be more than enough cooling for the condenser? We were cruising home from my mom's and testing it out the highway, 70-75MPH. It cut out and stayed out.

I put the gauge on it, and it was reading 70# at the low pressure switch and the relay had power, but the coil wasn't didn't close the other switch. The load side had no power. I swapped it with another relay of same P/N and nothing. The low pressure switch is working all quirky now. It was reading 12V disconnected(normal from other testing), but I still had a 12V signal at the relay. In all of my other testing, when that connection showed 12V, the relay had no juice, and whenever it read 0V, the relay had power, whether the connection was plugged in or not. Earlier today, it seemed to be working fine, but that was idle, not actually driving with various engine loads. I will update tomorrow night with new findings. I am also going to order an electrical diagram of the ac system, that way I know when it is supposed to read, and what it is supposed to read. I may not be out of the ball park with these cycling switches just yet. I am also going to make it easier on myself and get a proper high and low pressure gauge set, as well as some wire taps so I can stop pulling apart the weather pack connectors to get voltage readings!

 

I'm also new with a/c but been reading up alot on it too...Just got me a nice 6cfm pump, gauge set, 30lb tank, etc.....

Is it me or those low pressure readings (the 70psi anyway) is wayyy to high?

 

Thats the problem. When reading 70#, the compressor should be running full steam ahead! I know when reading 70# on the low side, that the AC relay is energized, but the compressor is not engaged. I think that is due to my problem, the high pressure cut-off switch, killing power to the compressor. When the system is cool, and the AC is on low, and the ambient temp is relatively low, the system will cycle normally between 25# and 45#, but then the system overheats and the high pressure cut-off switch kills power. This is the only logical situation I can think of.

As far as further diagnosing my problem. I am just now getting a hold of wiring diagrams, which if anyone needs one, I can get a copy to you. I have a 1 week access and they are trying to screw me over image and file sizes. I have found a way around this and am going to print tons of these, so if anyone needs one, let me know.

 

If the system isn't running (clutch not engaged or engine off), the system pressure will equalize over time, and what you see on a gauge will be purely a function of ambient temperature. The higher the temp, the higher the pressure will be. If you look at a P/T chart, you'll see the direct relationship between the two.
http://www.chillers.com/PT%20charts%...frigerants.htm
Using this chart, find the ambient temp in the left column, then scroll across to the R134A column, and that will be the pressure in the system with nothing running and after the pressures on the hi and low sides have had a chance to equalize.

(edit) let me add that the gauges used can make a difference. Most of the stuff pro's use will be + or - 10% reading. The gauges that come with a recharge kit, who knows...but I'd be surprised if they were closer than +- 20%.

 

I understand that, and that is exactly what has happened, but with reading 70# on the low side, the system should be running. There are no user inputs telling the system not to run, just internal safety systems or malfunctioning electronics preventing it from running. The low side gauge is pretty accurate, as it was cycling perfectly between 25PSIG and 45PSIG, as specs said it should.

I am really leaning towards the condenser not effectively evacuating the heat absorbed from the cabin/refrigerant. As that temperature continually increases, pressure increases to the point that the high pressure cut-off switch steps into action. I cannot verify this until I get a gauge on the high side, which may be somewhat inaccurate, but better than nothing.

I also have the wiring diagrams for this vehicle, so I can determine what electrical signal I should be getting and when. Also, the site is limited to vehicle, so I can only get diagrams for a 2000 Chevrolet Silverado 1500, sorry.

 

Another thought, if you have a high gauge reading that doesn't correspond to the P/T chart, you probably have air in the system. Boyle's law says that all gases in a system exert their own pressure, and the effect is additive.
Example: Car off, and has been overnight...all pressures will be equalized. We'll assume an 80 degree day. We see from the chart that system pressure should be 86.8 psi, but what the gauge shows is 115 psi. Assuming the gauge is right (+- 20%), the max it can be is 103 psi. The rest is air in the system. Your gauge probably isn't that far off, and if it is then I'd get another and see what it says. Now, how to get rid of the air? 2 ways. If you have a Schraeder vavle on the accumulator, you can vent it in very short little spurts until the pressure comes down to what it should be. The reason for very short spurts is that every time you release pressure, the refrigerant will start to boil. It does this to try and maintain the pressure. You want to not release the refrigerant, just the air. Wait a couple of minutes and do it again. Doing it this way minimizes the amount of refrigerant you release. If you don't have a Schraeder vavle on the receiver, you can't do this at all.
The only other way is to completely evacuate the system with a vacuum pump and recharge it.

 

This could be a problem, but no one has messed with the system since the mechanic did a few years back. I would assume it would not wait this long to give us problems, do you think?

 

The only way would be if the charge leaked out, and it had to be refilled. 134A systems don't normally run in a vacuum like the old R-12 systems would, so it shouldn't have been able to suck any in just from running.

 

OK, good, that means it is even more less likely as it hasn't been touched since however long ago, or if it has, no one asked me anything!