Re: ice age

Posted by HH56 On 2012/8/12 11:21:14

That clutch bit was the cheap way out for OEMs -- both for fuel efficiency and for cost since they could eliminate some components and raise the CAFE number with one quick change. A lot of the aftermarket mfgs had always used it for cost reasons.

Remember some pre 60's thru early 70s systems where the clutch was engaged all the time AC was on. Evaporator temp control was by POA valve, suction throttling or the like where the evaporator was kept at a constant just above freezing temp all the time -- 40 degrees or so. Ice couldn't form yet with evaporator being cold, air never got humid. Heater went all the time as well so after air was cooled and dehumidified, it was partially reheated to a comfortable temp controlled by the blend door per the operator setting.

With the gas crisis and cheapening the system in the mid 70s they got rid of extra components sometimes down to just an orifice instead of even an expansion valve. Cycling the clutch takes the load off the engine for a few minutes so fuel economy was boosted.

The clutch thermostat is supposed to cycle when the evaporator gets to just above freezing but usually around 35-38 degrees or so minimum. Air flow also is supposed to help keep evaporator relatively stable. In the several minutes it takes for evaporator to warm to start the cycle over, water will drip off the coils, air will get well humidified and since many still use a form of blend door, you feel a bit of heater warmth as well until the clutch kicks in and cools again.

If yours is icing up, wonder if your thermostat might be acting up or air flow is somehow restricted.

The evap temp control brings up a question on the heat powered units. Think refrigerators just turn on/off the small burner when needed. Wonder how hard it is to control the heat applied to the refrigerant material if residual heat is used or if there is another way to control evap temp. Maybe some kind of bypass system like they used in the early days?