" /> Bill's Corner: August 2016 Archives

« July 2016 | Main | September 2016 »

August 8, 2016

1969 Mark III AC Questions & Follow Up

Hi Bill,

I wanted to wait a while before getting back to you to make sure the HVAC system was working properly. Sorry for the length of this email.

I took your advice and had the high-range relay re-built at Lincoln Land and it restored proper function of the blower motor and it now works on each of the four settings! I'm very happy! I do have a couple of concerns related to the operation of the system though at this point.

The AC does work, but I'm not sure it's as cold as it should be. When I first charged the system, I was getting cold air out of the registers at approximately 38 degrees F. Now, I'm luck to get air between 42 and 45, and only while driving steady for a long stretch. When I slow down or come to a red light, the air temperature at the registers jumps to about 60 degrees. I should mention that I have used R-12 and used the proper amount of refrigerant. If memory serves, 36 ounces?

Now, I understand the basic concept of the AC system and how it works so I know that on LOW, it draws outside air and that the air in the engine compartment is very hot and I'm sure that has a great effect on the output temperature. But on HIGH, isn't the air intake closed to outside air and recirculates interior air? I would imagine that on this setting, the air coming out of the vents should be "ice" cold as the evaporator is further cooling already "cool" air. Or is it simply that the compressor is that inefficient that it only works very well at higher engine speeds? I know I can't compare it to a new vehicle's system, but I would at least expect output temperatures cooler than 60 degrees at an idle.

I can only speculate that the main function of the ATC box is to maintain a comfortable air temperature inside the cabin and that maybe the box adjusts the temperature blend door and what I'm getting is a mix of hot (heater core) and cool air. I know there is an adjustment on the bottom of the box but does this make the blend door favor more cold or hot air (respectively), or does it simply keep the blower motor at a higher speed for a longer period of time to essentially provide cool air longer before it reduces speed?

Lastly, with the system set to DEFOG, I am getting air out of the floor, right defroster nozzle and only a SLIGHT amount of air from the left nozzle. However on DEICE, I get equal air from both dash nozzles. Is this proper function and if not, what would cause this?

Thanks for your continued help. Your advice and knowledge is second to none!




Hi Dan

From the many questions it sounds like you have some shop manual reading along with much diagnosing to do. The refrigeration section and the control section of the Climate control each have many areas that must be in good working order to cool a vehicle properly.

If the refrigeration section has in fact deteriorated since you charged it with the r12 it can be due to several factors. Along with increased ambient temperatures some of them are:

* The r12 could be leaking and slowly lowering the cooling capacity.

* The cooling fan clutch may be defective and not engaging properly to pull plenty of air through the condenser at low speeds. Possible incorrect fan blade.

* The expansion valve could be faulty.

On the control portion the issues could be:

* The water valve may not be closing when required due to no vacuum at the valve when needed or it could be operating properly on the outside but not sealing on the inside ( must be removed for visual inspection on the inside and bench tested to be sure ).

* Rear vent at rear window package tray or left cowl floor vent may not be closing.

* Control head could be defective.

Any missing ductwork or blower air leakage at any ducting outside under the hood at the firewall or inside under the dash will result in less air being delivered to the inside passenger area. The left a/c outlet is well known to disconnect on many Marks of this era. On High the air supply will be on Recirc. for most of the maximum cooling if the ATC box and the control head are functioning. Do not adjust the control at the ATC box as it is only for minor adjustments for customer preference. Adjustments here will not cure the real issue or increase the maximum cooling performance unless it was already turned far away from its center position. For your deice/defog problem you need to inspect that vacuum motor under the dash for vacuum at the two ports that are on this motor. For deice no vacuum should be present. The vacuum motors can also be tested with a hand held vacuum pump. The shop manual has a very helpful chart that shows the vacuum as directed from the ATC box to the vacuum motors for the various functions. For all we know vacuum lines under the dash could be crossed along with some unknown repairs by previous owners or mechanics without using the proper Shop Manual. We have seen many cars in for repair with misarranged a/c and heater parts. The above advice is based on a portion of the past problems that we have found here at Lincoln Land. When you have done some inspection on some of the above items let us know what is found. We can provide further advice depending on what is uncovered if necessary. We wish you the best of luck with the HVAC diagnosis and a speedy repair.




Hi Bill,

Thanks for all the helpful diagnosing tips. It's been so hot here lately, I can't really do too much diagnosing on the freon side of things. Ambient temps have been well above 85 here, but the system appears to be fully charged.

On a hunch I went ahead and bypassed the water valve to cut off any and all circulation of coolant through the heater core. While driving the vehicle after doing this, the air outlet temperature dropped significantly which tells me that I am having a problem with the water valve.

When I bench tested the valve, it seems to be sealing completely with vacuum applied but I am not sure I'm able to apply enough pressure through the tubes to accurately test this. Stupid question, but at what pressure is coolant flowing through the system with the engine running and how does excessive pressure not build up with the water valve completely closed? I would imagine some coolant should be allowed to bleed by to avoid build-up of pressure.

I've read in the ACC System handbook that with the control lever in the OFF position that NO vacuum should be applied to the water valve. However, in my shop manual, it says that vacuum SHOULD be applied with the system switched OFF. I am currently getting no vacuum to the water valve with the system switched off. I only get a vacuum reading with the system in HIGH and LOW (temp lever at 65). My question is where does this vacuum come from with the lever in OFF as there is only one vacuum line to the water valve and that is from the ACC box. Am I to assume that my box is defective if it is not supplying vacuum to the water valve with the system OFF?

Lastly, should all of the vacuum readings leaving the ATC box be lower than the source coming in? From what I can see, these vacuum motors need 10" to operate but on my AC/Heat door motor, I'm getting readings as low as 7" HG. When I operate this door motor with a vacuum pump, it is holding vacuum so I know the motor is ok. My source vacuum is between 15" and 17" going into the box.

There is also this port sticking out of the plenum just next to or below the AC/Heat door motor which air comes out of all the time. It is roughly 7/8"-1" in diameter and no matter what setting the control head in on, air is always coming out of this port. I can't find on any diagram what this may be.

Thanks Bill, I hope to get this system working properly soon enough. I'm very close, I can feel it!



Hi Daniel -
To answer your recent questions. I have no idea what the coolant pressure is at the water valve and really have no reason to find out. When I think of that it seems to be similar to a water tap in a house. When it is turned off the pressure and flow only increases at other areas that the water line is routed to and no harm is done. There is NO provision to allow some flow to continue through the water valve in order to reduce built up pressure as you state. Some coolant may bleed or leak through on some worn or even new valves. Defective valves that require replacing have large leaks or do not close at all. In any case coolant pressure at the water valve is not adjustable, will vary with engine speed and is a none issue in my opinion. The water valve either operates "as designed" or if not it is deemed faulty and replaced. The radiator cap will of course relieve excess system pressure if necessary. You state that the outlet temperature dropped significantly when you bypassed the water valve. What exactly was the before and after temperatures at the center duct. The possibility also exists that the blend door inside the plenum may have lost its sealing material and is allowing some air to flow inside the heater core area when it shouldn't if the ATC is in the full cooling mode. The sealing and insulating materials have long decomposed on many of our collector vehicles after all of these years. The inside duct area and damper doors are very difficult areas to inspect and renew if the seals have in fact disintegrated and are causing decreased cooling performance. If some of the above is actually happening ( water valve not sealing completely and/or plenum seals decomposing ) then bypassing the heater core as you have experimented with will improve the cabin cooling somewhat I would imagine. Of course the original system does not include any design to automatically bypass the heater core in any mode. Some improvising and special underhood plumbing would be required.
You also have not stated any refrigeration pressures within the system. These are important pressures to be aware of in a properly charged and correctly operating air conditioner. Did you record your pressures in a warm ambient for reference when you recharged the system? One other item to consider is the deicing switch at the evaporator. It must cycle the compressor as designed to avoid ice build up in the evaporator and remain open only long enough to deice before re-engaging the compressor.
The FoMoCo shop manual is incorrect regarding vacuum to the water valve in the OFF position of the control. There is no vacuum to the water valve in the Off position and the valve will be fully open.
I would expect that the vacuum exiting the ATC box to the various vacuum motors should be the same or very close to the vacuum reading of the supply line entering at the box inlet port. There could possibly be a component leak or a vacuum hose leak within the box. I have repaired loose and defective hoses etc. within the boxes but have never taken a box apart to only repair a drop in vacuum reading. Of course and as a rule no vacuum leaks are allowed.
From your description the air leak under the dash sounds like part of the aspirator for the "in car sensor" in the dash above and to the left of the glove box. Its operation should be shown and explained in one of your manuals. The air escaping under the dash is necessary and part of the venturi that draws air past the dash sensor via a vinyl hose.