One more A/C question

[Nick Triesch]

I have been working on my Porsche A/C for a while. When it was working I remember that when I turned the left knob (thermostat) all the way to the left the compressor would turn off. Is this a correct function? If it does not turn off, do I have a faulty thermostat?

[Neel]

What year car?? I've updated the A/C on my car-81SC, so my controls are different, but I'll see what I can find out.. Easy enough to find out.. Someone with a working stock A/C, could easily chime in on this one..

[Bob Kontak]

Originally Posted by Neel

Someone with a working stock A/C, could easily chime in on this one..

Waiting on my fuel pump to arrive so I can't help.

Assuming fan is on, could ambient temp drive compressor on or off at the full left position?

Long story short, I am subscribed. Don't know.

[wwest]

All the way CW is generally accepted as being so close to evaporator freeze-up that it is not often left in that position for extended periods.

Full CCW might allow the compressor to cycle on and off but the "on" will be fairly short if the cabin atmosphere is 75dF or below. On the other hand if this is during an initial cabin cooldown from an elevated temperature, say 90dFor above, the compressor will not be cycled off until the cabin is cooled down to the 75dF range.

[kuehl]

Nick,

The original OE (no longer available) thermostat in the full CCW (left) position would not break the circuit (open) to the compressor and front condenser fan motor relay. The minimum "on" threshold at the full CCW was 65F M.O.T. (mean operating temperature) at the evaporator core.

Subsequently the M.O.T. on was changed to 55F near the full CCW position and a "dead zone", or "off" , was built into the design such that when fully CCW to the switches stop the circuit is broken and will not allow power to travel to the compressor and front condenser fan motor relay regardless of evaporator core temp. Hence, new switches have the dead zone.

Cabin atmosphere does not directly affect the AC thermostat, rather it is the evaporator core temperature, and evaporator core temperature is relative to many variables.

Trouble shooting the ac system.

Griff

[Neel]

Originally Posted by kuehl

Nick,

The original OE (no longer available) thermostat in the full CCW (left) position would not break the circuit (open) to the compressor and front condenser fan motor relay. The minimum "on" threshold at the full CCW was 65F M.O.T. (mean operating temperature) at the evaporator core.

Subsequently the M.O.T. on was changed to 55F near the full CCW position and a "dead zone", or "off" , was built into the design such that when fully CCW to the switches stop the circuit is broken and will not allow power to travel to the compressor and front condenser fan motor relay regardless of evaporator core temp. Hence, new switches have the dead zone.

Cabin atmosphere does not directly affect the the AC thermostat, rather it is the evaporator core temperature, and evaporator core temperature is relative to many variables.

Trouble shooting the ac system.

Griff

Nick,
What Griff tells you, you can take to the bank.. I've been working with A/C since the 60s.. Porsches about 10yrs.. I've learned so much about Porsche A/C from Griff!! And he is willing to help you out with his proven, documented information.. His products are top drawer also!!

[GH85Carrera]

Nick, on my 1985 the compressor will never turn completely off with the thermostat in any position. Since I drive in autocrosses on very hot days I like air from the evaporator fan blowing on me. Even warm air is better than no air.

I put in a manual toggle switch for the wire that turns on the compressor. The switch is in the engine compartment and I have to manually flip it to run the autocross and flip it back on to drive home in AC comfort.

[Nick Triesch]

Wow! Thank you everyone!, Nick

[ganun]

Yep, I still have the original temp switch in my 87 and sure enough all the way to the left the a/c holds a steady 55F. ....On the road cruising BTW.

[ganun]

All the way left, on the road.

[wwest]

Originally Posted by ganun

Yep, I still have the original temp switch in my 87 and sure enough all the way to the left the a/c holds a steady 55F. ....On the road cruising BTW.

"..holds a steady 55F..."

"steady" for, to, you doesn't mean "steady" for the thermostatic switch, compressor, etc.

The thermostatic switch will be cycling the compressor on and off, mostly off, to maintain your average vent temperature of 55dF.

Charlie's post is a bit confusing in that way.

"The original OE (no longer available) thermostat in the full CCW (left) position would not break the circuit (open) to the compressor and front condenser fan motor relay. The minimum "on" threshold at the full CCW was 65F M.O.T. (mean operating temperature) at the evaporator core. "

I think what Charlie was trying to convey is that the fully CCW would not break/(open), fully disable the A/C. The switch would still cycle the compressor OFF and ON, just less "ON" time vs partially CW.

Of course if the evaporator core temperature is already well below the thermostatic switch setpoint then the switch will remain open constantly. But then you wouldn't have need of A/C anyway.

[ganun]

I don`t recall if the comp was cycling, .... I think it would be slightly annoying, ie the surging on and off....., I would think.

[wwest]

Originally Posted by ganun

I don`t recall if the comp was cycling, .... I think it would be slightly annoying, ie the surging on and off....., I would think.

As Charlie says, the MOT of the evaporator core would be maintained low enough to keep the vent outflow at 55dF. Cycling, or no, would depend on the temperature of the cabin atmosphere being "pulled" into the cooling plenum.

If the cabin atmosphere was already at your comfort level, which was apparently the case since you turned the control fully CCW, the compressor would likely be cycling on and off.

[kuehl]

Think of the difference between to original thermostat and the replacement
thermostats like this:

Original Thermostat

When the knob is positioned in the full CCW position,
and evaporator core temperature rises to 65F, the circuit makes contact
and the compressor clutch and front condenser blower relay get power.
The thermostat will try to maintain 65F evaporator core temp by turning the system on and off +/- "x" degrees.
When the knob is positioned in the full CW position, and the evaporator core drops to 30F, the circuit breaks contact, and
again tries to maintain 30F +/- "x" degrees by turning on and off.

Replacement Thermostat

When the knob is positioned in the full CCW position,
the sensing side of the the system is shut off. However, if you turn
the knob a few degrees CW and evaporator core temperature rises to 55F,
the circuit makes contact and the compressor clutch and front condenser blower relay get power.
The thermostat will try to maintain 55F evaporator core temp by turning the system
on and off +/- "x" degrees. When the knob is positioned in the full CW position,
and the evaporator core drops to 27F, the circuit breaks contact, and again tries to maintain 27F +/- "x" degrees by turning on and off.

The pre C2 911/ 912/ 914 and 930 AC thermostat does not and cannot guarantee a
mean or average cockpit operating temperature.
To do that it would need a sensor in the cockpit alike what was designed, say, for example the 911 Autoheat, or the 928 HVAC system.

This form of thermostat control is a "feel like" design.
Set it to what you want to feel like.

Porsche 911/930 AC

[ganun]

Thanks