Any A/C experts on here?

[Cat]

Most recommend charging a r134 by using the weight specification. R134 systems are very very sensitive slight overcharging and slight undercharging. Its not like the old r12 systems where charging by the pressures was very accurate. Your pressure can be very close to a chart spec and not cool properly. There is no sight glass on r134 systems. On Cat equipment in AZ I had techs that said they could charge R134 system with gauges but we would still receive complaints. When charged by the correct weight r134 we had no further complaint. Safe Travels

[D Gardiner]

As others have already stated, gauges, temperature chart, and a vacuum pump is minimum needed items. After reading a couple posts, I wonder if some old school R12 freon service knowledge (verses 134a) got mixed in.

Example: To date I have never seen a sight glass on a 134a system (unless it was a retrofit system). My elderly mother's 1988 Mercedes 300TE has a sight glass, but it had a retrofit R12 to 134a service performed.

Good advice was misting the condenser to simulate driving. However we only did this on very hot days while charging a system. I will state 134a operates at a higher head pressure than R12, which is why the temperature chart is important.

Also great advice was adding freon (to a running system) is via the low pressure side. Why? The freon cans you buy at the store may not be able to handle 300+ psi.

Regarding leaks, and the PO not running/exercising the engine A/C system. 10 years is a long time, and freon can naturally leak out over that period of time, especially noting the system rarely if ever was operated. You can look for oily residue at any hose connection, compressor pulley, including at the Hi & Low fill valves. On our coach both the Hi & Low Schrader valves were old, and the freon leaked out.

If you find the leak and replace any component, try and do it on low humidity days, seal any openings with plastic and tape tightly. If you get too much moisture in the system it can damage the receiver dryer, requiring its replacement. Basically stated do not leave the system open long. It also goes without saying, if you must open the system, you will need to evacuate the system with the pump. Once pumped down for a good 30 minutes, close the gauges and note the vacuum level, then recheck after 30 miuntes. If you have no leaks the gauge should hold the vacuum reading, or at most drop a 1 inch of mercury. If good, then fill the system to manufacture specs.

You also stated your system still had freon, so with any luck your receiver dryer is just fine. As with any repair, some good knowledge and common sense will take you a long way. Good luck, and report back what you find, repair and especially your success.

[bpu699]

Alright it worked, not sure if I worked the gauges right...

Ambient temp 65 degrees...

Hooked gauges up, motor off, pressures 40psi hot and cold side.

Turn rv and compressor on, low side went to 0psi, high pressure went to 50 psi... compressor didn’t switch on....or did so intermittently for a couple seconds.

Hooked up 1 can of r134 a, opened the low side valve on my gauge set, Freon started going into the system. After one 12 ounce can, compressor now running. Still 0 psi low side, but only 50 psi high side. High side didn’t budge...

Based on the charts I should have a high side pressure of 130-140 psi...

High side barely budged after a can. I was bummed. But ac worked great... 37-41 degrees out the vent...

Shut the ac compressor off (ie turned ac of in the rv). Now looked at the pressure gauges: 140 high side, 60 psi low side. Pretty much exactly what it’s supposed to be?! Then, within with motor off for 10 minutes equalized on both sides to 70 psi with the motor off...

I’m confused, how can the psi on the high side be LOWER with the compressor running? And higher with it off???

It’s not like you can hook the gauges up wrong...

My gauges while filling Freon: both sides open where it attaches to the RV. High side closed, low side open.

Are you supposed to check the pressures with the compressor off???

Running pressures with compressor spinning: 50psi high side, 10-20 low side...

[Old Scout]

...pressure readings with compressor not running are meaning less....with compressor running [engine at 1000RPM and inside fan on 3/4] [fan or misting the condenser coil if engine fan is not running or aux fans not running] hi and low pressures should be in the ballpark with the ambient air charts for 134a, eg 30-40 low side and maybe 150-175 on high side at 65 degrees....and perhaps 45 degrees air temp out of vent inside rig.....

[bpu699]

Originally Posted by Old Scout

...pressure readings with compressor not running are meaning less....with compressor running [engine at 1000RPM and inside fan on 3/4] [fan or misting the condenser coil if engine fan is not running or aux fans not running] hi and low pressures should be in the ballpark with the ambient air charts for 134a, eg 30-40 low side and maybe 150-175 on high side at 65 degrees....and perhaps 45 degrees air temp out of vent inside rig.....

Can you help me understand why the pressures with the compressor on were LOWER than with it off? Could I have been measuring wrong somehow?

I was checking at idle, 500 rpm or so...

Compressor running peak high side pressure was 50 psi...before or after adding 1 can of r134a...

Compressor shut off, pressures checked immediately after, high pressure 150 psi...

Adding coolant made absolutely no difference to the high side pressure with compressor running...

How???

[Old Scout]

...am sure others will jump in but with compressor off--the hi and low sides will eventually equalize, so a "normal" compressor running pressure of say 40 low side and 175 high side will equalize on both sides at maybe 70 or 80...based on earlier readings and fact that compressor didn't run until one can was put in, you didn't have much left in system....Totally guessing here, but figure yr system will hold 3 or 4 cans total.....the fact that hi side now goes to 150 with compressor off is not normal....hopefully, someone will chine in with an explanation....again, engine should be at 1000RPM and condenser coil should cooled with fan or water misting while charging system--though at 65 degrees ambient, this is less of an issue....

[bpu699]

Here’s my PSI numbers:

Motor off. Hot side 70. Cold side 70
Compressor on. Hot side 50. Cold side 10

Now, turn the compressor off, check immediate pressures:

Hot side 150. Cold side 50.

Let it sit 10 minutes:
Hot side 70. Cold side 70.

Makes no sense...

[Spk64]

Perfectly normal for system to equalize after 30 minutes of being off. The static measurement at that point only needs to be high enough for low pressure switch to activate.

The low side is the suction side of compressor, so yes it will go lower when compressor is running.

[bpu699]

Originally Posted by Spk64

Perfectly normal for system to equalize after 30 minutes of being off. The static measurement at that point only needs to be high enough for low pressure switch to activate.

The low side is the suction side of compressor, so yes it will go lower when compressor is running.

Makes sense... but why does the high side go down with the compressor working??? That just can’t be right.... especially since the system blows cold...

I can add 12 more ounces, but the first bottle didn’t raise the high side pressure at all...at least not while the compressor runs...

[Spk64]

My guess is there are addition problems with your system besides a leak.
Failed dryer with desiccant loose in system?
Plugged or failed expansion valve?
Improper freon amounts?

[triplewide]

Didn't see what chassis you are working on, may have missed it. My 2015 F53 18K chassis took 2.1 pounds of freon from empty. Thought that was not a lot, maybe a small system, cooled like a small one.


When in your situation, not knowing what you had to start with as far as freon and air contamination I have had best results pulling a vacuum on the system and adding fresh freon by weight. Of course remove any existing freon legally first. In the past you could get a loaner vacuum pump at many auto parts stores. You pay a deposit that is refunded when you return it. Not sure if they still do this. You can buy a cheap electronic dope scale on Amazon that will weigh the cans of freon adequately. Here is the scales I used:

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1


I fall into the category of shade tree mechanic, so consider the source.

[bpu699]

Was just thinking, the gauge lines have Schrader valves in them... could I have attached the lines backwards?

Manual didn’t suggest going one direction or the other...

I can’t see how that would matter...

[Old Scout]

….nope--the low side is a different dia. from the high side so no way to reverse the gauge hoses….no offense and I do appreciate your enthusiasm but your skill sets for this effort are very minimal...do you have a friend or neighbor who might could help you?....based on what you are saying, your gauge pressures don't appear to be normal--I suspect you are going to have to dig a lot deeper, eg, perhaps flush system, replace exp. valve, replace filter/dryer, evac the system, add for compressor oil, check for leaks, etc..... not just adding 134a.....

[153stars]

Your not opening both valves to take readings are you. They are for filling and vacuuming if both open you create a slipstream /bypass. Only fill low side. Did you ever find data plate with weight.