Pauerman

Does anybody know if there are conditions where the A/C clutch engagement gets turned off - above a certain RPM or boost?

931guru

Nope. Of course, you could wire in a momentary off switch into the AC compressor clutch circuit and manually control that.

I did that once on an Audi (2.2L 100 bhp, and it did seem to help acceleration).

V2Rocket

On the early 944, the AC compressor clutch disengages when there is low vac/atmospheric or positive pressure in the intake manifold, and a flap closes at the evaporator outlet to keep air from blowing out the vents and keep the coil cold.

I would guess the late cars and 951s had a similar setup.

Pauerman

I'm looking at the factory schematic now and I not able to see a pressure sensor (other than the low pressure switch) related to the clutch control. There is an "output" sensor that is tied into the outside and inside air temperature sensors, but nothing related to pressure.

Pauerman

I can already control the A/C clutch manually - I just press the A/C switch to off!!

968turbos2

probably uses TPS signal to determine load. under load turn it off (if it is on - your manual switch). If using stand alone then a pressure signal (map) / Throttle % may be used. In your case I would use TPS voltage levels.

Pauerman

I run a standalone system and I'm wondering if the factory put a provision into the DME for disconnecting the A/C clutch over a certain TPS / RPM / boost threshold. If so, I want to tap into that circuit to control it with my ECU.

The A/C request output comes from the DME directly to the clutch, so it looks like I will be managing this direct output to the clutch rather than a separate relay or switch. This will allow me to disengage the clutch based on either TPS position, MAP or injector duty automatically.

968turbos2

Yeah I did for my 968 turbo the same and use drive by wire. TPS / MAP / Coolant temp. That 115 degree Celsius switch also prevents A/C. Not sure if the 944 has it but I think so The TPS signal I pull right off of the PPS (pedal position sensor which is essentially the requested TPS, requested by my foot )

Mangonesailor

I think a while back Rouge_ant had said the factory function of the A/C kicking off under WOT was going to still be in his chip sets he sold. My guess is that yes, there is a function and it's TPS related.

Adker

The AC clutch power goes through the low pressure switch (on/off depending on Freon pressure) and then to the AC relay. The AC relay is driven by the AC controller. The DME is not in the loop and it has no signal going to the AC controller. What the DME does look at is on DME pin 29, it looks to see if the clutch is on/off and adjusts the idle accordingly.

If you want to control the AC clutch, you can put a relay inline with the wire going to the clutch (about 4A draw), or put a relay in line with the AC relay coil, much lower current draw.

Rogue_Ant

Adker is correct. The DME has no influence over the AC compressor - it only monitors the AC clutch engaged/disengaged.

Pauerman

The issue I'm coming across is that A/C relay G17 connects pin 3 (signal from A/C switch) and pin 4 (output to low pressure switch) when the relay is relaxed or not energized. Pin 2 is the ground signal that latches the relay to the open position. So, when energized no power is sent to the clutch.

Initially I thought I'd tap into pin 2 because it seems that this ground signal dictates clutch engagement / disengagement. I was wondering what controls this ground.

So from your comment, do you believe that the clutch is not controlled by the DME to where it will disengage at WOT?

Rogue_Ant

The DME does not control the AC compressor, only monitors the clutch...

Pauerman

I have been incorrectly labeling the items - by controlling the compressor, I have been referring to the clutch engagement / disengagement.

Pauerman

Any thoughts about pin #2 (ground) at the G17 relay?