Automotive Air Conditioning Information Forum

Folks,

I have an old Ford truck with dealer AC installed many years after new. It is nothing like the factory setup. It has an H-block TXV and a thermo switch buried in the evaporator.

Well today the thermo swtich gave up completely. It would not open no matter what the adjustment and the whole thing is a solid block of ice.

Does anybody have a recommendation for an adjustable aftermarket switch? It needs to interrupt clutch current directly. I could add a relay if necessary but always for simple=better.

I am imagining installing it on one of the exposed evap lines rather than digging inside. The plastic ductwork is in rather shaky condition. I am not sure whether the inlet to the evap or the outlet from would be better. I imagine the inlet would be the place to just keep it above freezing. But maybe there won't be so many swings on the outlet and it can be adjusted to freezing+superheat.

Thanks so much.

If I'm understanding you issue.

Replacement of the thermostat. As the switch itself, it can be mounted anywhere.
Thermostat probe has to go into the evaporator core to work correctly.

In the interest of furthering the discussion I will post an answer to my own question.

After I posted this I remembered the small AC shop guy that works out of his garage so I went over there.

He dusted off a couple of old thermostats, some with levers and one with a D-shaft for a knob with the right length capillary. Used on some cars (never seen one) that had a dash temperature control, right for colder, left for warmer.

I bought that one off him (it was real hard finding any on the internet that looked like the right thing) and installed it on the evaporator outlet tube going into the TXV about 6" of straight section.

I pre-set the switch in a bowl of ice water so the opening point is right at 32° and the closing point a bit higher. It is almost all the way down at this point.

I bent the capillary into a U so it has a good 10" contacting said tube and covered it with electrical tape to hold it there and then the black goop with cork particles to insulate. No idea how much is supposed to be in contact but I am sure that is plenty of contact area.

I took a trip into the flat, hot, humid California valley and it seems to work as best it ever had. The old switch was always flaky and would alternately freeze up the evap or not keep it cold enough. This one once it has reached the cycling point is keeping the air much colder because it keeps the range tighter.

I know it is less than ideal location and the AC guy said so too but in this vehicle I think it will be good enough. The plastic ductwork is so rotten I do not think it will hold up well to another cycle of opening and closing it.

I will probably turn the temperature all the way down on the knob and see if I get any icing later but it is already pretty good.

Well, after some days of use (never gotten that hot here yet) I can say that this thermostat has made a world of difference.

Before, it would work OK on the initial pull-down of the cab temperature but never get real cold. I see now that the problem was that on anything other than full blast, icing starts to occur and this causes cooling to suck. Or if I turned the old one up higher then the evap would stay so warm it would not work well either.

Now with this new thermostat installed, it is much more aggressive about cycling off and on. Even with the cab still somewhat warm and the blower on 3 (of 4) it still cycles off once in a while to stay above freezing.

I am not really seeing any disadvantage to having it located on the evap suction line before the H-block, insulated.

I have tweaked with it so the evap feed line stays below freezing about 75% of the time when idling with the fan on low. It does not seem to build up significant ice nor leave large puddles when shut off so I think I got it about as cold as I can without fear of eventual blockage.

Just wanted to put this out there in case others have a similar problem or system. I know this is kind of old fashioned but it is better than being hot.

Sounds like it's working !!!

I never realized how much a malfunctioning thermostat could lead to warm air. Even if the 'stat was running the core too cold.

Now I see that as soon as I turned the blower from 4 down to 3, the core would start icing and performance suffer quickly over a matter of minutes rather than half an hour or so, which was my guess before.