Couple of questions on AC in a 68 - Vintage Mustang Forums
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post #1 of 17 (permalink) Old 08-17-2019, 08:53 PM Thread Starter
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Couple of questions on AC in a 68

Got a 68 6 cyl which had factory AC. New aftermarket compressor and condenser. Uses all the original controls which I restored 6 years ago. At that time it worked like expected but never got as cold as I think it should. I even put on a fan shroud which it never had but that didn't make much difference.

I'm back playing with it again and the AC was still behaving just like it always had. I bought one of those Arctic Air cans of 134A or whatever. The gauge showed it was at the very bottom of the blue range which is "Filled" so I shot in some more to get it to just under the top of the blue range. It feels much colder now but I'm seeing an intermittent problem where the compressor is not coming on when I think it should. In particular sometimes if I have the controls on MAX and the temp control lever on Cool the compressor is not on but then I can move the control to FRESH and the compressor kicks on and usually I can move the controls back to MAX and it stays on.

When the compressor is not on I can check that there is no power to the clutch and I can jump a power lead to the clutch and it comes on so somewhere prior to the clutch it's not getting power. Looking at the schematics I see 2 controls that look like they have to do with the clutch power. They are the AC Thermostat Switch and before that the AC Control Switch. I'm not sure where those are located or how to test them out. Are either of them vacuum controlled some way?

I'm also thinking about wiring up another power supply to the clutch. I was thinking of one of those little toggle switches with an LED light. Most have 3 poles. A negative, a positive in, and a positive out to whatever you are controlling. Would it hurt to wire one of those like that if the clutch is sometimes powered correctly and sometimes not? I wonder if the LED light will be on if the switch is OFF but the clutch has power to it from the normal source. I like the idea of an LED light showing me the compressor is on. Also this would provide the ability to have the compressor on when in defrost mode which I don't think it is designed to be.

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post #2 of 17 (permalink) Old 08-17-2019, 09:00 PM
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It should have a pressure switch that will prevent it from coming on. Did you have the system evacuated and filled to the recommended refrigerant level?

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post #3 of 17 (permalink) Old 08-17-2019, 10:17 PM Thread Starter
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When installed it was a new after market compressor, condenser, receiver etc. I think it was brand Olde Aire. The only parts from the original system used was the evaporator and controls (vacuum motors, cables etc). I had the evaporator checked by a radiator shop and it is good. A friend filled the system and soon after I put it in the shop and they said it was filled correctly. That was 6 years ago. They were the ones that suggested a shroud might help.

As I said I shot some more freon in yesterday from ones of those Artic Air cans that has a gauge. The gauge now shows it is in the Filled range about 10% from the top of that range. Could I have filled it too full? Is this the pressure switch? If so I don't suspect it because when there is no power to the clutch I checked that there is no power into that switch so it must be not coming from somewhere further back.
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post #4 of 17 (permalink) Old 08-17-2019, 10:21 PM
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Yes that's the pressure switch. Those generic gauges are worth about half what you pay for them. You'll need to do some diagnostics and see where you're losing power. But if the pressure switch has no power on either wire then it has to be from there to the switch inside. Break out your test light and find where the power stops.

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post #5 of 17 (permalink) Old 08-17-2019, 10:34 PM Thread Starter
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OK. Looking at the schematic the next thing to check going back I think is the A/C Control Switch. It has 2 prongs. I'm not sure where to find it or what it does. (Unless that is the original system pressure switch?) . I haven't found any description of it yet in the Shop Manual. Going back from there the next thing is the A/C Thermostat Switch. I did find a write up about it and how to find it behind the glove box. I can get in there to test it the next time the compressor is not on. If I have power in and out of it then I'll have to find that A/C Control Switch.
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post #6 of 17 (permalink) Old 08-18-2019, 09:04 AM Thread Starter
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Here is that compressor control switch. Is it vacuum operated? I got to get under there to find it. The way it's acting I'm guessing it isn't getting vacuum at those times the compressor is not kicking on. If that's the case the problem might be in that rotary control switch which I'm pretty sure isn't reproduced and very pricey for a good used one.

Looking closer at the vacuum diagrams it appears that amber colored round thing at the end of the switch is not a vacuum port but a push in button. It looks to be controlled by an arm that swings down on it when either the A/C Heat or A/C Defrost door is engaged by it's vacuum motor.
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post #7 of 17 (permalink) Old 08-18-2019, 09:51 AM
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My '68 A/C does not have a pressure switch. I don't believe they had one in the stock configuration. There is a thermostatic switch as shown on the attached at the top front of the HVAC box. IIRC, you can easily get to it by taking the glove box liner out. This switch will turn of power to the clutch if it gets to cold in the HVAC box to prevent the evaporator from icing up. So, that switch could be bad.


Once you get your switches sorted, I would take it to a good A/C shop to do the work of properly charging the system. I have never had any luck with those cans of refrigerant on daily drivers.
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post #8 of 17 (permalink) Old 08-18-2019, 10:03 AM
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Ok, on a 68 the power comes from the ignition switch through a Circuit breaker to the A/C control switch. The CB is on a little plate mounted to the Fuze box. The Control switch is attached to the A/C controls on the left side of the dash. When you slide the control lever to a position that turns on AC the switch provides power to the A/C system and vacuum is routed to the vacuum controls to open up the correct air source. This is the same power fed for the fan switch so if you can turn on the fan then you have power to the controls.

When the vacuum rotates one of the doors in the A/C housing a lever presses on the button of the switch in the pic posted above. The switch in turn routes power to the thermostatic switch. This switch controls cycling of the clutch to prevent the system from freezing up. As long at the temps are within acceptable limits the thermostat allows power to go to the clutch turning on the compressor.

Originally the system did not have a pressure switch. When your system was redone with a new condenser as binary switch was added. It should be connected between the thermostat and the clutch. It prevents the clutch from getting power if the system pressure is too high or two low. It doesn’t control the system at all. It is just a safety switch to prevent damage to the compressor.

To properly test the system you need to get an A/C test manifold. You can borrow them for free from your local auto parts store. It connects to the service ports on the system and will tell you the high and low pressure in the system. Comparing these values to a chart will tell you if the system has too low or too high a coolant level. It can also indicate issues like a restriction in flow or a bad evaporator valve.

It is important to remember that your system was designed for R12 and when updated to R134 may not run as efficiently. Classic Auto Air has improved design condensers and evaporator comes that we’re redesigned for R134 which may help.

When moving to R134 you have to also replace all the hoses with Barrier Hoses as the R12 hoses will leak with R134.

I feel your pain as I just installed a completely restored A/C in my 68 but have no power to the clutch. I’ll be doing diagnostics next week to track down the problem.

For you I would look at you vacuum controls. There is a vacuum switch behind the A/C control. You could also have a leak on a vacuum line, in a vacuum switch, or the vacuum canister. The check valve could be leaking as well. Also confirm the hoses are routed to the control properly. Is am guessing the switch is not getting depressed correctly when you go to max thus cutting power to the thermostatic switch.

One other important consideration is the water cut off switch. When you turn on the A/C the heater water must be shut off. If not your A/C won’t get as cold as it should. This vacuum controlled switch is often bad or missing. I have seen many with vacuum leaks.
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post #9 of 17 (permalink) Old 08-18-2019, 11:30 AM Thread Starter
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Well I went out and started it this morning. Turned on A/C to MAX and the compressor did not come on. So I got under the dash and I remember now how hard it is to get to that switch. But while reaching up there to try feel it the button was pushed in the compressor kicks on. I don't think any vacuum operated doors moved so I suspect I have a loose connection to the switch. Maybe a bad switch. Not sure how I'm going to tackle it yet but I think I have a handle on where the problem is.

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post #10 of 17 (permalink) Old 08-18-2019, 11:53 AM
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The A/C control switch is located on the driver's side of the dash vent plenum (above the ashtray) and is mechanical. The vacuum which opens and closes the dash vent door also moves a lever which in turn presses or releases the plunger in the A/C control switch. Check to make sure the switch is mounted firmly and the two electrical wires are firmly attached. Essentially the switch is either "on" or "off" and continuity can be confirmed with a standard DVM. It must be "on" for the compressor to receive power. One component you have not mentioned yet is the expansion valve, which is plumbed into the A/C hose located on the engine side of the firewall. If it is gunked up or otherwise not functioning correctly, the pressure within the system may be out of spec. Not sure how to test for that. Perhaps someone wiser than me can chime in on that point.

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post #11 of 17 (permalink) Old 08-19-2019, 11:01 PM Thread Starter
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I'm a little confused on which air door has the arm that actuates the control switch. I think it is the A/C Defrost door. The attached pic does not show the arm or the switch but it does show when the clutch is ON (by A/C Defrost Door Arm). It is also interesting to note that the attached pic does not jive with the same chart on page 16-15 of the Shop Manual. In that book, when the functional control lever is in positions MAX, FRESH or OFF it shows the A/C Defrost Door has vacuum but the attached pic shows it does not. I verified it does not have vacuum in those situations.
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post #12 of 17 (permalink) Old 08-20-2019, 09:37 AM
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I'm a little confused on which air door has the arm that actuates the control switch. I think it is the A/C Defrost door. The attached pic does not show the arm or the switch but it does show when the clutch is ON (by A/C Defrost Door Arm). It is also interesting to note that the attached pic does not jive with the same chart on page 16-15 of the Shop Manual. In that book, when the functional control lever is in positions MAX, FRESH or OFF it shows the A/C Defrost Door has vacuum but the attached pic shows it does not. I verified it does not have vacuum in those situations.
Nice detective work! When I replaced all the vacuum hoses and vacuum motors under my dash, I used the attached diagram as I did not have a shop manual at that time. Now, I am glad I did not!

I can't remember if the switch is on the AC/Defrost door or the AC/Heat door. My guess would be the AC/Heat door. But, those two vacuum motors are close to each other in back of and above the ashtray opening. When I did my under dash work, I had the seats and console out as well as the glove box, ashtray instrument cluster and of course the HVAC controls and the right hand register. I didn't do this initially to make this work easier but I was going over everything under the dash and knew I was going to replace the instrument bezel and lenses. Frankly, I do not know how I would have done it (especially putting in the new vacuum motors in question) without all this stuff removed. I also removed the left flexible duct to the left outboard register. Not sure if that was to better get to those two vacuum motors.

If you have any inkling that you may want to remove the instrument cluster in the future to work on it, do it now. Not sure if you have a console but that will really get in the way if it remains in place. If no console, taking out the radio will help. I know it is a lot f work but I had all winter and I am retired. I did buy about 18 feet of vacuum hose connected to the reserve tank inlet and used my Mercury Mountaineer as my vacuum source as I did not have a vacuum pump. It gave me a nice constant 15 hg. of vacuum for all the testing I did.

Good luck. The reward is that my whole HVAC system now works flawlessly.

Dave
'68 GTCS
8R01J
C4/PS/PB/AC
2/8/1968

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post #13 of 17 (permalink) Old 08-20-2019, 09:52 AM Thread Starter
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I went back over all the pics I took when I initially restored my system. In this pic you see the vacuum motor for the A/C Heat door on the left. The switch is on the component on the right and you can see the arm. It is on the A/C Defrost door. The vacuum motor for the A/C Defrost is not in this pic as it was shot and I had to replace that one.

I did have everything apart as you mentioned when I did this. Now I don't think I'll go that far. I am still considering adding a toggle switch with an LED light so I can manually turn on the clutch when I want and the light should show me when it is on.
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post #14 of 17 (permalink) Old 08-20-2019, 10:11 AM
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Originally Posted by Winch View Post
I went back over all the pics I took when I initially restored my system. In this pic you see the vacuum motor for the A/C Heat door on the left. The switch is on the component on the right and you can see the arm. It is on the A/C Defrost door. The vacuum motor for the A/C Defrost is not in this pic as it was shot and I had to replace that one.

I did have everything apart as you mentioned when I did this. Now I don't think I'll go that far. I am still considering adding a toggle switch with an LED light so I can manually turn on the clutch when I want and the light should show me when it is on.
Got it. You can certainly do as suggested. I would first however, jump the two wires going to that switch to make sure that solves the problem. You never know what evil may be lurking in the actual wiring with the switch not being the issue.

Dave
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post #15 of 17 (permalink) Old 08-20-2019, 11:58 AM
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If you bypass the A/C switch make sure you still run power through the thermostatic switch. This prevents the system from freezing up and provides protection to the compressor.

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