Automotive Air Conditioning Information Forum (Archives)

Bobby78 on Thu May 31, 2012 7:00 PM

Year: 78
Make: chevy
Model: el camino
Engine Size: 350
Refrigerant Type: R12
Ambient Temp: 75
Pressure Low: 37
Pressure High: 225
Country of Origin: United States

Help with A/C 78 elky

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Trying to get the A/C working in my 78. First some background info. New 350/290 HP crate engine. New 4 row aluminum radiator, new carb, intake, heater core, radiator and heater hoses, new alt, new p/s pump & box, new water pump,new belts, new heater control valve, new fuel pump, rebuilt TH350 trans.
Rebuilt 4seasons A6 compressor, new A/C hoses, new accumulator, new evap.

So only things related to A/C and heat that weren't changed are the condenser and the fan clutch.

Vacuumed system added oil and R12. Used 12oz cans of R12 and it took about 3 1/2 - 4 cans.
Did this work when outside temps were in the upper 60's low 70's & low humidity.
System blew ice cold air. Only problem was after being driven about 15 minutes, the accumulator, evap, and suction lines are iced up. So bad cycling switch, right? The 78 has the switch with the sensing bulb on the long aluminum wire. Previous owner had this in a horse shoe bend mounted with a small clamp on the inlet tube of the evap. That's the tube with the orifice.
Last few days it has been in the 90's with very high humidity. A/C blows cool not cold. No freeze-up. Low Side Pressure 49psi High Side Pressure 250psi. Rev the engine to about 2000rpm and hold it there., the pressures are Low Side 37 psi and High Side 275 psi.
Cools off in the evening. Temps drop to low to mid 80's still humid but not as bad. Low side pressure 45psi and High Side 225psi. Blows colder air out the vents.

Today temps in the low 70's and very low humidity. Freezing up again. Pressures are Low side 37psi High Side 225psi.

So aside from the cycling switch being bad, why is it not cooling in higher ambient temps and high humidity? Is it overcharged? Under charged? Not enough air across the consenser?
thanks

mk378 on Thu May 31, 2012 7:34 PM

If it's a TXV system, the valve's sensor bulb needs to be on the outlet of the evaporator not the inlet.

A pure CCOT is controlled by pressure at the accumulator (low side), there is no temperature sensor.

Your high side looks kind of high for R-12. Make sure the gap around the condenser is sealed so that all the air pulled through the radiator also has to go through the condenser first. Inadequate condenser performance can lead one to undercharge a system. If TXV, there should be a sight glass. Many times a replacement receiver-drier has tape over the sight glass to mask it while the whole thing gets painted black, it's up to the installer to remove the tape.

Bobby78 on Thu May 31, 2012 7:52 PM

NO TXV. It has an orifice tube in evap inlet. There is no receiver, this has an accumulator. Cycling switch is a box mounted to the front of the A/C box below the accumulator. It has a long aluminum wire with a sensing bulb on the end. I'm pretty sure this isn't working anyway since the system doesn't cycle. It just fereezes. So if the freezing is caused by a bad cycling switch that's not really a problem with inadequate cooling when outside temps are hotter, right? I will change the switch. But I'm also trying to get cold air on hot days.
Cycling switch is Factory Air part # 35720. I already swapped the location of the sensing bulb to the outlet of the evap,(suction side) still doesn't cycle and lines freeze up on cooler days. So bad switch.
So bad condenser too? Or air flow to and around condenser?
thanks

mk378 on Thu May 31, 2012 8:09 PM

Make sure the compressor drops out with the switch unplugged. There may be a wiring issue.

I suspect the fan clutch is shot. How does it do at highway speed (there, you get plenty of ram air, the fan isn't a factor). If you aren't concerned with authenticity, fitting a parallel flow condenser would improve performance even with R-12.

Bobby78 on Thu May 31, 2012 8:12 PM

Highway speed is when it freezes. Compressor shuts down if connecter is pulled off cycling switch.

Bobby78 on Thu May 31, 2012 8:42 PM

As far as I can tell the fan clutch could be original from 78. I'll replace that and the switch and report back. Thanks for the help!

Bobby78 on Sun June 10, 2012 8:34 PM

Replaced the fan clutch with a new "severe duty" fan clutch. Didn't make much difference. Temps today mid 80's, hazy, hot, and humid. Fan on High A/C on Max. Cool and comfortable but not cold like when temps are a few degrees cooler with less humidity. So any way to get colder air in hot humid weather?

Also replaced the cycling switch. Installed it as it came. Didn't adjust it. It has two screws for adjustment. Took it for a ride yesterday with the new switch. Temps in the 70's low humidity. Drove about 15 minutes on the highway at speeds from 60 to 80 mph and then back at the same speed. Got off the highway and drove about 10 blocks to the house. Blowing ice cold air the entire ride. Opened the hood at the house and the outlet of the evap was a bit frosty but not frozen. Inlet tube was sweating as were the suction line at the rear of the compressor. The evap was partially frosty and so was part of the suction line near the evap. So I'm not sure if I drove it long enough to freeze. If it does freeze again out on the highway how much do I adjust each screw? The tough part is, the switch has to be removed to adjust since the screws are in the back.
thanks

Dougflas on Mon June 11, 2012 4:38 PM

can you post a picture of your cycling switch? The capilliary sensing tube of the cycling switch should be on the INLET of the evaporator and well insulated with cork type tape.

Edited: Mon June 11, 2012 at 4:42 PM by Dougflas

Bobby78 on Mon June 11, 2012 6:52 PM

On the inlet? That's the smaller tube with the orifice tube inside? That's where it was when I started this project and everyone tells me I had it on wrong. They all say on the suction side of the evap, big tube, close as possible to the core in an 11 o'clock position. Someone else told me to "tar tape"? Not sure where to get that or cork tape either? Anyway the previous owner had it buried under dum dum. I just used the same dum dum but moved it to the bigger pipe.
Where can I get that tape? How about if I use aluminum tape and then cover that with duct tape or electrical tape? Will that insulate it enough?
thanks

Dougflas on Tue June 12, 2012 5:35 AM

Tar tape, cork tape, all the same thing. Available from a HVAC supply. Mitchell manual for 1978 states the input to the evap. which is the point after the orfice tube. I just looked this up again and it is located on the input tube; not at the accumulator which is the putput of the evap.

This innocent face would not lie to you!!!

Bobby78 on Tue June 12, 2012 1:35 PM

OK thanks. I did get it adjusted where it drops out at 21psi low side and kicks in at 41 psi low side. But the bulb is on the outlet of the evap. Guess I should switch it back.

JACK ADAMS on Tue June 12, 2012 3:19 PM

This is a "G" body, Fist year of this body style and uses an CCOT, sounds like you had or adjusted the cycling switch for 134a at one time and forgot to re-adjust it. also make sure your hot water shut off valve is also working. the vacuum lines tend to crack and leak, not allowing full vacuum to the valve.

Bobby78 on Tue June 12, 2012 3:47 PM

It did have 134a in it before. But now everything is new and back to R12. Heater core is cool on both inlet and outlet. What about sensing bulb location?

Dougflas on Tue June 12, 2012 4:07 PM

Trust me....I can read the book. :-)

Bobby78 on Tue June 12, 2012 4:47 PM

Yeah I'm sure you can. That's the way it was when I took the old one out. But some "experts" insisted it was wrong, so.. Anyway it gets mounted on the inlet pipe close to the evap and covered in tar/cork tape. How many layers of tape, 1, 2 ?
What do you think about the adjustment I've made to the switch? Will I have to adjust it again if I move the bulb? Should read the same no?
Thanks for your help.

Dougflas on Wed June 13, 2012 7:41 AM

Just use a few layers of tape. You may not have to adjust it again but bear in mind that there is a tolerance to gauges, the switch,etc. so possibly adjustment may be needed, maybe not.

Bobby78 on Wed June 13, 2012 7:47 PM

Ok got the cork tape. 30" roll! I'll have that for a thousand years! LOL! Anyway I moved the bulb back to the inlet. And I needed to adjust it again as it went from cut out on the outlet at 20 psi to cut out on the inlet at 5 psi. Cut in on the outlet was at 41 psi and then on the inlet it went to 51 psi. Anyway playing with the screws trying to get it back closer to what I had on the outlet, I lost the screw from the cut out port. It didn't seem to be making a difference as I was trying it in or out anyway. So back to the auto parts and instead of the Factory Air switch, I got an AC Delco. This one doesn't have 2 adjusting screws. It also has a strip of aluminum tape over a hole that might have a screw inside. Didn't remove the tape to check.
Anyway installed this on the inlet and w/o changing any settings on it the pressures are out at 5 psi and in at 50 psi. But at 5 psi the evap frosts over. Not thick heavy ice like before but still somewhat of a mild freeze up.
So do I need to pull off that tape and look for an adjustment or just leave it as is? As soon as the compressor cycles off the ice melts almost immediately.
And more important if it has a screw and I need to adjust it, what is the proceedure? Is like a half turn this way or that way? Will the one screw control cut in or cut out or both? Does it raise both settings or bring them closer together? I mean say I get cut out at 20 psi will that make cut in at 65 or 70?
thanks again

Bobby78 on Wed June 20, 2012 12:44 PM

Ok think I have all that straightened out! No more freeze ups even on cool nights. When outside temps are mid 80's or less AC works Good. Nice and cold.
Today outside temps are 99-104 and humid. Out on the road it's not bad but not COLD. Takes a while to cool down. Low Side is 32 psi and High side is 210 psi in idle. Are these in range? Is it overcharged or under charged? Lines and accumulator are slightly cool but not cold.
thanks again!

Bobby78 on Sun July 01, 2012 7:32 PM

Ok took some readings again.

Outside Air Temp 95* F
Low Side 27-29 PSI at idle
High Side 215-225 PSI at idle
Vent Temps 46.2

Accumulator and lines don't even feel cool and are not sweating. There is some condensation dripping out of the A/C box. I think it should be colder?
thanks again

Dougflas on Sun July 01, 2012 7:40 PM

what is your idle rpm?

Bobby78 on Sun July 01, 2012 9:18 PM

IDK didn't hook up a tach but with the idle up solenoid on the carb engaged for the compressor I'd say around 1000 RPM maybe in Park.