a/c clutch recycling system OR NOT

[Harry L.]

I replaced the evaporater, I added a new rebuilt stv valve, a new expansion valve a new dryer a new compressor, and my a/c guy added a thermal static switch, to regulate the temp. My a/c works great coming out at 44 to 50 degrees all summer long. So as I am investigating, is it the fact that I do not need the thermastat. that the stv and the expansion valve will regulate every thing properly and keep things cold but not freeze up. Is there anyone out there that has a non clutch cycling system that work well that I can discuss this matter? thanks DUTCH

[Joe R.]

Re: a/c clutch recycling system OR NOT

I replaced the evaporater, I added a new rebuilt stv valve, a new expansion valve a new dryer a new compressor, and my a/c guy added a thermal static switch, to regulate the temp. My a/c works great coming out at 44 to 50 degrees all summer long. So as I am investigating, is it the fact that I do not need the thermastat. that the stv and the expansion valve will regulate every thing properly and keep things cold but not freeze up. Is there anyone out there that has a non clutch cycling system that work well that I can discuss this matter? thanks DUTCH

Hi Harry:

I'm more familiar with the 1967 system that uses a POA valve instead of an STV valve, but I think the POA and STV functions are similar. In the 1967 systems, some people replace the POA valve with a thermostatic switch that turns the compressor off when the evaporator is cold and turns the compressor back on when the evaporator warms up.

This is done mainly to avoid replacing or repairing the more complex POA valve, although it is also advertised as being more "efficient" since the compressor does not run continuously.

In terms of the actual cooling you get, the optimal cooling will be obtained with the continuously-variable POA/STV valve. The on-off thermostatic switch simply approximates a continuously-variable control system by turning the compressor on and off. The only "efficiency" gain you get is a theoretical improvement in gas mileage, not cooling.

On the surface, I do not see any advantage to adding a thermostatic on-off switch to a properly functioning system with an STV. However, I am not an expert on these systems. I would be interested to hear what your A/C guy says was the reason he installed the thermostatic switch.

[Harry L.]

Re: a/c clutch recycling system OR NOT

without the clutch recycling system, how does the stv valve work to keep the evaporator from freezing up? Is there an adjustment on the stv valve?Or do you turn the heat on to adjust the temp? I AM HAVING A TIME TRYING TO FIGURE THIS SYSTEM OUT. Are there any a/c experts out there?

[Larry M.]

Re: a/c clutch recycling system OR NOT

The STV is simply a back pressure controller. It will ensure that the evaporator pressure does not get lower than approx 30 psig. At this pressure, R-12 has a condensing temperature of 33F, so water/moisture in the air cannot freeze.

The low pressure switch in the compressor suction line on CCOT cycling systems does basically the same thing....it shut off the compressor if the evaporator pressure get below approx 30 psig. When this happens, refrigerant flow stops and the evaporator pressure increases.

The STV valve can be (slightly) adjusted. The Chevrolet Service Manual has an excellent explanation of how everything works.

It is possible that your AC mechanic installed the low pressure switch as a safety device to make sure that the compressor would shut down on a loss of refrigerant.

Larry

[Joe R.]

Re: a/c clutch recycling system OR NOT

without the clutch recycling system, how does the stv valve work to keep the evaporator from freezing up? Is there an adjustment on the stv valve?Or do you turn the heat on to adjust the temp? I AM HAVING A TIME TRYING TO FIGURE THIS SYSTEM OUT. Are there any a/c experts out there?

Hi Harry:

On 1967 systems with a POA valve, the function of the POA valve is to keep the temperature of the evaporator *just above* the point where condensation would freeze on it.

I believe that the STV valve used in 1966 does the same thing, but not quite as well as the newer style POA valve. The STV valve is rebuildable and adjustable, but the adjustment has to be done carefully.

As designed, the GM A/C systems worked very well, and it's probably a waste of time to try and make them "better" than what the designers did. As I noted in an earlier response, the only advantage to the clutch cycling systems is a theoretical improvement in gas mileage.

The only time it might make sense to alter the factory settings is when some component of the factory design has been changed, such as a conversion from R12 to R134.