Air con clutch operation

[JulianEdgar]

I am trying to use the air con request function to drive the air con compressor clutch.

I have a dig input working and the AC request status shows in the status page when the input is triggered.

I have also tested the output (working through an E888 expander) and the clutch is triggered correctly. I can also manually change the polarity in the output set-up and engage the clutch.

So the input and output are both working - but they don't talk to each other! Activating the a/c request does not result in the clutch engaging.

I have appropriate values set in the rpm and throttle position settings for the a/c control.

What am I missing? This one has me absolutely stumped.

[Scott@FP]

Check the OEM polarity of the A/C request, they aren't all pull to 0v to activate. Also check logic of the system.

Normally its pull to 0v or switch to B+ on the req line and pull to 0v on the A/C clutch relay to turn on but this isn't always true. There are some weird ones out there that require a different approach, some German cars come to mind.

Also check the dual pressure switch or aux temp switch, by pass it and see if it works, in many cases the A/C clutch is wired thru the DP switch, or at least the A/C control logic uses it, and the clutch will not turn on if the system is discharged.

There may be some additional things going on in the A/C system control outside of the ECU preventing operation, review the OEM setup/wiring to see if something else is going on.

[JulianEdgar]

I think all of that is OK because I can manually activate the clutch by changing the polarity in the aux output set up.

Also, when I just feed the output to a multimeter, I cannot get it to change with the activation of the air con request status.

It almost appears that there is a bug in my version of the software - I even tried it with a normal ECU aux output (not the expander) and it still didn't work.

[Scott@FP]

Are you testing with engine running, or stopped? Some of the outputs logic either ECU or A/C control head may not operate unless the engine is running. The A/C control head may not ever send out req signal if there is a basic fault with the A/C. Best to refer to the FSM in this case and make sure everything is hunky-dory.

[JulianEdgar]

Got back to this: it appears the M400 a/c clutch output doesn't work until engine is running. That fixed it.