A/C low side pressure fluctuates and compressor cycles

[Jack Schmidt]

I'm posting this just to document a symptom I didn't find on TRS. Long story short: solved by carefully adding refrigerant.

Symptoms:
1. Air blowing just barely below ambient temperature
2. Compressor clutch would engage for 3 seconds, disengage for 5 seconds, and repeat, even with all A/C settings on max.
3. Low side pressure would drop from 80 PSI to 20 PSI while clutch was engaged and then rise back to 80 when clutch was disengaged.

I worried that something else was wrong besides low refrigerant, searched for fluctuating low side pressure but found nothing, and decided to take a chance and add refrigerant:

Whenever the clutch engaged and the low side pressure was below about 35 PSI I added refrigerant, and when I heard the clutch disengage I stopped filling. It took about 20 minutes but it worked. The clutch slowly stopped cycling and I used almost the whole can of refrigerant. The final low side pressure was only about 32 PSI. I couldn't get higher pressure, because, probably, the residual pressure in the can was only about 32 PSI. But the system is stable and the air blows really cold.

Case closed.

Edit: Just want to add that this site is great and you regular TRS bros are awesome. Thanks for all your help.

 

[RonD]

Just some FYI

AC has 2 settings, ON or OFF there is no "temp" adjustment, this is the same for home AC units or refrigerators or freezers, on or off
MAX AC just closes the Fresh Air Vent, so fan pulls in already Cooled Air from the cab, under glove box, cooling 80deg inside air is faster/better than cooling 100deg outside air

Temp is controlled by mixing in warmer air with cooler AC air, in a Ranger that's done by the "Blend Door" in the cab, it diverts air from the fan either thru the Heater Core(HOT) or around the Heater Core(COLD)
All air from the fan passes thru the AC Evaporator, part that cools the air, before coming into the cab

AC itself is done by compressing/heating a liquid and then releasing that pressure which allows the liquid to cool down to lower than ambient temperature
AC has a high pressure side after compressor and then a low pressure side after pressure is released
There is a high pressure sensor/switch, so hoses and fittings don't blow off, lol
And there is a low pressure sensor/switch so compressor doesn't "run dry", there is oil in the liquid that lubes the compressor, so if it should "run dry" it would burn out pretty quickly

The pressure is released thru an "Expansion Valve" or on Ford systems an "orifice tube"
Either is basically a small opening(orifice) that allows a fixed amount of fluid to flow thru
Expansion valves were often adjustable
Orifice tubes come with different hole sizes

Yes, if low pressure side was getting too low it usually means system is low on fluid, but can also be caused by blockage, in filter or orifice tube

The fluid doesn't get "used up" so you probably have a slow leak in the system
Refills can have a dye mixed in, so a slow leak can often be found if you refill using a can with dye mixed in, usually a UV dye so not visible without using a UV light, so no visible "stains" anywhere

 

[franklin2]

I will add something here; Ford uses the low pressure switch on the older units as a control for cooling also. Some units have a thermal expansion valve that controls the cooling, but Ford used the low pressure switch. As the evaporator coil gets colder and colder, the pressure on the low pressure side gets lower and lower. It will get so low that the low pressure switch will cut the compressor off.

You have to control the amount of cooling in the refrigeration system in some way. If you don't the system will keep going till the evap coil (the coil in the dash) turns into a huge block of ice. This is separate from what Ron D was talking about. He was talking about controlling the temp in the interior of the vehicle. You also have to control or throttle the refrigeration system.

This is why they tell you when filling the system, to turn the A/C on max and roll the windows down. I prefer to put the A/C system on normal and turn the fan on high with the windows rolled down. The objective is to keep very warm air flowing through the evap coil. This will keep the pressure higher on the low side of the system and you can fill it more accurately.

Half this stuff I have learned by mistake. I was filling my wifes Honda up, and I had mistakenly put the A/C on max and had the windows rolled up. I was watching the guage as I was filling, and I kept adding and adding, but I could not get the low side pressure to come up properly. I finally just quit and left it. Later that day she was in town in the heat of the day and the compressor locked up making the belt squeal. Thankfully she reached up instantly and turn the A/C off, ,she knew I had been working on it. After I figured out what happened I let some of the gas out and it's been fine ever since.

 

[Fast Eddie]

Thank you for posting this. I'm experiencing the same. I'm going to try your technique for filling only when the clutch is engaged and see if that gets me out of the woods. Maybe I'll just be kissing another can of $25 refrigerant good bye but it beats the alternative.

 

[Fast Eddie]

I will add something here; Ford uses the low pressure switch on the older units as a control for cooling also. Some units have a thermal expansion valve that controls the cooling, but Ford used the low pressure switch. As the evaporator coil gets colder and colder, the pressure on the low pressure side gets lower and lower. It will get so low that the low pressure switch will cut the compressor off.

You have to control the amount of cooling in the refrigeration system in some way. If you don't the system will keep going till the evap coil (the coil in the dash) turns into a huge block of ice. This is separate from what Ron D was talking about. He was talking about controlling the temp in the interior of the vehicle. You also have to control or throttle the refrigeration system.

This is why they tell you when filling the system, to turn the A/C on max and roll the windows down. I prefer to put the A/C system on normal and turn the fan on high with the windows rolled down. The objective is to keep very warm air flowing through the evap coil. This will keep the pressure higher on the low side of the system and you can fill it more accurately.

Half this stuff I have learned by mistake. I was filling my wifes Honda up, and I had mistakenly put the A/C on max and had the windows rolled up. I was watching the guage as I was filling, and I kept adding and adding, but I could not get the low side pressure to come up properly. I finally just quit and left it. Later that day she was in town in the heat of the day and the compressor locked up making the belt squeal. Thankfully she reached up instantly and turn the A/C off, ,she knew I had been working on it. After I figured out what happened I let some of the gas out and it's been fine ever since.

Can somewhat relate to this. I refilled the system with the AC off about about 6 years prior - not knowing any better - and it filled fine. Had cold air for the following 5 years. Last year it was dead again and did what youtube suggested - running max AC, windows down and fan on high. That attempt fell to wasted time and money.

I'm going to find middle ground between your post and @RonD's feedback. I'll attempt a refill with dye induced refrigerant. That way, if it doesn't stay contained, I'll have 'tracers' and can rent a UV light from O'Reilly's. I'll have to go back and research where to look. I think the seals have a tendency to dry rot over time and truck is 18yrs old. The other more likely culprit is the compressor.

 

[Bgunner]

No need to try and rent the UV light as they sell a pen light and glasses to see where the UV dye is leaking from. They are cheap and when I bought them it was around $15 at the auto parts store.

I found my leak buy using this method and mine is the compressor leaking at the input shaft so I have to get a new compressor to stop my leak. My research says that when I change the pump the condenser needs to be replaced along with the orifice tube along with a full flush of the system. more money than it is worth to me but I live up north so the heat isn't as bad as down south.

 

[Fast Eddie]

No need to try and rent the UV light as they sell a pen light and glasses to see where the UV dye is leaking from. They are cheap and when I bought them it was around $15 at the auto parts store.

I found my leak buy using this method and mine is the compressor leaking at the input shaft so I have to get a new compressor to stop my leak. My research says that when I change the pump the condenser needs to be replaced along with the orifice tube along with a full flush of the system. more money than it is worth to me but I live up north so the heat isn't as bad as down south.

You can understand I hope you're wrong. All the same, thank you for the info. Some days, it's required functional equipment here.

 

[franklin2]

No need to try and rent the UV light as they sell a pen light and glasses to see where the UV dye is leaking from. They are cheap and when I bought them it was around $15 at the auto parts store.

I found my leak buy using this method and mine is the compressor leaking at the input shaft so I have to get a new compressor to stop my leak. My research says that when I change the pump the condenser needs to be replaced along with the orifice tube along with a full flush of the system. more money than it is worth to me but I live up north so the heat isn't as bad as down south.

You do not have to change the condenser. The only reason to is if you have a catastrophic failure of the compressor. A mechanical failure of the compressor will shove debris right into the condenser, and the new style condensers cannot be cleaned properly.

Your compressor is fine, it's just leaking (most likely). If you get a new compressor, most of the compressor people want you to put a new orifice and accumulator to keep your warranty on the new compressor.

Tip; Look over all the lines, the condenser in front of the radiator, and behind the pulley on the compressor. If you see any oily areas with dirt stuck to it, that is your refrigerant leak.

 

[Bgunner]

You do not have to change the condenser. The only reason to is if you have a catastrophic failure of the compressor. A mechanical failure of the compressor will shove debris right into the condenser, and the new style condensers cannot be cleaned properly.

Your compressor is fine, it's just leaking (most likely). If you get a new compressor, most of the compressor people want you to put a new orifice and accumulator to keep your warranty on the new compressor.

Tip; Look over all the lines, the condenser in front of the radiator, and behind the pulley on the compressor. If you see any oily areas with dirt stuck to it, that is your refrigerant leak.

My compressor started to knock so its at the point of a catastrophic failure and why I haven't bothered to recharge it since it started to knock. For me it is the only way the warranty on the compressor would be valid.

 

[franklin2]

Knocking is not good. You can do what I did. I took the compressor, condenser and all the lines out. I then found some manual shut-off valves for the heater core and when it gets warm outside, I turn the valves off. Turn them back on in the winter. This really helped the heat inside during the summer. I adjusted the temp blend door so it shut, and it still leaked warm air. Shutting the water off to the heater core fixed that right up.

I am on a minimalist pathway now. I want to pull the evaporator out and modify the piece sticking out of the firewall and make it flatter. I also want to go to manual steering and get rid of the power steering pump and box, and lines, and continual leaking from rebuilt store pumps.

 

[Jack Schmidt]

It will get so low that the low pressure switch will cut the compressor off.

Maybe they shut down the compressor to save it from starving of lubricant as well as preventing ice-up.

I prefer to put the A/C system on normal and turn the fan on high with the windows rolled down. The objective is to keep very warm air flowing through the evap coil. This will keep the pressure higher on the low side of the system and you can fill it more accurately.

Thanks for that tip. I did it today and the low side pressure was clearly higher, more toward the center of the good range. Also, my refrigerant can was fully back to ambient temperature, so it had a little higher pressure and I could fill more, up to the center of the good range.

 

[Fast Eddie]

Also, my refrigerant can was fully back to ambient temperature, so it had a little higher pressure and I could fill more, up to the center of the good range.

That's a great idea - thanks!

 

[franklin2]

Maybe they shut down the compressor to save it from starving of lubricant as well as preventing ice-up.

You would think that lubrication would be a problem, but in my experience it is not. Most of the oil in the system stays in the compressor from my experience. When you take a system apart and dump the oil out of everything,, the compressor has the majority of the oil in it.

To make filling faster, I always take the low pressure switch wiring plug off, and jump it out so the compressor runs all the time. I have done many, many systems and it has never caused a problem making the compressor run like that when filling.

 

[Fast Eddie]

I jumped the accumulator/dryer switch with a paper clip and refilled the refrigerant. It seems to be holding in the 90+ degree heat here. We'll see for how long.

Used the UV stuff this time so I should get tracers for any leaks.

The compressor has developed a chatter/clatter. Is it the clutch or the compressor internals? Does it matter? Is the clutch replaceable?

TIA.

 

[Jack Schmidt]

The compressor has developed a chatter/clatter. Is it the clutch or the compressor internals? Does it matter? Is the clutch replaceable?

The clutch can make a grinding noise (bearings). I don't know about chatter. The clutch is replaceable. Roughly $120 at NAPA in S. California. I replaced mine about a year ago. The clutch comes complete with its engagement coil. Clutch replacement is an easy job if you leave your old coil on the compressor. It's kinda hard to replace the coil. It's a press fit to the compressor-- a bit hard to get off and a bit delicate to put on.