Factory Air Question

[davischevy]

RE; the Heater/Air Conditioner control panel, When the center lever is moved to the left (Air Cond.), position, on my 71, the blower fan comes on, no matter what position the blower switch is in. Also the compressor has constant power.

If I'm not mistaken the resistor inside the plenum is the "switch" for the compressor. If this is correct, and the resistor is bad, could this make the fan come on when the lever is set to Air Cond.?

Thanks,
Larry

[70cst]

Ttt

[1972SUPERSB]

I just got done redoing my entire factory AC. The resitor you are talking about is for the heater. There is a plunger type switch on they back of the AC duct where the flapper is that has 2 wires to it. Mine was missing. When the plunger is pushed down which happens when you set the controls to AC It sends power to the compressor. If the compressor has power all the time, possibly the fan switch is bad or the blower relay is acting up. Is it blowing on hi or low when the fan switch is off?

[72_Cheyenne_Super]

Originally Posted by davischevy

RE; the Heater/Air Conditioner control panel, When the center lever is moved to the left (Air Cond.), position, on my 71, the blower fan comes on, no matter what position the blower switch is in. Also the compressor has constant power.

If I'm not mistaken the resistor inside the plenum is the "switch" for the compressor. If this is correct, and the resistor is bad, could this make the fan come on when the lever is set to Air Cond.?

Thanks,
Larry

The compressor should not have constant power, there is a normally open switch that senses system pressure and closes to provide power to the clutch when the pressure drops. First place I would start is to throw a set of gauges on to see if it is low on charge as it is not uncommon for older systems to have small leaks that over the winter will drop the charge. You can also get a sense by feel on system charge level, the compressor discharge line should be very hot to the touch when running, if it is just warm to hot that is another indicator charge is low.

On the fan question, even though the fan comes on when the selector is moved to a/c, can you still change the fan speed or does it stay the same no matter where the switch is moved?

Have you done anything recently to the truck with wiring such as adding anything or replacing anything (not just a/c related)?

The resistor is for fan speed control, not just heat as stated in the other reply....it has two resistors, both are in series for low speed, only one for medium and neither for high speed.

[davischevy]

FOA, thanks for the responses guys.

I found the problem with the fan running in the AC position and compressor being powered up all the time. It was the relay behind the dash, behind the cigarette lighter. Pic below.

Now I have a vacuum leak near the switch panel. I will have to chase it down tomorrow. My hand held vacuum pump has been invaluable in this quest.

[Andy4639]

Check the plunger switch also. Bottom of the picture.When the middle arm is all the way to the right the lever pushes down on the switch behind the duct work which operates the AC compressor. The cable has to be adjusted for this to happen.

[davischevy]

Thanks for the great diagram.

I had the ducts and diverter out for service and actually checked the compressor plunger switch for adjustment and function before I installed the diverter. That's when I discovered the problem with the compressor. But, as I said in post #5, the problem with constant power to the fan and compressor turned out to be the Anti Dieseling Relay behind the cigarette lighter.

BTW, It appears the 1972's didn't have the Anti Dieseling Relay behind the cigarette lighter. I checked both the 71's I have here and they had it, and all the 72's I have here didn't.

[davischevy]

The Air Conditioning had been ignored for quite some time on the old Longhorn. As I sorted it out, I could hear a vacuum leak behind the control panel when the top lever was in the center part of its travel (inside Air).

It turns out the hoses were hooked up backwards.

I pulled vacuum on the system and it looks good.

[davischevy]

Update;

I replace the 1/8" vacuum lines from the vacuum canister throughout, pulled a vacuum on them and all checked out. I checked the heater valve in the sink and it seemed to be closing under vacuum, so I reinstalled it.

I installed a new dryer and pulled a vacuum test and charged the system with R134A refrigerant. The expansion valve was working, but the tube coming out of the top of the evaporator was warm. The back side of the POA valve was freezing ice on the outside. My low side pressure was about 60 lbs and the high side pressure was only l20 lbs.

I had an NOS POA valve, so I installed it, recharged the system, and had low side pressure of 35 lbs, and high side pressure of 210 lbs. The air from the vent still wasn't as cold as it should be, so I replaced the heater valve and I now have 60 degree air at the center vent.

My fan switch worked on low and mid range but on high it actually slowed down. I replaced the relay on the right end of the plenum (post 6, View A) and now have all three speeds on the blower.

[engineer_gregh]

Last year I replaced my POA valve with a high low pressure switch, replaced the expansion valve, receiver dryer and completely flushed the system. Now it cools at 48 to 52 degrees even with the outside temps at 95. I'm happy with it other than you feel the compressor cycling at highway speeds. I'm running a BB at roughly 350 HP, so I would think it would be even more noticeable with less engine power. Next year I'm going to replace the old style condenser with a modern design like the ones that ship with the vintage air set up. The older style don't work well with the increased pressure of 134a. Lots of good reading on the subject on the board and Google. I'm just sharing my own experience.
Check out the link below.

http://www.classicindustries.com/197...s/ap2551a.html

[hdff]

Subscribed.. I'll need to reference this valuable info in the near future. Thanks

[hdff]

Andy4639 what Manuel is this diagram pic from?

Originally Posted by Andy4639

Check the plunger switch also. Bottom of the picture.When the middle arm is all the way to the right the lever pushes down on the switch behind the duct work which operates the AC compressor. The cable has to be adjusted for this to happen.

[davischevy]

ANDY can correct me if I'm wrong, but I think it originally came from the assembly manual.

[davischevy]

BTW: ANDY4639 has posted some very valuable info regarding air conditioning. Search his posts.

[hdff]

Would this be the service manual you are talking about? http://www.ebay.com/itm/1971-CHEVROL...-/221197029940


Edit: I found a PDF file of the manual and see what I need to see.so now I have a couple of more questions. To you guys that that have pulled your heater core and evaporator boxes, what would you rate this as far as difficulty? I have some mechanical ability and can do lots of things but I don't think I am ready a frame off resto or rebuild a motor. Were all of the parts you needed available, I will need everything from the evaporator to the front of the truck, hoses,vacuum can, dryer, hot water shut off valve ect; I saw the poa valve gregh mentioned above and will probably go that route. Also what about the electrical stuff in the cab, resistors actuators ect. This will be a learning experience for me so please excuse any dumb questions I have. Thanks in advance and sorry if I hijacked this thread......

[coppertone]

I'm in the process right now of updating my factory ac also. When bought the my truck it was completely factory and I used it like that till my compressor gave out. Bought a new compressor and replaced the poa with a delete poa valve,used that system for about 6 years till my compressor locked up. Now I am rebuilding and replacing everything that has to do with my factory ac. I installed an evaporator update kit,pro6ten compressor, heater hoses,heater shut off valve,vacuum lines. Just ordered the new parallel flow condenser. I installed the evaporator myself,it was pretty straight forward. That was probably the most time consuming, had to drop the wheel well,remove the passenger side hood hinge and loosen the fender so I could have room to put the ac box back on.Once I removed the the ac box I found out the metal part on the back of the box was rusted out at the bottom so I had to replace the part that was rusted with a piece of galvanized sheet metal. I took it to a shop for an estimate before i did it myself and they told me they would call me back with the quote ,that just told me it was gonna cost me $$$$$ so I decided to do it myself. Good luck saved myself some money and did myself and now Iknow it was done right.

[hdff]

Where did you get the evap update kit and new parallel condenser, I would like to read up on them. I have a rebuilt stock compressor that the PO gave me with the truck that has never been installed. I know I need a brackets for the compressor, heater core, heater shut off valve, vacuum canister, ac hoses with dryer, pov I think, I have a stock condenser but it is questionable, evap core has yet to be inspected, and who knows what else. I am a little intimidated about this job, but I have the attitude that if others can do it then there is no reason that I can't.. Was it really necessary to remove the inner fender and hood hinge, how difficult was it? Damn I wish you were closer so I could come by and check out your progress

Originally Posted by coppertone

I'm in the process right now of updating my factory ac also. When bought the my truck it was completely factory and I used it like that till my compressor gave out. Bought a new compressor and replaced the poa with a delete poa valve,used that system for about 6 years till my compressor locked up. Now I am rebuilding and replacing everything that has to do with my factory ac. I installed an evaporator update kit,pro6ten compressor, heater hoses,heater shut off valve,vacuum lines. Just ordered the new parallel flow condenser. I installed the evaporator myself,it was pretty straight forward. That was probably the most time consuming, had to drop the wheel well,remove the passenger side hood hinge and loosen the fender so I could have room to put the ac box back on.Once I removed the the ac box I found out the metal part on the back of the box was rusted out at the bottom so I had to replace the part that was rusted with a piece of galvanized sheet metal. I took it to a shop for an estimate before i did it myself and they told me they would call me back with the quote ,that just told me it was gonna cost me $$$$$ so I decided to do it myself. Good luck saved myself some money and did myself and now Iknow it was done right.

[coppertone]

I bought the compressor,evap update kit and the gasket kit for the ac box from old air products. As for the condenser I contacted radiators r us just to make sure how they measure the condenser. Well I just received it and it was the wrong size and one of the installation kit screws punctured the condenser. Have to wait till Monday to talk to somebody. Think Im gonna have it ordered here locally by an ac shop so I don't have to worry about shipping and returning.

[Andy4639]

Yes that is the manual and yes they have PDF files of it now also. I have both the 71 and 69 manuals. I use them all the time in the garage.

davischevy thanks

I will help anyway I can.

[hdff]

Originally Posted by engineer_gregh

Last year I replaced my POA valve with a high low pressure switch, replaced the expansion valve, receiver dryer and completely flushed the system. Now it cools at 48 to 52 degrees even with the outside temps at 95. I'm happy with it other than you feel the compressor cycling at highway speeds. I'm running a BB at roughly 350 HP, so I would think it would be even more noticeable with less engine power. Next year I'm going to replace the old style condenser with a modern design like the ones that ship with the vintage air set up. The older style don't work well with the increased pressure of 134a. Lots of good reading on the subject on the board and Google. I'm just sharing my own experience.
Check out the link below.

http://www.classicindustries.com/197...s/ap2551a.html

engineer_gregh i am just curious was the replacement of the POA valve with the pressure switch pretty straight forward?

[Bigdav160]

I don't think just removing the POA and turning the system to a "cycling" system is the best route.

Do you plan to change to an orifice system also?

[Andy4639]

I have swapped all of my system over. New 134 a stuff for better expansion and it works great. The AC runs at 40* all day long. I took the bottom line loose and inserted the orifice in the line. I took the POA valve and trashed it because it sucks and it's worthless. I added the acumalator with no drier, I also took the cowl vent out of the cowl and closed it off. I put the passenger side kick panel back to manual. My system is a cycling one also ran by the computer.
I open the passenger side kick panel and the AC runs 40* after it runs for a little while because it just keeps on cooling the cab air off.

[davepl]

Sorry if I missed it skimming this very long thread, but in case someone didn't throw it out there already:

If you had a stuck AC relay (up by the pass foot vent) that will supply a full 12V right from the fuseblock directly to the fan. Non-AC trucks don't even have that, I don't think.

In other words, the whole fan resistor thing is not even used when on HI. But I'm wondering if perhaps there's a short in your switch that is causing the relay to be triggered in other switch positions.

If it's running at high in the medium position, unplug the relay and see what happens. Nothing should change. If it does, it's either the relay itself or the switch that triggers it or a short to 12V in the relay trigger.

Not a guarantee, but hope it helps!