a/c problem
 

hi I have a '77 3L with an aftermarket a/c converted to r-134 by the first owner ( i'm the 2nd). The system performs normally for 45 min and then the evaporator freezes. r-134 pressure is within limits and front condenser fan is functioning. What's up?

try reducing the refrigerant by 20%

Not sure, but I seem to remember that there is a calibration adjust screw on the thermostatic control switch regulating the evaporator core temperature. Insert the capilary tube end into an ICE BUCKET "standard" and adjust the screw such that the contacts always open at 32F.

Make sure that your evaporator fan is running on high speed. If not adding enough heat to the system you will be prone to freezing. More causes of low air flow could be a restricted supply or return to the evap or a dirty or partially plugged coil or even air bypassing the coil due to a bad seal between the evaporator and the housing.

When the system is working well, what is your outlet air temperature from the vents?

try the above if you can adjsut the cap tube, or at least verify it works. they are filled with refrigerent and if it leaks out, they dont work. also, make sure it is even inserted into the evap. some people will leave them out thinking their system will blow colder.

the few 930/911 AC systems i have worked on, and from what i have been told by my porsche mechanic friend, 911 systems run higher pressures. if you charged based pressure alone, it may be under charged.
when the system is under charged, the evap freezes

Actually the screw on the thermostat offers very little in adjustment, the range and "Hysteresis" are set at the factory. We have talked to the factory engineers. Although you may find a post here in the forum about someone whom did play with the adjustment, a new thermostat properly installed does not have freezing issues. Most issues with the thermostat and freezing relate to either:
A) The capillary sensing tube is not making good contact in the evaporator (the aluminum fins have been pushed away from the probe or the probe in its brass sleeve).
B) When the thermostats get old they can fail in a contact closed position.

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Question:

I have some thermal paste left over from a computer build I did some time ago. If I used that paste when inserting the capillary tube, would that be a good thing? Or a bad thing?

There isn't very much range of calibation required. Put the "probe" in a ice bath, adjust the "screw" so the switch is distinctly open with the control knob FULLY clockwise..MAXIMUM cooling. The switch should close with the slightest CCW of the control.

An ice bath can be as simple as large bucket filled with shaved ice, or findly chopped ice, and filled with water below the level of the ice an inch or 2. Insert the capillary tube/bulb into the center, well into the water/ice mixture and allow a few minutes to stabilize.

This is NOT rocket science.

Wouldn't matter.

Just so you understand, you don't have to "adjust any screw in the thermostat".... that is SNAKE OIL..... LOL. You can monkey around with the screw till the cows come home and if the probe is not making good contact with the fins in the evaporator in the right location it won't make a difference. They are set at the factory and seldom drift from that setting either. But, Wwest might be willing to fly over to your place and adjust it for you... lol, he sounds like an eager puppy tonight. Down Boy!

I can tell you this because we have installed many in the past 20 year and we have sold 100's. We sell new units because they sometimes break when you bend the aluminum tube extensively or when they fail in an open or closed position.

And, location of the probe varies from evaporator to evaporator.
OEM pre 86 coils are different than 86-89, and as well when it comes to
serpentine units from various manufacturers.

Does thermal grease help, YES.
Will thermal grease solve your issue. Don't think so in this occasion.

Location & Contact.

Hmmm, I don't know anything about any "screw" - I don't know what you're talking about there.

I was just thinking of making sure my capillary tube was making full thermal contact with the fins of the evaporator. Since I had some paste left over, I figured I'd put that paste to use. :)

when operating the vent temp is 45 deg

Concur on the getting screwed thing.

Moving on...

What you can do is, if you still have it, inject the thermal grease in the brass sleeve, about half full (you can use a syringe) and then insert the cap tube into the sleeve to its bottom.

The cap tube with sleeve on a 77L vintage evap, assuming it is a Behr evap, goes in 3" straight down vertical I believe. If you still have the freezing issue you could drill a 1/4" in hole (adjacent to the current hole in the top of the Behr box) placing this hole as close as can nearer the fan's shroud (shroud is the large round plastic riser that house the evap fan blower cage), this might get you into some 'fresh' fins in the coil; but be careful not drill more than 1/4" into the top of the box so you don't drill in a coil (ouch).
If you decide to relocate the cap tube like this you will want to take a thin piece of say welding rod or a coat hanger (something thinner than the diameter of the cap tube or brass sleeve) and create a 'pilot' hole into the new section fins. You'd press this rod down into the fins and without too much effort so you don't puncture a refrigerant tube... follow?

Or remove the top of the evap box, exam the area where you have been trying to insert the cap tube presently, and you'll see the fins have been pushed back.

You can go to our website, find our phone number and call us if you need more direction.

Removing or adding refrigerant to the system should not have any affect on the "function" of the thermostat; more common a procedure on home central air, industrial or commercial refrigeration units that don't have freeze out switches.

That is over a 13 degree loss of cooling through the vents... hmmmm.

What fan speed was the temp taking and what instrument was used to note the
vent temp.

Since the factory system does not rely on the "close" thermal coupling that would be provided via the paste the only result might be a slight reduction in the hysterisis "span".

No, but if your last resort is sending it to me for calibration I would be glad to be of help.
No, the last resort is to send them to you and tell you where to stick it! lmfao

"Sold hundreds.." Obviously with no feedback methods to ascertain the failure mode of the unit being replaced, gas leakage, contacts welded shut, nor calibration drift.
No. wrong again son. Time to take your xanax.
If a thermostat is 30 years old and it fails because the contacts are shot, or because the aluminum tube broke when handling it... because you did not read the directions, and you know the 'calibration' does not drift, wtf would you have a client send you a broken 30 year old thermostat and write a report? Do you return your worn tires or brake pads back to the mfg for an analysis? Rhetorical question.

"THIS IS NOT ROCKET SCIENCE" lol.

PS. our thermostats come with instructions. :D

Time to go while wwest is on his merry go round

What does fan speed have to do with evaporator freezing up? Even if the fan speed were ZERO the control system should prevent freeze up.

Either the probe is not properly positioned, the control contacts are welded shut, or for some other reason the control system is not cycling the compressor clutch OFF correctly.

Because you have been a bad boy, we are not going to tell you:)

When you recieve new thermostatic controls from your vendor what level of assurance do you have of proper calibration.?

BS, admit that it was simply an inappropreate question on your part resulting from your VAST experience but TOTAL lack of knowledge.