Gen Board/Under Development/Firm Ware/Air Conditioner

can provide enable / disable signal for AC (eg. provide ground for clutch relay), via misc outputs
can sense 0/5V AC-On signal (take care not 12V ! Apply through a diode, so diode anode goes in to a pulled-up mcp3208 analog pin.). Any changes can be applied through TableSwitch :
- higher iac_reference position/pwm values
- changed iac_PID parameters
- and other changes (lower redline maybe)

Things below are mostly outside the scope of ECU - they are controlled by simple sensors (widely available in the AC industry) and relays.

Air Conditioning clutch output:

Clutch active when signal +12V. appr 450mA (26 Ohm)

relay is the natural choice
stepper output would work too
standard low-side power-output (eg. a spare injector channel) would not work, unless with a relay.

Air Conditioning clutch output activation rules

Main fan activation rule

AC system pressure > 1700 kPa
or CLT temp > threshold (eg. 97C) ... note: histeresys
the main fan can usually be powered through a (big) series resistor under certain conditions (for light fan-RPM). Although, unknown (to me) condition.
- The only reason I have ever heard given for 2 speed fan is noise. Generally however, with AC on, fans run at high speed.

Note that the main fan usually draws 30..40A - almost always a relay is used and a 50A fuse.

The main fan is the 2nd biggest current eater (after the starter motor).

Input signals

AC system pressure signal

Naturally, a single pressure sensor can be used instead of the above range switches (eg. an MPX700A).

Other signals

Misc stuff

These might surprise the unaware, when debugging an AC system:

the AC "snow-symbol" is dimmed when the headlights are on
when the warm/cold dial is in most cold position (sensed by a end-position-switchbutton), a pneumatic valve (using intake manifold vacuum) activates a bypass valve that makes the coolant go around the cabin-heater. This way heat from the coolant will not sneak into the cabin, even if the air-valve "flap" is not perfect. What happens during long duration WOT when there is no significant vacuum ? Will the bypass operate correctly ? I don't know
the ECM is notified about the (AC controller intention of) AC clutch and the main fan, so the iac valve can be opened in time, and RPM target raised
Compressor clutch is usually disengaged (not powered) when in WOT.
Often the compressor clutch is engaged in defrost mode (even in cold temps) - Reason is to reduce cabin humidity to help eliminate fogging more quickly. Probably not something we should do, but worth mentioning.

Some systems are certainly different to some degree.

Automatic AC systems have somewhat more parts, eg.

Vasilis' suggestion

i think i have a solution for a very stable idle control for the audi

engines that can have a good solution for the A/C function as well,

and can make it work with the IAC valve+diode

Here is what is needed:

Under idle settings, put a dialog, IF A/C turns ON, then decreace Idle

PW by X number.

If you cannot control the AC function through VEMS, ( i mean when it

comes ON or OFF) then you can actually do this by using battery

voltage:

SO if Vbatt = 13.00 then IAC PW decrease by X number

so you can make a simple table of 5 values (i dont think more are

needed) so that the IAC PW can change depending on Vbatt.

This can work in combination to the IAC PW/ CLT.

So for example when CLT=80 and IAC PW=20 if vBatt=14.00

BUT if CLT=80 and vBatt = 13.40 (just when the fan kicks in) then IAC

PW=18 etc.

That way you can possibly eliminate the problem with idle drop AND use the IAC succesfully.

We '''actually do that since 1.1.4x : apply higher IAC PWM duty with lower VBATT voltage. (It's done automatically, unconfigurable atm).