Brian Scudder wrote to the list:

>> My wife would like to drive my '87 (and I want her to), but she refuses until the AC is fixed. The Freon (R12) checks out fine about 300 psi on the high side and 40psi on the low side. The problem is that after 20 minutes or so the system seems to shut itself off. The air blows warm and the return vent closes up. After maybe another 20 minutes it turns itself back on. And so on...
>> How should I go about debugging this system?


Symptoms sound a lot like the anti-freeze switch is stuck closed. This is a thermostatic switch designed to keep the evaporator temp above about 35-40F.
and therefore avoid getting a bunch of ice formed on the evaporator. As your symptoms indicate, the system works great, until moisture in the incoming air is condensed then frozen on the fins and tubes of the evaporator. When the switch is working correctly, as the temperature gets down to 35 or so, the switch opens, and current to the AC clutch is interrupted until things warm up just a bit. So no ice will form, and things stay fine. Switch stuck closed, ice forms and eventually blocks airflow through the evaporator. This is the "vent shuts off" symptom you describe.

You can do a quick-and-dirty function test by turning the AC off for a few minutes as soon as you notice that the airflow is reducing and/or temp is increasing. I get a little musty smell too as the ice block melts and drains. After that few minutes, you can turn the AC back on and go for another while before you have to do it again. Living in the coastal desert areas of southern California, I can go over an hour without thinking about the switch. In Austin, with the higher humidity, it obviously ices up really quickly.

The fix:

The switch itself is in an aluminum box that lives under the hood, and under the rain shield at the top near the wipers. It's almost dead center in there. Open the hood, pull the rubber molding that holds the front of the rain shield down, and carefully extract the plastic shield. Look for that aluminum box in the center, just immediately to the rear of the firewall panel that the rubber trim was sitting on. The box has two wire connections on it, and a thin capillary tube that runs from the little box towards the driver's side, then down through a hole in the panel into the evaporator space. On my '89, there's a yellow insulating sleeve that covers some of the exposed tubing. There's also a little clamp that holds the tubing right where it passes through the hole.

Once removed, the switch can be "tested" with a glass of ice water, a thermometer, and an ohm meter. Put cool water in the glass, stick the end of the capillary tube and the thermometer in there, and attach the meter to the switch terminals. It should be 0 ohms, indicating continuity in the circuit through the switch. Now add the ice, and watch the thermometer as the water cools down towards freezing. Between 35 and 40 degrees, the switch should open (infinite ohms, no continuity through the switch circuit). I'll speculate that yours doesn't open. At this point, if it doesn't open, you may decide to open the switch housing and see if you can adjust the setting on the switch. You have nothing to lose at this point since you were going to get a replacement anyway. You may be able to adjust the switch contacts so they open correctly. This is very delicate stuff.
Once you have totally screwed up the little parts in there, you can get on the phone to Jim at 928 International and get a new switch on the UPS truck headed for your house. Installation of the new switch is exactly the reverse of disassembly, keeping in mind that the capillary tube can't be kinked or 'reformed' too often.

Hope his helps! I have pictures in my archives somewhere that show the switch location if my description wasn't clear enough. let me know and I'll dig them out for you.

dr bob


>On our way back from SITM my HVAC blower was not blowing very well. It was hot and very humid driving to GA.
>I turned on the rear air and soon my son and I were being pelted by by ice chips followed by very cold water. Quite refreshing.
>1. I thought there was a freeze switch that should prevent ice up.
>2. Why did it freeze up and what can I do to prevent it doing so in the future? I recently recharged the R12 after fixing a leaky low pressure switch.
>Best regards, Kevin
There is an anti-freeze switch on the front evaporator, but not on the rear.

Freeze-up can be caused by:
- Faulty anti-freeze switch. It can be replaced without losing the refrigerant.
- Faulty installation of the anti-freeze switch - the temp probe must be in intimate contact with the evaporator, so that it can sense the evaporator temp.

Some (not all) anti-freeze switches are adjustable. These have a screw slot inside a locking nut. You use ice and water to set the switch.

Wally Plumley
928 Specialists




I've seemingly forgotten--what exactly is the logic that determines whether the climate control blower draws interior or exterior air? I've suffered from evaporator icing any time I've been on the road for more than a couple hours at a time this summer (refrigerant pressures are spot-on according to the shop manual), and I was surprised to discover when I got home last night that it was drawing exterior air. But after a few seconds of idling in the driveway, the recirc actuator was activated and it switched to interior air, so the mechanical bits seem to be working. I was always under the impression that with the aircon running it would always be in recirc mode. (Doesn't the second set of contacts in the compressor relay energize that actuator directly??)

Joe Elliott
'88 S4

The recirc flap is activated based on the temp mixer. You can reach under the dash above your right knee and find the electric mixer motor. There is an arm on the right side that moves the mixer flap up and down. Full down = full cold. Recirculation occurs when the mixer is within 10% of full cold.

To test things - set the a/c temp to full cold. The mixer arm should go all the way down. Then move the temp to full hot and the mixer arm should move all the way up. When the arm is at 10% of full cold the recirc flap should activate. Putting the temp in the middle should eventually have the arm somewhere in the middle once the temp regulates.

A problem I have is that my recirc flap is not being activated to open. It might be a bad temp sensor (the temp does not regulate - only works full hot or cold) but may be the head unit. Rather than get into all the money I wired a switch to the recirc flap solenoid on the vacuum console so I can manually control it.

John Pirtle
87 a/t 245k

A properly-functioning anti-icing switch will cut the compressor off when the evaporator gets to 32-33 degrees F, back on at 33-34 degrees F.
There should be no ice accumulation.

Some early switches were adjustable - there was actually an adjusting screw on the side of the switch, with a locknut. You can remove the switch without losing any refrigerant - there is no connection to the refrigerant. The sensing bulb is inside the insulation on the evaporator outlet pipe, or in contact with the evaporator, inserted into the fins.
You can check it by removing it, then checking continuity on the switch (should be closed at room temp), then see if it opens when you put the sensing bulb into ice water.

If the sensing bulb is not in good contact with the evaporator or pipe, it will not sense the temperature correctly.