Automotive Air Conditioning Information Forum (Archives)

Provided by

We've updated our forums!
Click here to visit the new forum

Archive Home

Search Auto AC Forum Archives

R134A not cooling well at idle in retrofitted system

bob234 on Tue August 17, 2010 3:43 PM User is offline

Year: 1992
Make: Chevrolet
Model: Silverado
Engine Size: 5.7
Refrigerant Type: R134A
Ambient Temp: 95
Country of Origin: United States

I had to replace the compressor on my truck last week because the clutch was slipping on it(R4 type). I had all the R12 evacuated and once I got back home, I did a full system flush to remove the mineral oil and possible metal shavings. I charged it up the proper way, meaning 80% of the R12 charge, and used a blue orifice tube as recommended by the clerk at Napa. He owns a similar truck and said with the factory white it doesn't cool worth a darn with R134A. It blows ice cold and the low side pressure stays at 36psi until the engine warms up. Then it goes all the way to 55psi and I get 60* vent temps and the compressor starts rattling. If I have water spraying on the condenser, the pressure will go back down to 36psi and I will get 40* vent temps and the compressor gets nice and quiet. While driving same thing, 40* while moving, 60* at red lights. Is there a different orifice tube I should try, or should I just go back to R12? I took it down to a A/C shop that was offering a free a/c check, and they said everything is working fine, but I should try a red orifice tube, and that's what they use in retrofits if the customer complains that it isn't cold enough. Is the advice I am hearing correct, or is there a better way to get better idle cooling?

iceman2555 on Tue August 17, 2010 8:16 PM User is offlineView users profile

It would be so nice to meet the guy who determined the % charge of 134a to 12. Over the years have heard everything from 65% to what ever. The correct charge is the amount that it takes to completely fill the system. The lesser amount is normally considered the amount IF the system was not flushed and a unknown amount of mineral lube remained in the system.
One of the other aspects....charging to a specific pressure and quoting pressures from an AC service manual as truth. There are not set pressures specifications for a retro fitted system. The system was never designed to operate with this refrigerant and its inherent operational pressures.
The function of an orifice tube system relies on the amount of refrigerant necessary to migrate lubricant from the compressor thru the system back to the compressor. The system operates from a 'flooded' evaporator. A good test for the correct recharge is to simply charge about 85-90% and then operate the vehicle for app 5 minutes....let the system stabilize and then conduct some basic temperature test. A good touch thermocouple is infra red may be contaminated by underhood heat and exhibit incorrect temps.
Operate the vehicle...MAX AIR/RECIRCULATE, DOORS OPEN,ENGINE AT IDLE, HIGH BLOWER. Measure the inlet and outlet temps of the evap....they should be the same or within 3-5 degrees of each other. If the outlet is warmer than the inlet, the system requires additional refrigerant. Add this is small increments...1-2 oz at a time. Allow the system to re stabilize and retest temps. If the outlet is a bit cooler than the inlet...this is acceptable. But stay within the 3-5 degrees.
Accomplish this task.....the key is NO MATTER WHAT YOUR PRESSURES ARE!!!!. If a perceived excessive high side pressure is encountered...DO NOT REMOVE REFRIGERANT....increase engine/condenser cooling. An extra fan (electric) or perhaps the purchase of a new fan clutch is needed.
The removal of refrigerant to achieve a 'certain' pressure is a good method to reduce lubricant flow to the compressor...and it will fail.....!!!
Oh, yes, go back to the correct GM orifice for this vehicle.....should be the black/white tip is installed in the condenser outlet.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

Dave in Texas on Tue August 17, 2010 9:40 PM User is offlineView users profile

"A good touch thermocouple is needed">>>>> Make one or buy? Is this a refrigeration-trade tool or have a specific name? U R right about the IF thermometer. I'd like to be able to read the temps more accurately than by relying on my old hands and suspect circulation in them. Thanks

At 2, I went home with a 10. At 10 I woke up with a 2 !!

Edited: Tue August 17, 2010 at 9:46 PM by Dave in Texas

iceman2555 on Tue August 17, 2010 10:08 PM User is offlineView users profile

Not sure if site sponsor has a source for this very useful tool are not. We can see when he replies. If not, then I shall contact you and offer several sources
We have used this type tool since the 70's and have found it to be very helpful in system diagnosis.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

TRB on Tue August 17, 2010 11:02 PM User is offlineView users profile

Feel free to post up some sources Ice.


When considering your next auto A/C purchase, please consider the site that supports you:

Dave in Texas on Wed August 18, 2010 10:29 AM User is offlineView users profile

Did a search using the correct terms and among other sites, found some on ebay. Most out of Hong Kong....(forgetabowdit!). Did locate an NOS affordable unit w/contact probe, made in the USA. I'd love a Fluke brand since I've used that brand for over 40yrs, but will settle for the less expensive USA-made product. No factory warranty>>a leap-o-faith purchase.

At 2, I went home with a 10. At 10 I woke up with a 2 !!

iceman2555 on Wed August 18, 2010 11:27 AM User is offlineView users profile

There are several sources that we have used over the years. A very good tool is marketed by CPS Products. CPS offers various models, I personally prefer the TM350A unit. UEI markets a great tool, we did some test research for them a couple of years ago. They offer a single,dual and quad probe unit.
Fluke meters are great, some of their DVM offer a touch probe also. Check CPS and UEI prior to making a purchase. The UEI is not an expensive unit and works great. The CPS unit allows for more information input....
Any questions, please post or contact me...

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

bohica2xo on Wed August 18, 2010 12:13 PM User is offline


I have a UEI dual unit, and it is a fine device. If I want data logging beyond it's capacity I have an Omega unit that plugs into a Palm Pilot.

Within the past few years, many DMM's now come with a socket for a thermocouple. For a basic single channel unit they are great. You can strap down several T/C's, and just swap plugs if needed. There are also a couple of plug-in units to fit older DMM's to provide temperature measurement. Some of the newer DMM's are incredibly versatile.

The results you get with a T/C are relative to the installation. I prefer to clamp the bead down to the surface, and insulate over it whenever I can. T/C's are cheap enough I leave them in place on my own vehicles sometimes, if it is a hard to reach spot. A simple cable tie & a bit of presstite tape can change the information collected.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Dave in Texas on Wed August 18, 2010 12:16 PM User is offlineView users profile

Thanks Iceman. I appreciate the help.

At 2, I went home with a 10. At 10 I woke up with a 2 !!

Dave in Texas on Thu August 19, 2010 10:26 AM User is offlineView users profile

Made the purchase yesterday and it's on the way. Should receive Sat. I'd hoped my Fluke would also do Temps with the correct probes, but like me, it is a discontinued model.

At 2, I went home with a 10. At 10 I woke up with a 2 !!

Dave in Texas on Fri August 20, 2010 2:57 PM User is offlineView users profile

Re'cd new Digital Thermocouple Thermometer this morning. The manual suggests using a bit of high-temp grease on surface to be measured after surface has been cleaned. I have HT wheel-bearing grease, dielectric grease, Hoppe's gun grease and chassis grease. Which (if any at all) to use?

At 2, I went home with a 10. At 10 I woke up with a 2 !!

NickD on Sat August 21, 2010 5:44 AM User is offline

Solid state type thermal conductive compound works very well, but if assurance is required that you are achieving solid liquid in the line feeding the orifice or TVX, can install a site glass. Yet another method is the outlet of the evaporator temperature should approach that of its inlet. What the trick is, to get that solid liquid without blowing up the system.

Yet another problem with R-134a is keeping up the low side pressure at idle, if you cannot get enough refrigerant in the system without exceeding high side pressures at the rated engine rpm, it kills the low side pressure where the cycling switch kicks off. Can only go so low with the trip point of the cycling switch, so have to do something to decrease the high side pressures at rated speeds and ambient temperatures. A more efficient condenser and more air flow.

Taking precision temperatures and/or installing a sight glass are just indicators, but you also have another indicator that is just as reliable. Your system doesn't cool the interior of the vehicle very well.

dreiher2 on Tue September 27, 2011 10:23 AM User is offline

I am getting a UEI dual input temp sensor (a 350). You said that the evaporator output temp should be equal to and definitely not warmer than the inlet temp. You also said there should not be a greater than 3-5 degree difference.

It obviously makes a big difference with the kind of condenser and the amount of air going over it. It also makes a big difference between a hot day in summer (i.e. 85 and up) and a cool day in winter (50 and lower) here.

Question: On a hot day (98), and a tube & fin condenser with a good fan on a retrofit should you aim for the same temp in and out of the evap, or should you still aim for slightly cooler temps coming out? If you take that same kind of retrofit car, and have to evac & fix the AC when it is 50 degrees out, what should the in/out temps look like? It seems like it is easier to overcharge in cooler weather because it does not cycle as much. On cooler days, how do you determine if the evap is "flooded?" How should you compensate on cooler days, so that on the hottest days that you are not overcharged?

What ambient temps is the "3-5 degree" rule most accurate for?

Thanks. . .

- Don R.

mk378 on Wed September 28, 2011 5:26 AM User is offline

If you drive in hot weather you really should stay with R-12 in your GM fullsize truck. Those models just don't convert well-- many people have tried.

Edited: Wed September 28, 2011 at 5:28 AM by mk378

wrenwright on Sat October 01, 2011 1:01 PM User is offlineView users profile

Sorry to threadjack, but would it be possible to use a laser thermometer instead of a thermocouple? I can get under the dash and right up to the evaporator inlet/outlet. This is in a car with an expansion valve system. Thanks!

Back to Automotive Air Conditioning Forum

We've updated our forums!
Click here to visit the new forum

Archive Home

Copyright © 2016 Arizona Mobile Air Inc.