A/C thermostat question
 

Rennaire system (barrier hoses, Sanden comp., new evap core/exp. valve, rec./dryer, aux. cooling fans on rear/stock condensor) is now installed along with a new Griffiths thermostat; making vent temps down to 38 degrees with no evap icing with 134 (95 degrees ambient temps). Not really a complaint, BUT, I'm seeing about a 10 degree vent temp swing between cycles....since I've never really had a legitimate operating A/C system in this car, is this about right or if not, is there something else I should look at before "closing the book" on this project?? Would appreciate opinions/thoughts on this issue.

Congrats, but not to rain on your parade, If it gets too cold it could freeze up and you don't want that either.

sounds like that is what is happenning, the cycles are long because it's freezing or almost freezing.

Maybe I did not phrase my question clearly....yes, I do realize that IF temps get too cold, there is a chance of evap icing; perhaps I'm trying to duplicate temp cycling of my Accord or Ranger (where there is about a 5 degree "spread" in temps), and that's not possible on an older Porsche a/c system. I'm getting this same 10 degree "spread" whether the bottom temp is 38 degrees (which if prolonged WILL ultimately cause icing) or the bottom temp is 48 degrees (at which temp evap icing will not likely occur). It's the temp spread I'm curious about....is that normal for an older Porsche a/c system or can that spread be brought to something closer to a 5 degree spread as on newer systems?? Thanks for your ideas!!

Draco is right.

Too cold is not good.

We have the same A/C configuration on our '86 (w/o the fans on the rear condenser), and did incur evaporator freezup on 1 hour+ drives.

High humidity is one of the major contributing factors which promote evaporator freezup.
And here at the beach, we are seldom below 80%.

You may want to check when your compressor engages/disengages as a function of center duct temperature.

This function is controled by your temp knob on the console, along with the temperature relay contacts on the back of the temp knob.

Ours is set to de-activate the compressor between 38 and 42F center duct temperature, and re-engage at about 46F.

Also, the location (depth) of the evaporator temperature probe is important.

I have known the folks @ Rennaire for many years. They are very knowledgable people, and make good products.

They also sell a device that will sense evaporator temperatures and will cycle the compressor accordingly.

Give this a try.

Good luck,

Gerry

I wish
 

New compressor, new front condenser fan motor, rear condenser fan, muffin fan for return air to evaporator behind passenger floor board, full R12 charge and all I can get out of the vents here in Arizona is 58-60 degree air maybe. Now before you reply to this remember thats a 50-55 degree drop over the temperature.
I wish I could get 50 degree temps out of the vents. I've taken the car to 2 different people and both put gauges on the car and said the pressures are within range and its probably as good as it gets. Another user on this board here in AZ is using Propane as a refrigerant and gets temps in the 40's, believable?

I'm skeptical that using propane would really solve much. The real problem is that 911 AC systems really lack condensor capacity. And personally I would not take the risk, yes I've heard the argument that we're driving around with 20+ gallons of flammable gasoline - but that's in a system that was designed to handle it safelty. AC systems never were.

To get lower vent temps you need to upgrade the condensors and/or the evaporator. I have the Rennaire system II with the procooler and desert duty front condensor running 134a and I can get upper 40's vent tems when its 105 out. If its only in the 90's out I can get low 40's upper 30's.

10 degree is on the high side. I know mine has more of a swing if the thermostat is turned up but more like 5-7. For a reference I just went for a ride in my truck and it has good A/C and it is dead on 6 degree swing every time like clockwork. Hope this helps.