My A/C is constantly cycling. Should it cycle on and off?

[bobwhiles]

EDITED : My title says constantly cycling IT SHOULD HAVE READ: Stays on constantly and does not cycle off. Thanks

Hi Ford Guys and Girls. I am new to this forum. I own and maintain 2 Aerostars - a 96 and 97 both w/ 4.0l and extended bodies.

My searching the forum did not return the answer that I am looing for.


I am rebuilding the A/C on both vehicles. I just finished the 97. The reason for the rebuild is the high pressure hose was leaking. I decided to replace the compressor with a new one, the condenser and the accumulaor. I flused and blew out all the lines. I pulled the system down for 2 1/2 hours (have rear air). It was a good 30 inches down and held tight.

I am asking this question because, it doesn't seem to run as it did before. So I am wondering is this how it is suppose to . . .

Q) Should the clutch cycle off when the a/c is on.

My clutch engaged during the first 12oz can of 134a. It may have cycled a couple of times. By the 2nd can it stayed on. The system is fully charged and blows "ice cold" air. BUT it never cycles off. The low side shows about 70 and does not drop. Before rebuilding the system the clutch would cycle on and off and blow ok cool air. Was this because the charge was low. Seems to me that I had recharged the system before and it still cycled.


I guess that I am not sure how this system (a 134a) is suppose to work. I understand that the cycle switch will engage the clutch when the pressure is above 24 (don't remember the number). Does the cycle switch only disengage the clutch then if it drops below this number? I found no reference to max number (so it's just a low side cycle??)

I heard that a 134a system does not cycle unless the evap starts to freeze up and the systems drops back down. I was told that the cycle switch in this case is working to keep the evap core from freezing - otherwise the compressor runs all the time.

MY CONCERN is that the pressure on the high side raises up. It was getting too high at an idle (outside temp high 80s). I sprayed some water on the condenser core and it dropped from the 300s back to about 250. So I replaced the fan clutch.

What ended up happening - the pressure blew off - I guess there is a dump valve. I was in the palm desert area yesterday with the outside temps above 100. During in town driving the system blew out. My conclusion was that the system should of cycled off which would have lowered the pressure - but I read here somewhere that ford didn't use a high pressure cut off swith in this system.

So should the AC be cyclling on and off?

Thanks Bob

 

[BrooklynBay]

Welcome to this forum! The system should go up to around 30 or more PSI in order for it to stop cycling. You have to jump the low pressure cycling switch with a piece of wire while feeding freon into it.

 

[bobwhiles]

Are you saying that the cycle switch has an upper limit that will cut out the clutch?

My impression from everything that I have been reading is that it engages the clutch when the pressure is above @24 but no reference to an upper limit. My low side sits at about 70 and may have risen up to as high as 90 at idle when the high side rose into the 300s

 

[BrooklynBay]

It has a low pressure cut off, and won't be affected by high pressure. It's possible that you might have a blocked orifice tube if you are seeing such high pressure.

 

[rv4fun]

Those pressures sound pretty high. I've never seen 70-90 on the low side of my 3.0L single evaporator system. Maybe with the extra evaporator it could get that high on your system. Over 300 on the high side sounds pretty high also. Too much refrigerant doesn't help. 300psi is equivalent to 160F degrees. That's getting pretty hot. As long as the suction line from the accumulator is good and cold, you've probably got enough refrigerant in the system.

(on edit: replaced "gas" with refrigerant - in the HVAC trade, refrigerant is commonly referred to as gas - not to be confused with gasoline. Although propane works as a refrigerant, there are some problems with it as BrooklynBay pointed out.)

 

[BrooklynBay]

You've probably got enough gas in the system.

I remember reading how some mechanics substituted propane (illegally) for freon. The condenser caught fire in a front collosion as a result of the impact.

 

[rv4fun]

Here are some scans from the '97 A/C & Vacuum Troubleshooting Manual showing the switches in the a/c system. It does show a High Pressure switch (too bad it doesn't show what it's supposed to open at like it does for the low pressure cycling switch) in the circuit.

(You can "right click" them and save them to your PC for reference)

 

[BrooklynBay]

Thanks for posting the diagrams. It appears that the 1997 requires a higher amount of pressure on the low side than previous models to keep the compressor clutch engaged (unless it's only because it's a dual A/C system).

 

[waskly]

if low side is 70 the system had too much freon.

 

[bobwhiles]

Here are some scans from the '97 A/C & Vacuum Troubleshooting Manual showing the switches in the a/c system. It does show a High Pressure switch (too bad it doesn't show what it's supposed to open at like it does for the low pressure cycling switch) in the circuit.

Thanks RV4FUN

That is real good stuff. I will save these Pictures for future reference.

In another post I stated what I found. Yes there is a high - pressure switch on at least the 96 and 97 aeros. I decided to go to the Ford dealership with the manifold off my a/c pump and ask if they could id the part and order me one. IT IS a High Pressure switch. I assume that the HP Switch is normally closed (NC) because if you pull the connection the clutch will disengage. So until proven otherwise, I suspect that there is a high pressure that will open the circuit and cut out the clutch so the head pressure will drop.

MY AC is now working fine:
1) Charged with 36 oz of 134a - Tag in engine compartment states 4.25 pds and the MAX guide says 54 oz (w/ aux rear air). I can tell you that 54oz is too much. At 36oz the system is blowing ice cubes.
2) Yes clucth engages and stays on.
3) FYI - I went through the entire system - flushed everything, repalced the condenser and accumulator. Pulled both evaporators and cleaned off the fins. BOth were covered with lint and other debris - will tell you that everyone should pull the covers and clean the exterior of their cores for air flow.

The high pressure on the low and high side was due to over charge at 54 oz -- Again, I went with the shop manual vs. other specs.

So I am happy. And the pics provided are icing on the cake - thanks

Bob

 

[bobwhiles]

Before leaving the post I looked at the diagram.

RV4Fun - the diagram does show what the High Pressure switch controls.

In series you have the clutch cycle switch, high pressure switch and then the WOT circuit. These three control the clutch. The cycle switch will engage the circuit or disengage if the core temp drops, the high pressure switch will open the circuit if HP rises to ????, and the WOT will open the circuit to disengage the clutch under hard acceleration.

Thanks everyone.

Bob

 

[rv4fun]

The valve on the compressor manifold is a high pressure relief valve set to open between 450-470 PSI. It vents refrigerant to the atmosphere. I cannot find the pressure setting of the high pressure switch. Picture #1 shows the wires going in and out of the high pressure switch are Dark Green with an Orange Stripe so if you can find those wires, you can trace them to the switch - one of the wires goes to the cycling switch.

The low pressure cycling switch cut-out and cut-in pressure are shown in picture #1.

Ford's '97 Ranger/Aerostar Service Manual shows the single evaporator system's capacity is 2.33lbs (37 oz) and the two evaporator system shows it's 3.33lbs (53 oz). Works is works, if it's doing fine with less and the pressures are more like 25-30 on the low side with a good cold suction line (no perceivable difference between the inlet on the accumulator and the outlet), that's all you need. If you were to measure it with a sensitive electronic thermometer with excellent contact to the tubing, you'd be able to measure a few degrees warmer on the outlet tube of the accumulator when the charge is proper.

 

[bobwhiles]

The valve on the compressor manifold is a high pressure relief valve set to open between 450-470 PSI. It vents refrigerant to the atmosphere. I cannot find the pressure setting of the high pressure switch. Picture #1 shows the wires going in and out of the high pressure switch are Dark Green with an Orange Stripe so if you can find those wires, you can trace them to the switch - one of the wires goes to the cycling switch. .

I searched for a pressure rating on the oem mfg side for the switch and could not find the limit there either. I also appreciate now knowing what the dump side of the relief valve is. I guess we can only assume that the pressure cut out switch is below this. I do know that it has to be above @375 PSI as head pressure did rise that high the other day at idle after charge. I did not let it rise any higher though as it was "our aniversary" and I wanted AC for my wife and I for when went to dinner that afternoon.

The low pressure cycling switch cut-out and cut-in pressure are shown in picture #1.

Ford's '97 Ranger/Aerostar Service Manual shows the single evaporator system's capacity is 2.33lbs (37 oz) and the two evaporator system shows it's 3.33lbs (53 oz). Works is works, if it's doing fine with less and the pressures are more like 25-30 on the low side with a good cold suction line (no perceivable difference between the inlet on the accumulator and the outlet), that's all you need. If you were to measure it with a sensitive electronic thermometer with excellent contact to the tubing, you'd be able to measure a few degrees warmer on the outlet tube of the accumulator when the charge is proper.

Maybe I read the manual settings wrong -- now I will have to re-read as I have "that shop manual" and thought the capacity was 36 for the w/AUX. BUT as you said if it works it works. AND it is working real fine.

I was gettting @ 114 deg and @90 deg on the evap with a non-contact temp gun. I was told to try for a 10% difference but I didn't want to push it any closer as I really don't know which way the numbers get pushed as the charge increases. I also can't say what the vent output is but it feels like it has to be 50% of ambient temp. That isn't saying much as "IT FEELS" isn't to factual - so I might buy a temp guage of sorts and test it this weekend.

All of your input RV4FUN is apprecaited as I couldn't find any substantial input from any other post. Even a friend who is a FORD guy and did AC didn't know that about the HP switch. Guess we can't know everything though and that is why these forums are so important.

I am sure that others will benifit from this post and your input.

Bob

 

[rv4fun]

I searched around to find out what "normal" pressures would be with R134a. Here's some of what I found:

http://www.aircondition.com/tech/questions/82/
When charging, what should my low and high side pressure be?

Ah, this is the most asked question there is. There is no magic answer for this question though. There are too many variables. Compressor (engine) RPM and airflow across the condenser are always changing, thus engine speed is always affecting pressure. System design, blower speed, mode setting, refrigerant type, all cause variance in high and low side pressure. For this reason we simply can't say 30 on the low side and 200 on the high side. Though I might add, that's about where you'll usually end up. The reason 30 psi on the low side is just about right is because that translates into an evaporator temperature somewhere around the freezing point of water. Look at your low side R12 gauge and you'll see a temperature scale right next to your pressure scale. That low side pressure translates into evaporator temperature. Since moisture collects on the evaporator, we would like to keep the evaporator temperature slightly above the freezing point. R134a low side pressure will be be slightly lower (27 PSI) at this temperature. Again, refrigerant type is one of those variables we have to consider.

Then I found this "rule of thumb":

Air Conditioning Temperature Basics
In a Nutshell:

A good High Pressure Side Reading for 134a is equal to twice the ambient temperature + 50.
For Example at 85° F, you would expect a high side pressure of 85x2=170+50 = 220 psig.

A good Low Temperature reading should be between 30-40 psig.
This means the temperature of the refrigerant entering the evaporator is between 35 and 40° F.

These may vary depending on ambient temperature.

This chart goes along with the "rule of thumb" information. If temperatures are higher, pressures will go up. If air flow is restricted through the evaporator, it's pressure will go down - if it's restricted through the condenser, it's pressure will go up.

Temperatures and pressures inside the system can be charted - when you read the pressure with the gauges, you can go to a chart to see what them temperature of the refrigerant is. Use this link to calculate the pressure when the temperature is known - there is pressure<->temperature chart below: R134a Refrigerant Pressure Temperature Calculator

Originally Posted by bobwhiles > I was getting @ 114 deg and @90 deg on the evap with a non-contact temp gun. I was told to try for a 10% difference but I didn't want to push it any closer as I really don't know which way the numbers get pushed as the charge increases. I also can't say what the vent output is but it feels like it has to be 50% of ambient temp. That isn't saying much as "IT FEELS" isn't to factual - so I might buy a temp gauge of sorts and test it this weekend.

Those temperatures don't sound right for the evaporator - not sure why they would be that high.

How much trouble was it getting at the evaporator core? Cleaning it out sounds like a great idea and something I'd like to do - if it's practical.

 

[bobwhiles]

Lets say what you did early - what works works BUT . . .

I pulled the evaps, flushed and blew them out when I had the accumulator out (for the front as the rear access is easy). Condeser is new. The numbers that I am getting are at a low idle which I would expect at a stop light. When in motion all the factors change and the pressures drop - simulated at about 1200rpm and higher. I do believe that the "books" refer to running the idle at 1200 rpm during the proceedures - or at least I saw a reference to this.

I installed a new fan clutch and fan after discovering that I could spray some water on the condenser and the pressures would drop. (the test for effect of air movement over the condenser).

My concern was high pressure at a low idle as this was when the pressures went up and dumped the 134a. BUT remember I had that charge of 54 oz in the system also - which I am now thinking is too much and gave less room to expand. I also had the bad high pressure switch which would not have open the cirucuit which would have dropped the high pressure before the dump valve opened.


You were right about the capacity charge - but so was I - if you look in the shop manual in the previous section (2 back ) from the AUXILARY AC - I think it's the first section for Aerostar (then comes the section on the pump and then comes the AUX AC which has your numbers) Ford has another capacity setting for the Aerostar w/ Aux Rear listed as 36 ozs. The numbers for the oil are the same but it does list the charge for refridgerant as less and does say w/ Aux rear.

Go figure????

FWIW, my guages although new could have a bad low side guage.

For giggles - there was a TSB from FORD that suggested the cycle swith be adjusted. There was a problem out there - don't remember much about it although I recall it wanted techs to turn the adjustment screw 1/2 turn to the right I believe.

Bob