'''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.
- 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.
- temp sensor
- and control of warm/cold mixing (electro-pneumatic or electric actuation)