By on August 16, 2019

TTAC Commentator Stefan writes:

Sajeev, my old triple-black 1997 Town Car has developed some air conditioning problems and my trusted independent shop seems unable to duplicate it nor to understand it. Maybe you and the B/B can help?

The a/c always works when starting up the car but will abruptly cut out after a while, almost as if the clutch disengaged. This happens even when the system is fully charged. I was told by the technician to keep the fan blowing full speed but this merely delays the inevitable cutting out. There does not seem to be a refrigerant leak. Not being an air conditioning expert by any means, I am at a loss on how to proceed.

The car has only 170,000 miles on the meter and should be good for another 100,000. Not wanting to spend big bucks on replacing the entire system, where to start?

Sajeev answers:

This is a serious problem: please, quickly give me this fine dangerous Panther so that 1995-97 Town Car Fat Panther deliciousness can be mine! ALL MINE!

Okay, seriously: it could be incorrect refrigerant pressures, or possibly a blocked orifice tube. And while things like the A/C cycling switch and, in some vehicles, a Variable Control Relay Module can turn off the A/C Compressor’s clutch as you stated, the electronic bits might not be broken: they might be getting misleading information!

More to the point, the switching mechanism is freaking out because the evaporator froze, likely due to lack of airflow. Hence why you running the fan on MAX helped, somewhat.

What’s true for home HVAC applies here, as I noticed in 2009 when I ripped apart the dash to rebuild the HVAC (both hot and cold bits) in my 1988 Cougar. The Cougar was about as old (both age and mileage) as your Townie is now, hence why I am torturing you with the following story: this job was a nightmare because of personal/career issues, and because I was working in the mosquito-infested space you see below whenever I was lucid enough to turn a wrench!

1988 Mercury Cougar Dashboard removal heater core HVAC, Image: Sajeev Mehta

And no, that blurry spot isn’t a fingerprint on the lens — it’s one of many bitter tears secreted when removing the HVAC box in a non-Mustang, non-Lincoln Fox Chassis vehicle that you just had to upgrade with nearly every factory option from donor cars in the junkyard. Because plug-and-play wiring (i.e. why we had a la carte options back then) was so damn easy to install, and you’re never gonna remove it and subsequently regret your decision, right?

Back on point: the heater core was my main issue, but I replaced the fan motor and evaporator.  What I saw when removing the old evaporator was shocking: it was packed with dirt and leaves, apparently becoming a muddy mess over time. And that was likely due to the evaporator’s freezing/melting due to a lack of airflow.

Sure enough, the rebuilt HVAC box spewed painfully cold air during several blistering hot Houston summers. (Might need another clean now!)

I hope your evaporator isn’t as clogged as mine (was), but this job on a 1997 Town Car is likely much easier. And if A/C cleaners and smartphone linked endoscopes work as intended, you can inspect and then clean your evaporator without removing the dash.

Or just yank the dash: you know, for fun! It totally builds character, Son!

Send your queries to [email protected]com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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31 Comments on “Piston Slap: Hot Air on Frozen A/C Evaporators?...”

  • avatar

    evaporator coils don’t freeze because of lack of airflow, they freeze when the suction pressure in the circuit drops too low. either because of low refrigerant levels or a restriction somewhere in the system where it shouldn’t be. Filter/dryers are a frequent culprit.

    • 0 avatar

      In my experience yes they can freeze from lack of airflow in high humidity. I haven’t experienced in a car yet but have seen it happen in home and boat systems I have worked on. Now If your running the fan on high and getting good airflow I might question that diagnosis. My experience with it always occurred when people set the fans to low and ran at low temps in high humidity.

      On this problem my 2001 Volvo did this and it was the clutch on the compressor and the Volvo’s were notorious for it, so it seems possible. You could also have an issue with the dampers My old Dakota had a vacuum leak that caused the AC to kick in and out for example (moving the heating and cooling damper as it saw and lost vacuum). I don’t know enough about those Ford HVAC systems to go further then that.

      • 0 avatar

        I had it happen once over the course of a very long drive in 100-degree, high-humidity weather in a nearly new 2006 Honda Civic. The system just wasn’t designed to be running for the length of time it did in the extreme conditions that existed that day.

        I initially assumed the compressor had failed, and parked the car at a rest stop to give myself some time in a cooler building. When I came back out to the car there was a giant puddle of water under the car, the A/C worked fine again, and the light clicked on in my head.

        • 0 avatar

          Freezing of the evaporator most certainly can be caused by a low charge. Check that first. Second, the lower the fan speed, the longer the moist air is in contact with the evap coil, which means it can do a better job of wringing out moisture. If you have the right conditions – slow airflow and very high humidity – which is likely in a car, you can freeze the coil. Evaps with close fins, again likely in a space-constrained car, can exacerbate this problem.

          If the charge is ok, on a low(er) humidity day try to keep the system in recirc, which will keep out the humid air from outside. This might eliminate the freezing of the coil. Lastly, is there a reduction of air velocity (and change of sound possibly) when you lose the cooling? If so the coil is likely frozen. If not are you sure the clutch is not losing its grip on the compressor? My Sable was similar – the A/C worked fine on startout but would lose its cooling after use. it would return to normal on next startup. Replacing the clutch/compressor took care of that….

        • 0 avatar

          My ’84 Jetta GLI used to literally blow snow out the vents under those sorts of conditions. Turning the heat on for a few minutes would fix it for a couple of hours on a trip, or as you did, just parking it for a bit.

  • avatar

    Hopefully he is dealing with an overfilled system. The dashboard of a Town Car might not be as bad as a Thunderbird’s, but this is still one of the worst jobs you can do on almost any car. Also, when these cars were still on the road around here, I used to see them with clicking blend door servo motors because their owners couldn’t see paying to have the dash apart. They preferred being driven insane instead.

  • avatar

    An evaporator that is clogged with debris could certainly cause the A/C system to malfunction.
    Some more details about the problem could help in diagnosis. Does the air handler fan keep blowing? But it’s now ambient temp air? Is the compressor clutch engaged and not slipping? Are there any excess noises coming from the compressor?
    Using a gauge manifold for the system will indicate quickly if the orifice tube is restricted.
    Stating that the A/C “stops working” is like “The car WON’T START”.

    • 0 avatar

      pwrwrench, for us A/C dummies, what do we watch for on the gauges to determine a restricted orifice tube?

      • 0 avatar

        Low suction side pressure. High High side pressure. On my Mopar products seeing under 25 on the suction side is an issue either low refrigerant (which I assume the shop checked) or a clog etc.

        • 0 avatar

          But Chrysler products don’t have an orifice tube, they have a thermal expansion valve. So they should stay at that ~25 psi. They don’t have to be clogged to go bad, sometimes, much more frequently on Chrysler products than others they just go bad.

          • 0 avatar

            Not an expert on auto AC but I did work on marine systems as a mechanic. Just my experience with my own cars. In my cars (mostly mopar) with low refrigerant I will see somewhere between 20-25 on the low side, When the rear AC expansion valve on the wife’s durango clogged it did drop into the teens.

          • 0 avatar

            I’m just saying with a TXV they can just go and fail to work properly even if they aren’t clogged. I’ve seen them fail, ie stick, and have zero signs of any debris.

          • 0 avatar

            OK sorry. Yeah I agree the added complexity of the valve vs orifice may not be worth it. Of course in Theory you get better performance and efficiency with the valve.

          • 0 avatar

            The orifice tube style is more efficient, from a mpg stand point but definitely provides less consistent output, than a TXV which when working properly matches the flow to the demand and maintains a tight pressure range.

          • 0 avatar

            On the efficiency this comes from my experience with Marine refrigeration. The guys that built valve systems claim greater efficiency as they stay on target more and can reduce time in the on cycle. (basically they run less often). Other manf would generally claim orifice tube was simpler and the efficiency gain was negligible. As some one fixing it the orifice the cut down on trouble shooting, but in new installs the valve handled more variation in the system with custom length lines etc.

          • 0 avatar

            Well on the automotive systems the orifice tube design allows the compressor to cycle off while on most TXV systems the compressor will run continuously.

          • 0 avatar

            Interesting. I know the compressor in both my Durango and old Dakota cycle and have expansion valves.

      • 0 avatar

        mopar4wd is correct. Also feel the high side line/hose from the condenser to the evaporator in the dash. If you find a spot where it changes from hot to cold, while the system is running, and that is not the orifice tube or expansion valve then there is a clog/restriction at that point.
        If there is that kind of problem it might be what A/C techs call black tar syndrome. This usually comes from failure of the compressor and the debris goes downstream. Fix is minimum; replace compressor, condenser, high side lines and accumulator/drier. Otherwise the gunk will work its way through the system clogging things and wrecking a new compressor.

  • avatar

    ugh… had a clogged evaporator on my ’91 Ranger… stuffed with Live Oak leaves that had fallen into the cowl… once they started burning (while doing 70 on the interstate), massive amounts of smoke & ash came pouring from the vents.

  • avatar

    I would check the orifice tube/valve if the system is not holding a vacuum. I just had this exact same problem on a 1991 Mercury Colony Park. I had replaced almost all of the A/C components last fall, including lines, condenser, evaporator, orifice (a $5 part), etc. Everything worked fine, pulled it out in the spring and had the exact problems you describe with the A/C system. Not being able to properly diagnose I took the car to an A/C specialist. They were baffled until finally checking the orifice and finding that the new part I had installed was defective. A $5 part, a $300 labor bill, and the A/C can now turn this vintage station wagon into a meat locker.

  • avatar

    One possibility is that the compressor cycling switch (if so equipped) is defective and causing the compressor to run continuously. This causes the evaporator temp to drop below freezing and get blocked with ice.

    A pressure gauge on the low side will show this. With a light load (low blower speed, moderate ambient temp) the compressor should run until the pressure drops to around 22psi, stop until the pressure rises to about 44psi, then repeat.

  • avatar

    So has anyone run the EATC self diagnostics? Both on start up and once it stops working? To do so press off and floor at the same time, release and then press Automatic within 2 seconds. That will do a test of the screen and then give codes if present.

    Far and away the most common problem with 80’s and 90’s Ford AC systems is the Nipondenso sourced clutch coil. When they get hot the windings inside either open or short and the clutch doesn’t engage. You used to be able to get the coil separately, from Ford, but for some applications you must buy the entire clutch assembly while others are only sold with the compressor.

    The problem is directly tied to underhood temps. So if it is a bad clutch immediately hoping on the freeway and cruising at 60 it may stay on indefinitely or take a very long time to stop working. At the other end driving in stop and go traffic or just idling and it will stop working much much quicker.

    A blocked orifice tube will not cause icing, it will cause weak performance as the evaporator will not have enough volume of liquid refrigerant to actually freeze things. You’ll also notice it when watching the pressures. The low side pressure will drop quickly if it is restricted and it will do that from start up, not just give a problem after running for a while.

  • avatar

    The a/c working fine and then the compressor not engaging after the car gets hot is a classic sign that you need to check the air gap on the compressor. Google “ford ac compressor air gap” and watch a video. Simple fix / adjustment.

  • avatar

    I have a 2013 Escape that has been having issues with the ac for the past 2 years (beginning at 60,000 miles) and nobody could find out what the issue was, so I suffered. The ac worked fine on the highway and after the initial start while I was stopped at a light or in traffic. After the car was running for a while it would still work while the vehicle was moving, but it wouldn’t blow cold at a light or in bumper to bumper traffic. I did find that if I revved the engine in park to 3,000 rpm it would blow cold ac. However, a few weeks ago I had the engine coolant flushed and replaced, and now the ac blows cold all the time. Maybe that could be your issue? Just a thought.

  • avatar

    The picture of that disassembled dash will give me nightmares for years. I like to wrench on cars as a hobby, but that’s one where I would waive the white flag.

  • avatar

    I love this body style Towncar and have a huge soft spot for it, I remember riding in my grandfathers when I was younger. Pure class, luxury in spades. While they are over 25 years old now I don’t remember them having stellar reliability when newer, typical Ford reliability. I would love a new one though.

  • avatar

    My first thought is that the AC compressor clutch is failing. You can diagnose this two ways. One is to look at it while the engine is running and the AC set to ‘max’. You should see the compressor clutch pull against the pulley and start to spin. If this doesn’t happen in a 2 or 3 minute time frame, then you might want to look into the relay for the compressor clutch. This year of Town Car probably has an individual relay in a power distribution box, but it was common for Ford vehicles to have a relay module in which the relays are soldered onto a circuit board. If the compressor clutch is not cycling on then you need to diagnose the relay. The second way to diagnose the clutch is to measure the air gap while the engine is off. The gap between the clutch and pulley should not exceed .030″. The compressor is probably the FS-10 compressor, which was very common in a lot of vehicles, and you can replace the clutch without replacing the entire AC compressor or having to open up the AC lines. You’ll need a special tool, which will likely not fit, if your compressor clutch has 6 holes in the clutch plate – you’ll have to use a lathe to reduce the diameter of the three cylinders to fit the holes of the compressor, or have a machine shop do that. You’ll also need external snap ring pliers.

  • avatar

    Why did he need to know it’s “triple-black”?

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