'''Air Conditioning clutch output ... subpage of
GenBoard/UnderDevelopment/FirmWare'''
Many ECUs control Air Conditioning (AC) clutch output. Currently there is no firware feature for AC in GenBoard/VerThree, but it seems fairly simple.
MembersPage/MarcellGal is not an AC expert, but studied his air-conditioning system (can post scanned schematic, but it's nothing more than described below, with some hardly readible Italian abbreviations).
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
- AC system pressure is 200 .. 2500 kPa
- cabin fan on (eg. switched in 1..4 position, not in 0)
- not agressive acceleration (optional)
- engine running for a few seconds at least (optional)
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
- AC system pressure within 200..2500 kPa range (active-low) signal
- AC system pressure > 1700 kPa
Naturally, a single pressure sensor can be used instead of the above range switches (eg. an MPX700A).
Other signals
- AC enable switch
- AC control dials (warm/cold/temp)
- headlight switch
- CLT temp
- ...
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.
Some systems are certainly different to some degree.
Automatic AC systems have somewhat more parts, eg.
- temp sensor
- and control of warm/cold mixing (electro-pneumatic or electric actuation)