AirCoolBill

My system air temperature varies 10 degrees in a constant cycle of decreasing and increasing air temperature.

Is this normal?

At setting 6 of the air temperature control ***, the low end of the temperature cycle is in the low 20's and stops cycling due to failure to reach the low end switching point. Thus establishing the conditions to ice the evaporator.

Is this normal? Can the range be calibrated to prevent this condition?

griffiths

On an extreme hot day the variation of temperature out the vent can have a large swing or range such as you have noted as the compressor cycles off and on.

If your evaporator core is freezing the first suspect is the thermostatic probe is not making good contact inside the evaporator core, the second suspect is a failing thermostat.

AirCoolBill

Is there a source for the operating specs for the thermostatic control?

griffiths

http://www.griffiths.com/achelp/achelp2.html

wwest

By design it is highly desireable for the mechanical thermostatic control switch to have a fair level of hysterisis, wide range between the compressor "on" cycle and the "off" cycle. The thermostatic probe, capillary tube, should be "buried" well enough into the evaporator vane surface area that when the temperature control **** is set for maximum cooling the compressor relibaly cycles OFF before the evaporator reaches the "freeze" level, 35-33F.

The hysterisis, off at 33F, on at ~40F, prevents the compressor from cycling on and off too rapidly but does often result in a 10F swing in ourlet airflow temperature.

wwest

The thermostatic switch hysterisis level quoted by griffiths in the above link appears to me to be wrong. Off at 26F and back on at 33F would put the evaporator's AVERAGE core temperature below freezing. That, with a high Rh and using a fairly low blower speed would, IMMHO, often result in airflow blockage through the evaporator due to moisture freeze-up.

Having the operational temperature of the evaporator just a tad above freezing would, in my opinion, provide more than satisfactory cabin cooling capability without the threat of icing up depending on local atmospheric conditions for which we have no "sense", method of ascertaining.

To be "safe" I would never adjust the "off" point below 33F.

AirCoolBill

WWest,

From your comments the 10 degrees of hysteresis I have is the normal and necessary cycle.

The "compressor on" temperature starts in the high 50's in position 1 and drops ~5 degrees with each increment of the control. This also seems reasonable.

The "compressor off" temperature also drops in a similar manor starting at position 1 in the low 50's. However, the "compressor off" temperature continues to fall with each increment of the control until it is in the low 20's and can go no lower.

I suspect the probe is not properly buried in the evaporator.

You mention adjusting the "compress off" set point, how is this done?

griffiths

When I first heard the specifications from the OEM I too thought it was odd.
However, it is what the engineer quoted. Now whether they "adjust" the switch to something other than that before it leaves the factory is another question, and we are dealing with a technology that is 50+ years old and design drawings as old as well.

Most cases of evaporator core freezing at maximum cold setting are due to the fact that the thermostatic tube is not making good contact with the evaporator core fins.

Update: found the OEM drawing, blew off the dust, the specs are as I noted.

AirCoolBill

I work for companies supplying the automotive industry and can appreciate the issue with old designs. Often, particularly during down turns in the business, senior engineers are retired and knowledge leaves with them. Prints may contain the information required to make the part but not the design criteria and function data for the system.

In the US this resulted in the inability to get approval for design improvements because the engineers did not understand the design requirements well enough nor did they have the time and money to develop it.

The Germans are best at documentation and engineering practices.

griffiths

Thomas Edison - German?
Warren S. Johnson (Johnson Climate Controls) - German?
Edward Weston - German?
Alexander Graham Bell - German?

llangston1

So if the fins on my serpentine evaporator are a little bent out of shape where the temp probe normally sticks in the evaporator case should I try to drill a new hole in s different location to get som undamaged fins? My probe still uses a brass tube and it seems to make decent contact. I can see and hear the compressor cycle on mOst days bt if I move my temp **** much past the 4 o'clock position the core will ice up. Once it starts turning the temp **** has no effect. It stays iced up until you turn off the system completely for 5 minutes. The last time it froze I got over 2 cups of water in a catch pan.

griffiths

So you are still having the issue?

There are a few different serpentine configurations so the distance
between the serpentine tubes in relation to the hole in the top of the box
could be your solution (moving the probe closer to the center of the evaporator core), or simply removing the evaporator box top, 'raking' the fins back into place and carefully re-inserting the probe.

I have at times come across thermostats that on occasion simply don't want to turn off as the mfg's cold set point: this you can test by removing the probe and either spraying it with a cold aerosol or insert the probe in a glass of ice with salt and checking the circuit.