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.

[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.

[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]

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!