StratfordShark

I first raised my aircon issues here:

https://rennlist.com/forums/928-foru...-then-hot.html

Briefly the aircon would work fine at first, then after about 10 minutes from first switch on it would very quickly go warm and feel as if there was less airflow too. If I turned aircon off it would work again when turned back on if left for a period - the aircon would then go warm again and it seemed like the longer I left it off the longer I had before the aircon went warm after turning back on.

Yesterday was hot for UK, and I drove 300 miles to attend a 928 gathering. the aircon problem persisted so i determined to research it thoroughly and look at it today.

After reading lots of threads it looked as if problem could be with my anti-freeze switch. My theory was it wasn't cutting off compressor when evap was almost freezing and so allowing the evaporator to freeze over.

Now here comes the baffling bit. I removed the switch tonight (first photo shows it in situ - connectors pulled off slightly so I could confirm 12V on each side with aircon switch on and ignition on). I was expecting to be able to test at what temp it switched - if at all - but there was no sensor tube! Where I expected tube to go into bulkhead the hole is filled by hard plastic.

Final pic shows the side of the switch which I expected to have tube sticking out. There's nothing and I can't see where tube would be in the first place!

Please can anyone with a deeper knowledge of the anti-freeze switch please solve this mystery for me. I am completely baffled!

Leon Speed

The tube is covered with yellow plastic, it's under the switch.

StratfordShark

Now I feel very stupid. I thought that was an earth wire and next question would have been why have it when the part is bolted to bracket? Doh.

So now I have to follow the yellow tube and withdraw it, then test the switch in ice/brine

Thanks for clearing up mystery!

WallyP

The tube is inserted into the evaporator fins - no direct contact with the refrigerant, so it can be removed/tested/replaced with no loss of refrigerant.

The manual details how to test the switch.

StratfordShark

Please can you point me towards the test details? I've gone over every page of Vol 4. I can see removal/install instructions for the switch but no test procedure,

Thanks

VehiGAZ

Be careful to note where the tube goes as you remove the switch - it is a very small hole and if you don't see it when you pull the tube out, you will have a lot of trouble finding it again.

Replace the switch. If it ever stuck, it will again.

Good luck!

Leon Speed

Hope you fix it. Then maybe you could help fix mine It won't blow cold air after sitting for 4 months during the winter. The air flow reduces when turning on the AC and I hear the typical hissing sound of an airco switched on, but no cold air. Reduced airflow and evap valve does get cold but not freezingly so. Hmm maybe just low on gas.

StratfordShark

Thanks Gaz. Once I withdrew the probe I marked the hole with a paper ring reinforcer so I have a clear target to aim at when re-installing!

StratfordShark

Well I removed the probe - it was inserted as far as a short piece of black tubing at junction of the straight probe and yellow covered wire so I presume it was in all the way.

Tonight I'll test it by seeing if contacts open when it's immersed in frozen water (with some salt to cool it below zero).

Just curious though about how does the freeze switch work? Does the heat at end of probe simply conduct all the way up the yellow wire? But then what - the wire is just soldered to switch case? I'm an old electronics hand but this has me stumped.

WallyP

The probe isn't electrical - it is a capillary tube that mechanically operates an electrical switch inside the switch body.

The test procedure isn't in the Factory Workshop Manual - it is in Service Training Manual P 800005, "Air-Conditioning Systems". This is one of the many manuals found in the Morehouse Tech Info CD set.

The appropriate info is:

"Two versions are used.
1 . Anti-icing switch with fixed setting
2. Adjustable anti-icing switch

The anti-icing switch is installed in the area of the evaporator housing or air duct.

Function
The sensor of the anti-icing switch is inserted between the evaporator fins and senses the temperature at the coldest location in the evaporator. The sensor, also called capillary tube, is filled with a medium and connected to a pressure cell. The pressure cell controls a switch which switches the compressor electro-clutch on and off. The switching point can be adjusted by means of an adjusting screw. A slotted screw is visible after removal of the cover plate. An arrow with the designation "cold" indicates the direction of rotation in which the switch switches at a lower temperature.
One turn corresponds to 37.4"F (3°C).

In the case of adjustable anti-icing switches, the switching point can be changed, thus permitting a certain amount of control of the air temperature. At the max. setting, the switching point is also off at 33.8°- 32°F (1° to 0°C) and on again at 37.4°F (3° C).

Maintenance
The sensor of the anti-icing switch must not leak or be kinked. The switching point can be checked by immersion of the sensor in ice water and connection of a pilot lamp. The switching
point must be at 33.8 to 32°F (1 to 0°C) at the max. setting."

The switches found on the 928 are the "non-adjustable" type - but may or may not be adjustable. If they have the slotted screw, they can be adjusted.

The "adjustable" switches actually have a cockpit-accessible **** to turn the adjustment screw.

StratfordShark

Thanks Wally that's exactly what I needed at exactly the right time - just come in from kitchen where I was testing the switch and was using iced water as the service instructions suggest (I have the wsm but not full Morehouse set).

I immersed the probe in a narrow glass with ice cubes and salt in the water as in the photo. Left it in for some time but the switch contacts never went open circuit. Aiming a little IR thermometer at surface of water it showed 1.2 C, so I think the probe must have been at zero.

I then put the whole switch in freezer (-18 C) and left it in for about 10 minutes. When I took out the switch had gone open circuit, and putting a meter on it it switched closed again after a few seconds with probe plunged in hot water.

So I'm wondering if the switch is not opening at correct temp and allowing evaporator to ice up? I think next step is to carefully open the switch to look for adjuster, and see if I can make it switch at a slightly higher temp.

dr bob

Aryan--

You don't have a freeze switch problem, by your symptoms.

The hissing noise and reduction in airflow are telltales of a vacuum system problem, specifically the center comb flap actuator is leaking. If the hiss is from the center console high, you have little project. Roger sells the diaphragms separately for a lot less than the cost of a whole actuator, and you'd end up pulling the diaphragm out of a whole actuator anyway. I suggest that you go through the whole vacuum system diagnosis and fix everything that shows up. What's likely happening is that the system loses vacuum from the comb flap actuator whenever the AC button is pushed. That in turn means less vacuum to hold the heater valve closed, so your airflow is reduced and you are circulating hot water through the system right about the time you are asking for cold.

Start a new thread on your circumstances and symptoms, since they are separate from the original subject of this thread. I won't feel so guilty delving into your issues there, vs hijacking the OP's problem here.

StratfordShark

Glad you suggested separate thread Dr Bob, was just about to say I hadn't complained about hissing noise then realised it was addressed to Aryan (not me, Adrian!).

Anyway I'm making progress in understanding but gone backwards in terms of fixing things.

I took cover off switch which went off like a jack-in-the-box! There is a sprung arrangement inside which presses against a little 'spike'. When the spike is pressed in there's a little click and the circuit is made (normal condition).

Alas only after everything sprang apart did I realise that there was a screw on case external which must adjust the switching point. There is also an adjuster screw behind the spring which alters position of plate and how much pressure it places on the spike. maybe one screw alters 'make' and one the 'break' temperature?

Anyway after about a frustrating hour of trying to fit it back together I've given up and will get another switch. You need a lot of force to put it back together and at some point before I can get the tabs secured back on the cover something always pops off!

A bit frustrating as if I'd experimented with external screw before taking it apart I may have been able to make it switch correctly at iced water temp anyway.

Photos show what's under the cover (you can see the 'spike' at lower left - normally it's pushed in by the plate). Also pix of the outer screw and the one behind the spring revealed when cover removed.

dr bob

Adrian--

What happened to that capillary tube, the one with the yellow sleeve?

StratfordShark

It's attached to the cover which I took off to examine internals. The tube ends in a mini copper concertina shape which presses down on plate and pushes it onto black spike to close switch when cover is on.

In fact the screw I photographed which adjusts spring length/plate travel is also accessible through hole in plastic plate at end of switch. So the whole unit is adjustable for temp (but no 'cold' direction marked).

Will try to reassemble today by holding unit in vice while I try to push cover back onto tabs.