Automotive Air Conditioning Information Forum (Archives)

BG1 on Thu August 12, 2010 7:45 AM

Year: 1976
Make: Pontiac
Model: Firebird
Engine Size: 455
Refrigerant Type: r134a
Country of Origin: United States

Did the load test on my 1976 Pontiac Firebird that has been changed to R134a. It was changed over to R134a several years ago and leaked down so I added some R134a to get it up and running but its not cooling as well as I would like. At 1500rpm for 5 minutes with the doors open the low pressure stayed close to 30 and the high went up to 360 and the vent temp was at 60 with the outdoor temp at 100-103.
The same test but with doors closed the low was 30 the high was 350 and the vent temp was 54-57.

padgett on Fri August 13, 2010 12:07 AM

Longer description under the 78 Corvette but sounds like you have a bad fan clutch. Insufficient airflow through the condenser will make pressures go through the roof and not cool much. Also check for blocked or dirty fins (or even a plastic bag between the radiator and the condenser). This will also show up in high coolant temperatures (my cars stay under 185F even at 95F ambient with 180F thermostats. In cool weather they sit on 180F)

76 Firebird really needs a lot of airflow assistance particularly with a low compression 455.

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Many Carpets

HECAT on Fri August 13, 2010 5:44 AM

Quote
Originally posted by: BG1
leaked down so I added some R134a to get it up and running but its not cooling as well as I would like.

So it now has enough refrigerant in it to engage the clutch?

Cannot diagnose without first knowing the proper charge is in the system.

Suggest to recover and recharge by weight.



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BG1 on Fri August 13, 2010 7:10 AM

The fan clutch is brand new and I just cleaned the condenser. I added new r134a and used a scale to weigh the amount, the system did not cool as well as it should and I thought that maybe the scale was not as accurate as it was supposed to be and that I over charged it so I took some r134a out. The compressor clutch is engaging.

iceman2555 on Fri August 13, 2010 7:32 AM

The validity of your test temperatures is questionable. A vent temp of 60 with an ambient temp exceeding 100 is a 40 degree drop....with the doors open....this is not too bad. A different test...same conditions with doors closed and the temp drop was basically the same....was the selector for MAX AX/Recir or set for Normal alr. Once the cabin cools down....the selector set for MAX, the system should drop the same 40 degrees which would normally indicate a much lower temp.
Also question, the excessive high side pressures and the low side pressures. The high side pressures could be in line given the RPM and ambient temp...however, the low side normally is a bit higher during this test also. It is possible there is a restriction in the system....possibly a restricted orifice tube screen.....however, this would limit the amount of refrigerant entering the evap and not normally indicate such a temp drop ((vent to ambient). Not sure what part of the US you are located....but humidity plays an important part in determining cooling efficiency.
A good start...would be to recover the refrigerant and then insure that the system is properly recharged....since this is a retro....app 90% of 12 spec. Do the same heat load test...but engine at idle....operate for app 5-7 minutes...then measure evap inlet and outlet temps....should be the same or within 5 degrees of each other. While the system is not charged...remove the orifice and inspect it.
To conduct a valid test....the system must be fully charged....adding a bit here and there is not an acceptable method.

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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

BG1 on Fri August 13, 2010 8:30 AM

I know the best way is to know how much coolant is in the system. I need to open the system and flush it out and replace the VIR but I was wanting to see how it worked now. How does the engine compression play into this? I know this engine really puts out the heat. I may have taken out a little to much r134a?

BG1 on Fri August 13, 2010 9:06 AM

I just ran the test again at 75 degrees outside with humidity at 85% with the vents open and the doors open the low pressure stayed at 30 and the high stayed about 290 with a vent temp of 53-54, with the doors closed the pressures stayed the same with the vent temp a little cooler at 51-52 degrees.

iceman2555 on Fri August 13, 2010 9:32 PM

Darn..hit the wrong key once more….and ‘blam’…all was gone. Wish these fingers were not so ‘fat'.
Not sure what is meant by the term ‘vents were open’. Does this mean that the AC was operation in the NORM mode? This would coincide with the posted temps. The lack of difference when the doors open and doors closed beings to become more relevant. Seems the vehicle is using outside ambient temp during both test.
If one were to look at a standard pressure/temp/humidity/rpm chart the follow information is supplied. The chart is in 10 degree increments…and the math could be done to more adequately match the posted operational conditions…however, it’s Friday PM…..spent most of the day flying and I’m tired.
At 70 degrees, 80% humidity, engine rpm 2000….vent temps of 63 degrees are indicated. Low and high side pressures indicated are 45 and 355. The posted operational conditions are within spec. Keep in mind that the charge rate is a major contributor to the correct pressures and temps. Also, this info is a guide and should not be considered as ‘cast in stone’
What temps are expected with this vehicle? If ambient temps are 100 degrees, humidity is 85 and center vent is 55 degrees….that is pretty darn good……calculate the heat index for the above information…..!!! It is darn hot !!!!
The post concerning engine compression….only known effect this would have on AC performance would the engine cooling system would have to be able to handle any extra engine heat. The post stated that the vehicle would maintain a 185 degrees with a 180 degree thermo. This is one heck of a cooling system….the rating of a thermo is the opening temperature, in this case 180 degrees. The thermo is fully open about 20 degrees later. So this vehicle is never actually reaching the desired operational temperature. Most Chevy engines (may not be a Chevy) at app 200 degrees cylinder head temp.
It would be best to insure that the system is totally charged with refrigerant and has the correct amount and type of lubricant. The fan clutch is new…..mind an inquiry concerning the part number and location of purchase. May be able to supply more information concerning this particular clutch assembly.
Yes, TRB, I did it the way you suggested the second time....sorta like installing a AC compressor....sometimes gotta do it twice to get it right!!!!!



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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