Automotive Air Conditioning Information Forum (Archives)

teletekman on Thu August 22, 2013 2:36 AM

Refrigerant Type: R134a
Pressure Low: 38
Pressure High: 215
Country of Origin: United States

I've been scouting this forum as a guest for quite some time and learning quite a bit about MVAC repair and maintenance procedures reading the various posts so thought I would join and be part of the group. I recently purchased everything I need to do a full and proper AC charge from an unpressurized system, R134a Manifold Gauges, Leak detector w/ Dye Injctor, single stage Vacuum Pump, refrigerant scale, 30lb R134a refrigerant cylinder and a few other items as well. I figured for the price I was going to have someone do this repair including recharging the system back up I thought of just buying the tools myself and learning how it works. Since I can't afford a full recycling machine to properly suck out the existing refrigerant, I have a local shop that for the cost of the refrigerant they remove they do it for free so I got all my bases covered.

I for the first time replaced my AC Compressor on my Honda over the weekend and successfuly vacuumed, and recharged the system with the required amount that was needed, including replenishing what oil was removed from the old compressor. Anyways, I was told that after the system receives the proper amount of refrigerant and you close the valve on my charging hose going to the R134a cylinder to open up the low side on the manifold gauge a bit more to purge anything that is remaining in the lines. I did that and removing the coupler afterwards was effortless. However no one told me about what liquid is stuck in the high side hose. After unscrewing the high side coupler to close the schader valve, I tried to purge the hose by slowly unscrewing it from the manifold but whatever is in the hose get's all over the place, not to mention how cold it can be on the fingers even with gloves on.

So my question is before removing the high side coupler, should I open the high side as well and let the system suck in anything sitting in the line? I was told to charge on the low side but no one ever told me to open the high side on a pressurized system so not sure if this is the proper way to do it. I am working on another car this weekend so wanted to hopefully get this part figured out before hand. Everything else has been working great.

Sorry for the lengthy introduction and thanks in advance for any info anyeone can pass my way.


Sean C.
Madera, CA

Leggie on Thu August 22, 2013 6:12 AM

You do not open the high side valve while the system is running, period.

emsvitil on Thu August 22, 2013 6:41 AM

I shutdown the AC and let the high side pressure drop before shutting off the valve at the high side port and disconnecting the line and gauge set.

Usually on get a slightly longer pffffft compared to the low side disconnect (pffft)

-------------------------
Ed
SoCal

GM Tech on Thu August 22, 2013 8:16 AM

Why not?? I always shut off high side at the vehicle port-(while a/c is running) then open both valves at gages- which allows the low side to suck in all liquid out of both hoses- then all that is left is just some vapor- done this for 30 years- the emptier the gage set is, the more that is in the system- that was weighed out to go into the system--

I then suck the vapor back into my recovery machine from both sides after disconnecting from vehicle- so I can start with zero pressure on the gages for the next job.

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Edited: Thu August 22, 2013 at 8:18 AM by GM Tech

teletekman on Thu August 22, 2013 10:58 AM

So if I understand this correctly, once the system is charged and while the AC is still running, close the high side coupler at the vehicle port while leaving the low side open, then open both valves at the manifold and allow the low side to suck in whatever refrigerant is left in both hoses before removing?

webbch on Thu August 22, 2013 11:33 AM

The little nuggets of information you post GM Tech! I had never thought to do that before - just assumed to keep the residual refrigerant in the hoses for the next job, but didn't much care for the bleed off...especially with a system that has a decent amount of dye in it.

Do you do the same thing when you're doing a basic pressure check on a vehicle as well....recycle the refrigerant in the high side hose back into the vehicle?

wptski on Thu August 22, 2013 11:48 AM

You are basically connecting the high and low side hoses to the low side port. It would be at a higher pressure than the low side alone but how much would it suck in? I can see recovering what's in the hoses that'll get all no matter what.

teletekman on Thu August 22, 2013 12:16 PM

teletekman on Thu August 22, 2013 12:21 PM

Yeah, I have been told that a few times also to use a recovery machine to suck what's left in the hoses. My Snapon dealer has been dying to sell me one of these but at $4500 for the cheap model I think I'll have to pass.

GM Tech on Fri August 23, 2013 8:39 AM

No such thing as a basic pressure check-( I don't use my gage portable set) If I hook up to a car- I recover what is in there-no matter what-- then I record the recovered amount- (usually low) then vacuum and recharge to spec- then I check pressures---and cooling ability-- then I leak test- dye check- then I pull all the charge out and fix the system- then I do the final evac and recharge.

I don't like fixing a leak just to find out the pump is no good-- so I go full charge before I fix anything to be sure the system can get it done- and there are no surprises after the leak is fixed....

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

wptski on Fri August 23, 2013 9:08 AM

Quote
Originally posted by: GM Tech
No such thing as a basic pressure check-( I don't use my gage portable set) If I hook up to a car- I recover what is in there-no matter what-- then I record the recovered amount- (usually low) then vacuum and recharge to spec- then I check pressures---and cooling ability-- then I leak test- dye check- then I pull all the charge out and fix the system- then I do the final evac and recharge.

I don't like fixing a leak just to find out the pump is no good-- so I go full charge before I fix anything to be sure the system can get it done- and there are no surprises after the leak is fixed....
You don't pressure test with nitrogen then?

GM Tech on Fri August 23, 2013 5:15 PM

I never use nitrogen- only dye check and sniffer- and basic lookingat wet spots....

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Leggie on Mon August 26, 2013 6:56 AM

Quote
Originally posted by: teletekman
So if I understand this correctly, once the system is charged and while the AC is still running, close the high side coupler at the vehicle port while leaving the low side open, then open both valves at the manifold and allow the low side to suck in whatever refrigerant is left in both hoses before removing?


On a standard manifold setup, there are three openings. Middle, low and high. If you block off the high and middle at the hose ends, then open both valves while "sucking" on the low by leaving it open to the low side, everything equalizes to low side.

So the loss is limited to internal volume of hoses and manifold at about 30 psi, which is insignificant.

When you disconnect it, you'll hear a hiss. No liquid refrigerant should escape and it shouldn't get cold at the fitting. The only way you can lose 2oz is if the hose is full of liquid refrigerant and letting this escape every time is just poor service practice.

Edited: Mon August 26, 2013 at 7:02 AM by Leggie

wptski on Mon August 26, 2013 8:40 AM

Quote
Originally posted by: Leggie
Quote
The only way you can lose 2oz is if the hose is full of liquid refrigerant and letting this escape every time is just poor service practice.
For someone without a recovery machine, what's the hose loss? I've read that it is 2oz for the hoses and 1oz/can if you are using 12oz cans.

Leggie on Mon August 26, 2013 8:51 AM

Quote
Originally posted by: teletekman
Yeah, I have been told that a few times also to use a recovery machine to suck what's left in the hoses. My Snapon dealer has been dying to sell me one of these but at $4500 for the cheap model I think I'll have to pass.

I assume you don't have one since there's no reason you can't use a basic recovery machine to do this. So the first question is what you'r doing opening the system without one. Not having it is a legal and ethical violation.

A basic recovery machine is a glorified window air conditioner unit with some gauges and valves that is approved for use as a recover machine. The only thing it can do is transfer refrigerant from a system charge into an approved container to prevent release into the environment. A shop that doesn't have one but works on an A/C anyways is like that guy who don't pay for trash service and go drive around at night dumping trash at parks and apartment complexes.

Decades ago, a complete idiot couldn't take a blood pressure. It needed a stethoscope, reasonably good hearing and a proper technique.

Fully automatic heavy duty blood pressure monitor costs thousands of dollars to do the same thing. Busy hospitals gladly pay this for the ability to push a button and have it all entered into the patient's chart some 15 seconds later automatically so expensive nurses are spending their time on more productive things than taking blood pressures and entering data.

It doesn't produce a result that you can't get with a manometer and a stethoscope. The ability to do so automatically, quickly and tirelessly comes at an expense of thousands of dollars.

A/C service station is not the same as a recovery machine. Recycling is a feature that is found on many machines which separates oil, sediments and moisture found in recovered refrigerant using filters with a process just like aquarium filter. Air purge removes air. Only thing it can't do is separate contamination by other refrigerants. It isn't required however. A garage can simply use recover only setup and take the container back to someone who has a recycle capable machine or take it back to places like Airgas. Reusing it without recycling isn't the best idea since it may have moisture in it.

Total service stations are engineered for maximum idiot resistance, operator deskilling and heavy automation and appearance of advanced diagnostic capabilities. Higher degree of automation and advanced features come at a cost. Much of the cost goes to idiot proofing.


It basically allows evacuate and recharge by nearly clueless workers aka "lube techs". Some advanced deskilling feature may be VIN driven or make/model/engine menu driven instead of entering in charge amount manually. If customer vehicle calls for 32oz, many shop bills out standard rate + 2 lbs at retail, regardless of amount recovered.

Customer comes in with a complaint "it doesn't feel like its cooling adequately". Worker kid connects the two hoses, hit ok.
System checks refrigerant identity and if it passes (i.e. not filled with wrong refrigerant), it recovers and document amount, evacuate/massive leak check, then wait to check for absence of major hole in the system. If it passes, it adds the proper amount of refrigerant by weight.

Idiot proofing reduces workers comp claims,contamination of shop refrigerant, customer dissatisfaction and damage to customer vehicle caused by idiot workers.

From there, some have the ability to diagnose by directing the worker to run the A/C, fast idle and suggest possible causes based on input (system design per database, temps, ambient.. asks worker "cold air present? "y/n" etc etc.