A/C question

Discussion in 'Troubleshooting & Diagnosis' started by 72ss4me, Jun 2, 2021.

  1. 72ss4me

    72ss4me Veteran Member

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    My 72 350 cu. in. ss has an A/C issue. The fan motor blows, I can hear some of the doors moving when I change positions on the selector, but no air is coming out of any vents. It used to work erratically when I would play with the selector switch back and forth and eventually, the air would come out the correct ducts, but the car's been sitting. My question is...where do I start?
     
  2. 72SS9681

    72SS9681 New Member

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    Its been awhile since I worked on my a/c in my 72 so I'm a little foggy on which doors do what. Most of the doors are operated by vacuum, but if I remember correctly the main door that directs cold air into the cabin is operated by a cable. It runs from the selector to the top of the inside box. You may be able to get your hand up there, but I think you'll have to pull the glove box. You should be able to see/feel the lever move at the box when you move the selector. If that's not it I would check and make sure you don't have a vacuum leak somewhere in the system, doubt its the problem I think you would hear an obvious hissing sound. Next I would drop the selector at see if anything is broken or disconnected.

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  3. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    ^ Good tips!

    The first thing I would check is the vacuum line running from the intake manifold, through the vacuum ball and then through the firewall to the control selector.

    If the system isn't getting vacuum from the engine, the actuators will go to the default position and you won't get air out of the dash vents.

    If vacuum is getting to the control unit, you nee to start tracing vacuum lines and testing individual actuator function.
    (This is a royal PITA as the lines and actuators aren't easy to access.)
     
  4. 72SS9681

    72SS9681 New Member

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    After thinking about your problem some more it occurred to me. With the car running, if your moving the upper lever (a/c, vent, heat, def, deice) and your hearing doors moving your getting vacuum. If your moving the lower (cold - hot) lever and your hearing the door your cable is probably functioning correctly. The vacuum controls the cowl vent (fresh air), the defrost, and the kick panel vent on the passenger side. The cable diverts air through the a/c box. You should be at least getting hot air blowing at your feet. You mentioned that it used to work erratically when you played with the selector and now it doesn't work at all. I think the problem is heater a/c control (selector) and I would start there. If it checks out good and the cable is functioning check the vacuum lines as Twisted Metal recommended. If you happen to have a factory service manual there's a schematic of the vacuum lines. If not I can scan a copy for you.
     
  5. 72ss4me

    72ss4me Veteran Member

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    Thanks for your feedback. Here's where I am today. I took apart the dash control, cleaned all the contacts, lubed everything and disconnected the vacuum tube bundle from the top with all the colored tubes. I figured that their must be leaks where it connects to the disc with the nozzles due to age. So I wrapped all the nozzles with a little teflon tape to make a tighter fit and reconnected the bundle. After I did that, all the vents worked properly if....the one vacuum diaphragm that's mounted on the A/C box under the hood on top of it is not engaged. I tested that diaphragm and it holds vacuum. I'm not quite sure why, but if the diaphragm is activated, none of my a/c selections work. If I move the lever as if the diaphragm is not engaged, (and it will stay there) they all work. So, with the diaphragm engaged, air does not come out of any of my vents or change flow. Puzzling.
    The other thing is my A/C compressor clutch is not engaging. So, I can't view the window of the receiver/dryer to see if any coolant is flowing. Not quite sure how to address this. I suspect maybe low refridgerant, but I see no signs of A/C oil anywhere or from the compressor.
    Ideas?
     
  6. 72SS9681

    72SS9681 New Member

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    I checked my shop manual and the diaphragm on the A/C box is for a "Purge Door", with the system in the off position this door opens (with vacuum) and directs air into the engine compartment at a low blower speed. So if I'm understanding correctly your system is working properly providing the lever is in the off position. Apparently this is a Camaro only valve in 1972, Interesting my 72 does not have this valve. As for the compressor clutch you probably have lost refrigerant. If the system is low it will blow the "Thermal Fuse" its located on or near the compressor. You can check it with a meter to verify. You wouldn't necessarily see A/C oil if the system has a leak, I converted my car to R134a and it bleeds off refrigerant through the compressor seal during the winter. I usually have to add more every spring. During charging you can jumper out the thermal fuse if necessary to charge the system. I attached schematics of the vacuum and wiring.

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  7. 72ss4me

    72ss4me Veteran Member

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    I"m not sure I follow you or you me. Lol!
    if the diaphragm on the A.c box is not activated, the system works fine with all positions directing air flow to where it needs to be. so, if I manually make sure the diaphragm is in the non activated position, meaning the plunger is not pulled inwards, the system works fine. If I move it to the non-retracted position, it will stay there, car on or off.
    If I manually push the plunger inwards, it will stay there and the system will not work properly. The plunger, even with a spring on it, will stay where I put it. I would assume that with manifold vacuum activated to it, it would normally be in the activated (plunger in) position. I dont' understand what's supposed to happen and when. The diaphragm does hold vacuum by the way.
    I have seen some posts talking about jumping the thermal fuse to activate the clutch while recharging the system, but they say it can ruin the compressor when there is low coolant. I've never messed with a/c recharging so I'm leary about screwing it up.
    By the way, any idea what the single green wire with the white stripe is that is near the a/c box. It has a plug on the end, but I haven't a clue what it's for.
    Thanks, by the way.
     
  8. 72SS9681

    72SS9681 New Member

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    Okay I think were on the same page... Lol. If I understand the manual correctly with the engine off and the A/C selector off, the plunger should be in the "non activated" position. As soon as the engine is turned on and with the selector in the off position the diaphragm should activate and closed the valve on the A/C box. When you move the selector to Max A/C position the diaphragm should move back to the "non activated" position and the rest of the system should work. It sounds like from what your describing something is obstructing the door and not allowing it to move as the return spring should close the door. I pulled up a picture of my engine (apparently my car doe's have that valve, I also have a 69 with A/C that doesn't). With the engine off the valve is in the non activated position. My car is at my storage unit so I cannot check right now but I have to go down tomorrow I'll start the car and test it.
    Regarding the compressor you should be able to test to see if there is power to the compressor by turning the key on, selector to A/C and blower switch on. You should have voltage at the solid dark green wire going to the center terminal on the thermal fuse. If there's voltage check the dark green with white strips wire that comes out of the fuse and go's to the compressor clutch. If there's no voltage your fuse is bad.
    I agree that you shouldn't run the compressor with no or low refrigerant but if you cannot get the compressor to kick on when charging you'll have to jumper it. If you have access to A/C gauges that will give you an idea if you have a charge and if you need to evacuate the system and start from scratch.
    Looking at a wiring schematic there's green wire with white strip that go's to the temperature switch but without seeing it hard to say.

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  9. 72ss4me

    72ss4me Veteran Member

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    Thanks for your help. I've been thinking to when I tested the plenum diaphragm. When I put the mighty vac on it, it might not have retracted, but when I manually retracted it and put the mighty vac on it, it held vacuum. I'm going to recheck it today.
    Maybe I should disconnect it from the door mechanism and see if the door mechanism is sticking. I'll try to take a pic of the green/white wire I'm talking about.
    By the way, where does the thermal switch mount? Mine has a metal clasp that looks like it would mount on an a/c tube, but its too small in diameter to mount on them, so I have it clamped to the compressor bracket.
    If I was to attempt to recharge, shouldn't I try to find the leak first? I guess I need freon in the system to do that. Do you know of any youtube videos that explain the process? Any suggestions where to get the gauges/freon. I have the R 12.

    Also, my spring on the plenum door looks much lighter than yours. My car was in pieces when I bought it. I had boxes of parts, filled up my garage. Took 2 years to put it together using the gm manuals. so, I may not have the right tension spring. mine is very fine wire.
    Again, thanks for your help. I'll post what I find.
     
  10. 72SS9681

    72SS9681 New Member

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    Had a chance to test my car this morning. I was able to confirm what I told you before, with the engine off the diaphragm is in the non activated position. As soon as the engine is started with A/C off the diaphragm moves to the activated position. When the A/C is turned on the diaphragm moves back to the non activated position and the other doors become operational. I tried moving the door by hand and it is very difficult. The spring is very stiff I was surprised the diaphragm is able to overcome the tension.

    I didn't any see green wires with a white strip near the A/C box. The only green wire near the box is a solid green wire going to my temperature sender on the right head.

    My thermal switch is clipped to one of the hoses going to the vacuum canister. This is where the assembly manual showed it should be mounted.

    As for the recharge I would determine if the system has any charge or not. You can simply take a screw driver and depress one of the schrader valves and see if there's any pressure. If there's none I would probably pull the system apart and replace the o-rings and seals. Before charging you'll need to pull a vacuum on the system if it holds in most cases your go to go, however some systems only leak under positive pressure. If there is still pressure in the system I could be that its leaking from the compressor seal. The original Fridgidare A6 compressors will leak through there shaft seals if there not run periodically. With mine It only leaks during the winter probably because of the R134a and also lack of use. My system works well with the R134a but if I had it to do again I would stick to R12. I had a lot less problems before.
    I recently pieced a Ferrari 308 A/C system back together for my brother and ran across a good book for the novice A/C mechanic. Its called "Just Needs a Recharge" by Rob Siegel and I got mine on amazon. Its a great book for someone like you that wants to do it yourself.

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