Does anyone know what the AC high and low side pressure should be on a 94 Explorer?

[luvmonterey]

I am troubleshooting some AC issues with my 94 Explorer after some years of it not working. I hooked up a pressure gage to the low and high sides to check static pressure and they were both zero. Anyone know what they should be? Neither my Hanes or Chilton manuals list these values. I'm going to replace the AC relay, as the compressor does not seem to engage, and this is the last of the original relays to be replaced. Once this is done, I will charge the system with coolant and dye to check for leaks. Thanks!

 

[Big Z]

Pressures will vary depending on ambient temperature, the compressor, flow restrictions & other variables. From what I can see, this is the 1st year for R134a refrigerant in the Explorer ('93 & prior using r12). A static somewhere around 100, & running low side PSI of around 25-45 @ 12-1500rpm are some ballpark figures, but not necessarily the ones you will observe with a properly charged system. System capacity from my search shows 36 ounces; conveniently, exactly 3 normal 12oz cans. It's better to charge by weight than pressure. The compressor will not engage [at least] because there is no refrigerant in it (gauges reading 0 PSIG). There are pressure safety switches that will not close the circuit necessary to send power to the a/c compressor clutch unless they detect pressures within certain limits. Both switches have 2 terminals inside & should give a low or zero ohm reading across the 2 prongs when they are closed ("on").
If this was me working on my vehicle:
Since this a/c is empty & has been inoperable for such a long time, I would remove the orifice tube (inside the line where the high & low sides are split) & inspect it for debris. It would be a good idea to replace it after it's been removed, but not necessary. They do have o-rings & they're only a few bucks at parts stores. Assuming it appears fairly clean, I would then replace the accumulator. Now would be the time to replace o-rings on any lines that are easily accessible. I would add an ounce of PAG 46 & 1/4oz of UV leak detection dye. Close everything up, & get ready for the refill.

After replacing the basic maintenance parts, you will need a vacuum pump to remove air and moisture from the system. Because the system is at 0 PSI, it's safe to assume it has a leak. Once the refrigerant leaked out, air & humidity made their way back in over time & it's important to remove it for the efficiency & longevity of the a/c compressor. I believe some parts stores will rent electric vacuum pumps out.
Hope this helps.

 

[luvmonterey]

Hey Big Z, thanks for the info and suggestions! To purge the system of air and moisture, do I hook up a vacuume pump to both the high and low sides separately, one after the other? Also, is the PAG46 the oil that is put in the condenser?

 

[Big Z]

I would add the oil & dye to the low side (suction) hose. Large diameter & flexible hose make it an easy place to add.

Vacuum through both high & low side fittings simultaneously. Hook up your gauge set to the high & low fittings. Attach the common line to the vacuum pump & turn it on. Then open both valves fully and let the pump work its magic. You can check for a vacuum leak after about 5 minutes by closing the manifold valves (you can leave the pump on during this if you want) & watching for a rise in pressure over time on your low side gauge. This will give an initial indication as to whether or not you have an obvious leak. Then re-open the valves fully & continue vacuuming.

 

[luvmonterey]

Thanks again for the info! I will get started tomorrow.

 

[luvmonterey]

I picked up a $20 AC vacuume pump from Harbor Freight today and hooked it up to my test manifold and it pulled about 1 bar for ten minutes. When I sealed the manifold and turned off the pump, the negative pressure creeped up to zero over the course of about five minutes. I'm not sure if that means there is a leak or if that is normal, but I'm guessing leak? I purchased a set of o rings and an orifice tube and have started installing the o rings but the set was missing the larger rings for the back of the compressor. I haven't found the location of the orifice tube, but I'm guessing it is someplace off of the condenser? Thanks!

 

[Big Z]

Correct. It's not normal to lose your vacuum pressure. And I would say at that rate, the leak is fairly substantial by a/c standards. Sure, I would expect a little rise on your first leakdown check after just the first few minutes of vacuum as moisture was likely still evaporating out. But in your case, the rise was much too fast & went much too high--it's a leak.

The orifice tube....hmmm. There's a line that starts on the bottom of your condenser & heads towards your firewall. Follow it. It should be just inside a joint on that stretch of line. The line might also have a step-up in diameter or a few dimples around it (that act as a "stopper" for the orifice tube) as possible giveaways. They're usually closer to the accumulator/firewall than the condenser, but some cars have them very near the condenser or even right off the condenser output (I'm gonna bet it's right near the accumulator). I recommend pulling the orifice tube especially to inspect its cleanliness. If it has black sludge on it, you have other problems. Sometimes the o-rings on the orifice tube are pretty stuck & the tubes break trying to pull them--thread a wood screw into it just a smidge to get a grip if that happens. Take note of the orientation--it matters. The big screen side goes towards the condenser.

It's possible that your new o-rings won't take care of your leak. This will be where the dye comes in handy for the home mechanic. Add it in one of your lines as you're replacing o-rings. Vacuum it as best you can & then charge 2 cans, or enough to make the compressor cycle reliably. Keep the a/c running for half an hour, then shut down, cool off, & start blacklighting for the leak.

 

[luvmonterey]

Thanks Big Z. Would you recommend that I replace the accumulator before I re-charge the system and leak test? I'm going to continue to replace o rings and can run vacuume tests in the mean time.

 

[Big Z]

I'd wait til you get it holding vacuum before replacing that. If it can't hold vacuum & you have to do a dye test to find the leak, replace it after you've found/fixed the leak.

 

[luvmonterey]

Thats what I was thinking, so thanks for confirming. I pulled the orifice tube and it actually looked relatively decent. I think I got all of the o rings but I'm not sure I got some the right size. Needless to say, it's still not holding vacuume. I will do the dye test tomorrow. From the looks of the oil I saw on the old orifice, it appears that there is yellow dye in the system already. I have an 8 oz bottle PAG46 oil with yellow dye. Do you think an oz of this will be sufficient, or should I go with the straight dye? I just want to make sure I have enough in the system when I test. Thanks again.

 

[Big Z]

Yeah. I'd probably go for 1.5oz of that oil+dye..max of 2oz, & don't add any more after the leak is fixed..be careful not to get it anywhere but in the hose--wipe off any drips with a rag & solvent so you don't get a false positive (I'd wear some nitrile gloves for this so you can just throw them away & not get dye everywhere after handling it) Vacuum it at least a few minutes before you charge it, despite the leak. Don't forget to purge your charging hose of air before sending it into the system. Even though we expect it to leak out, I still encourage some effort to get a fairly clean charge in the system. A good blacklight will help the dye pop out at you. The cheaper ones have a longer wavelength and can drown the picture in visible UV light, so you may have to use filtering glasses--the ones at autoparts stores usually come with the glasses unless you already have a good 365ish nm UV light. Be thorough & use an inspection mirror to help see everything, especially the main seal around the compressor.

 

[luvmonterey]

I checked the engine compartment with a black light to get a baseline reading, and found trace amounts of old dye in a couple spots. However, I did find spray patterns (old) around the compressor. I put some additional dye in the accumulator and managed to spill a couple drops. I washed down the engine compartment to remove the old and new dye, but that stuff is really sticky and I was only semi successful. I purged the system for 30 minutes, which probably did nothing, as I was just sucking new air into the system, and then charged the system with two cans of refrigerant (24 oz). This did not go well. I could not get the compressor to engage after installing the first can or the second, despite having a replacement AC relay. By the time I finished charging the system with 24 oz, and got a gage on it, there was zero pressure. None! There was too much ambient noise for me to hear any leaks when I was charging. I checked with a black light, and it appeared that there may have been some new dye around the accumulator, but I couldn't be sure. I could find no other leaks or the "smoking gun" I was looking for. At this point, I'm not sure what to do. Do you know of a way to bypass the pressure switch on the compressor or to test the compressor?

At this point I'm thinking that I just may have to bite the bullet and replace the whole system. It's 22 years old and I could spend weeks trying to diagnose and repair. It appears that I can get all of the parts, including hoses from Amazon for $450-$500. Still, I hate to replace parts that are still good and would prefer not to spend money unnecessarily, but I would have a "new" system for a quarter of what I would be charged by a shop here in CA. I also need to decide if having AC is that important. Climate on the coast is mild, but drive 50 minutes inland and its 105 degrees. And....it kind of bothers me when things on this old truck don't work. At least I now have a fairly strong understanding of AC systems and they no longer seem daunting. Thanks for your assistance!

 

[Big Z]

Hmm, wow. I think you have a leak I'm surprised it took even a few minutes to lose vacuum at that rate. There's a couple ways to proceed. If it were me, ideally, I would hook a nitrogen bottle up to one of the service ports & start running air into it. A leak that fast should be audible, but could be pinpointed with leak detection solution (bubbles). Option 2, caveman style: put your quick-connect onto one of the service ports without a hose attached to it. Depress the schrader valve. Have a helper run compressed air into the fitting on the quick connect with a rubber-tipped air wand. The same principles will apply. Keep the air pressure regulator turned down so you can keep a seal with the nozzle. Use soapy water to pinpoint it if it's not obvious.
Judging by your notation of fluorescence around the compressor, I'm going to guess that's where you'll find your major leak. It makes the most sense to me for the leak location after you swapped o-rings.

You can easily bypass the pressure switches. In this case to see if the clutch can engage (and prove the electrical portion works), you would remove the harness from the low pressure switch. It looks to me like it'd be right on the accumulator from the parts pics I'm looking at. Normally it plugs onto a 2-prong switch. When the switch senses pressure high enough, it closes & the 2 prongs are connected. When it doesn't sense enough pressure, the switch opens & the circuit can't be completed across those 2 prongs. So to trick the harness into thinking the low pressure switch is closed, you're going to need a special tool. Get a paperclip & bend it into a long skinny "U" shape and jam the ends of it into each of the 2 receptacles in the wiring harness. You have officially bypassed the low pressure switch. If it was me, I would turn the a/c controls in the cab on, THEN do the paperclip trick & watch the compressor. Remove it immediately if that works. Make sure your fuse is good too.

I wouldn't throw parts at it just yet. I think you'll find the leak. Once the refrigerant portion of the system is taken care of, you can worry about any electrical issues that show up. And all that really boils down to is getting positive voltage to the a/c clutch.

 

[luvmonterey]

Hey Big Z, thanks for the advice and information. I will try to charge the system with air this week. I would prefer nitrogen, but I am not sure where I could find it. I will let you know how it goes!

 

[larrydd999]

Big Z obviously knows a lot about A/C so all I can add is that I also rebuilt a '94 A/C unit last summer and it wasn't all that bad. Even the compressor isn't all that expensive. I changed the orifice tube, the evaporator, the filter/dryer, and the compressor. I also used a can of flushing chemical since the system had been out for at least a couple of years. While I don't use A/C all that often (near the coast slightly north and a lot west of Seattle) I like it in the summer to cool the car down and in the winter to help clear the windshield. Take your time, don't get impatient (especially with the *^$% push-lock connections) and you'll get it done.

 

[luvmonterey]

Thanks Larry. I actually grew up in Pullman and went to College in Olympia, so I can relate to the weather where you are. It's nice to know that you were able to get your AC working even after a couple of years. I'm great full to Big Z for his knowledge and assistance. How did you tackle the evaporator replacement? Seems there is two schools of thought on this; open up the plastic housing in place or completely remove the housing and evaporator as a unit first and then remove and replace the evaporator unit and put the whole thing back. What did you use to replace the foam insulation between the firewall and the plastic housing? Thanks!

 

[Centaurious]

I just repaired my system from a 48hr leak to not enough pressure to activate the low pressure switch. Vacuum checked the system and it tested great. Turns out the low side port's schrader valve was not tight and caused a leak. Replaced the low and high side schraders, vacuum and charged and it works great. At 95+ outside temp I can see 38 degrees at the center vent. I'm so glad Ford put a bypass valve in the heater hoses, it really helps.

 

[markhenry63]

I know this thread is old, but I'm starting to work on my 94.
The AC works and cools ok in mild weather, but it will soon be in the 100 degree range here and the first thing I noticed is the sight glass in the line on the drivers side next to the front of the chassis is busted and you can see some dye/oil has leaked out there.
Does anyone know where I can get a replacement sight glass module? It appears that is simply screws down on the tap in the line.