A/C air temp (Page 1/2)
donuteater306 JUL 24, 01:57 PM
What’s the typical air vent temp for a 134a retrofitted Fiero? I know that there are many variables such as ambient air temp and humidity, etc., what are we seeing at the center vents? 102° yesterday, low humidity I got down to 50° on Norm at freeway speeds. I was expecting 40s.

Mine was a full retro 12 years ago: compressor, receiver/dryer, orifice, pressure switch. Recently noticed I could smell the evaporator warming in-between cycles so I had the guy who charged it 12 ago Inspect, evacuate and and charge. It was a couple of ounces low, but surprisingly no leaks (except for one retrofit Schrader that he replaced). Anyway, it’s steady cold again but not arctic.
RWDPLZ JUL 24, 02:41 PM
A properly working systems should be seeing vent temps from 34-42 F. See if the condenser is clogged and the fan is spinning.
donuteater306 JUL 24, 04:14 PM
Sigh, ok. Fan is working, condenser fins are straight and clean. Pressures were all good. Maybe (hopefully) the cheap thermometer I bought is not calibrated.
sanderson231 JUL 24, 04:24 PM
You said the pressures are normal - give us some numbers. The low side pressure is key to getting cold air out the vent.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

donuteater306 JUL 24, 04:53 PM
Unfortunately I have no specs. Guy's been doing AC for many many years (all he does and he was last to charge it). Went to hook up the gauges and saw that a valve had been leaking. He ran the system and watched the gauges while it cycled. Discharged the system, pulled vacuum for about 45min-1hr and it held. I saw that he used a scale to measure the refrigerant. Ran AC on Max during the recharge and he was watching gauges. He said that the pressures looked good. He went around the system with a sniffer and it didn't pick up on any leaks. Commented on the lack of condensation dripping from the car so we searched for the drain tube (honestly the one thing i've never looked for on that car). I took it for a drive while he worked on my other vehicle. Seemed cold but it was a typical 60deg SF Bay kinda day.
theogre JUL 24, 08:10 PM
you measure AC output air, center vents for most cars, vs Ambient Air.
So Ambient Air — Vent out = X
think ~ 40°F dif is about max. Not digging thru note.

AC converted R12 to R134 have "rules" to follow.
Highlights See my Cave, AC Conversion

Convert w/o trying to "dump" old oil often have problems w/ too much oil. Old oil won't move w/ R134 but still takes up System Volume for new oil and R134 to work in.

Many old Compressors won't work right to outright hate R134 and die and can have "Black Death" that is a huge problem.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

sanderson231 JUL 24, 09:19 PM
Here is some actual data I just collected from my 4.9L Cadillac Fiero. The system was converted to R-134a. The 4.9L uses basically the same compressor as a 2.8L V6. It is a clutch cycling type of control. Compressor clutch engages when the low side pressure climbs high enough. Compressor clutch disengages when the low pressure drops to a certain.

Ambient conditions - 97 'F and 24% humidity
Car in the shade idling at 600 rpm

Test Condition 1
HVAC fan on Hi
A/C control button on Normal
Compressor low side pressure - 40 psig
Compressor high side pressure - 280 psig
Vent air temperature - 57 'F
Driver's side door open

Test Condition 2
HVAC fan on Lo
A/C control button on Normal
Compressor low side pressure - 27 psig
Compressor high side pressure - 238 psig
Vent air temperature - 43 'F
Driver's side door open


Test Condition 3
HVAC fan on Hi
A/C control button on MAX
Compressor low side pressure - 38 psig
Compressor high side pressure - 270 psig
Vent air temperature - 55 'F
Cabin Air - 68 'F
Driver's side door closed

Test Condition 4
HVAC fan on Lo
A/C control button on MAX
Compressor low side pressure - 30 psig
Compressor high side pressure - 260 psig
Vent air temperature - 45 'F
Cabin air - 72 'F
Driver side door closed

I thought that when the A/C was set to max it recirculated some of the cabin air back to the evaporator and reduced the amount of outside air coming in. However it did not seem to make much difference in the vent temperature if the A/C was on
NORMAL or MAX.

The compressor was maxed out for all four test conditions. It did not cycle on and off. At normal cruising rpm the compressor will have about 3X the capacity than it has at idle and so it should cycle. The vent temperature will vary depending on where the low side pressure is in the control range. At the high end of the range, 55 'F vent temperature would not be unusual. I didn't take it for a drive but the A/C in this car has no problem keeping the cabin comfortable even with 105 'F ambient.

As you can see the vent temperature will change with the HVAC fan speed. This is normal. But the system cools the car better with the fan on Hi with a 55 'F vent than it does with the fan on Lo with a 45 'F vent.

Even with only 24% relative humidity the accumulator and the lines in and out were condensing water out if the air. The boiling point of freon is 30-45 'F at normal low side pressures. If the aluminum line from the evaporator to the accumulator is not cold it is a sign of restricted freon flow. In all likelihood this is a partially plugged screen on the orifice tube. It is a good practice to change the orifice tube whenever the system has been evacuated and has been in service for a long time - especially if there are complaints of poor cooling.

Autozone et al will lend A/C gauges for free and I'm sure they will show you how to hook them if you are not familiar with their use.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

[This message has been edited by sanderson231 (edited 07-25-2022).]

Eliredandblack JUL 29, 02:40 AM
I took my 88 GT to a very reputable AC repair shop that is well known for repairing vintage cars. They discovered that two of the fittings were leaking and that the blend door (hot/cold) was not closing completely. Since I had just replaced the heater core, they felt that the problem was caused by me. I had to remove the bass speaker to get to the heater core, then removed the plastic cover. It was easy to see that the blend door was not closing all the way when the lever was slid to the “Cold” position. I found the problem was that a small plastic clip was not holding the blend cable taught enough thereby keeping the blend door from closing completely. I ended up adding a small metal ring collar to the cable which acts as a spacer so that the blend door closes completely. I wonder if you are having the same problem? The shop must have seen that my AC was getting cold enough, but the heat from the heater core was making the cold air … warmer. You know our cars don’t have a valve to shut off the hot water going to the heater core when running the AC?
Eliredandblack JUL 29, 02:47 AM
Here are a few pictures of the blend door cable with the added collar that make the door close completely.
sanderson231 JUL 29, 10:46 AM
Update with some on-the-road data

Ambient conditions - sunny and 100 'F

Vent temperature held between 47 'F and 50 'F with fan on Hi and mode on Max while cruising at about 1800 rpm.

This is an 88 coupe with a 4.9L Cadillac with an HR6 compressor. From what I know this compressor is the same capacity as the DA6 compressor that came with Fiero V-6 and at least the 84 4 cytlinder.

Update: the 88 V-6 came with an HR6 compressor.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

[This message has been edited by sanderson231 (edited 07-29-2022).]