So I have been trying to nurse my a/c back to health... compressor changed, o rings replaced, full a/c charge, clutch engages off a 9V battery and a/c will run cold, evaporator will go sub 10 degrees no problem. With the regular electrics connected then I can see the a/c request kicking in within a few seconds of ignition start (nanocom, read from the ECM). I can also toggle the a/c request on and off with the a/c switch, so the nano reading is good, and live. Request is an active-low 12v connection.

The A/c grant is 5V, also active low. As far as I can see I don't get the grant back from the ECM - does anyone have the exact GEMS criteria for grant? BBS talks about temperature, load and acceleration... but what specifically? The nanocom HEVAC connection is pretty unreliable once the car has started (it says it can't connect, but still reports most input variables), but it reports no grant. When I have time I will pull the ECU and confirm that by testing the pin. I tend to believe the nano for now re lack of grant, but not 100%. Because the grant is active low then there can't be a wire break

As a plan B I was thinking to join the A/c request output from the HEVAC, via a 5V regulator to the a/c grant input to the HEVAC - then they would both go active low together automatically, and would be 12v/5v inactive....would this do as a work-around?

Thoughts welcome

Rgds, Rob

I always start the engine first before trying to connect to the HEVAC and it will often come up with a can't connect error but if I back out of it and try again it almost always connects the second time. You should be getting 12V out of the HEVAC when it tries to engage the compressor clutch but it is 12V and not full battery voltage so I suspect there is a regulator in there (or the volt drop through whatever is doing the switching drops it a touch). Check at the multiway behind the RH kick panel and see if you are getting a voltage there. Also have a look at the readings from the sensors, someone on .net found that he had an open circuit evaporator temp sensor which meant the HEVAC wasn't trying to engage the compressor as it thought the evaporator was cold enough already.

Tnx Richard, I have the a/c request, so there shdn't be a sensor problem with HEVAC... But that's why I'm asking about the ECM conditions - although what's on nano looks normal enough - for some reason the ECM isn't returning the a/c grant, so if I knew the exact conditions I could make sure they're all reading ok and move on through the diagnosis.
Everything i've tested seems ok, but the only odd thing is I see 11 something volts at the compressor plug even before the ignition comes on...but it's only 0.1 or 0.2Amps so not strong enough to pull the clutch in. That wd only have come from 2 sources, i think: i. the Fuse box to the dual pressure switch, or ii. HEVAC to dual pressure switch. Maybe the pressure switch is playing up.
I'll test all the pins on C102/202 next, didn't get time today before the rain started....

romanrob there's a technical service bulletin for this case, IIrc ... which I now realize have posted as a link before eyesroll

That TSB updates the system to that used on the Thor where instead of the HEVAC driving the clutch directly, it powers a relay which switches battery voltage to the clutch. It needs the big resistor as the early HEVAC will detect if insufficient current is being drawn (the amount needed by the relay compared with the amount needed by the clutch) so will assume the pressure switch is open due to a lack of refrigerant and not try to operate the clutch again.

Yes ... but it mentions as well the second positive lead to rise the amount of ampere current for the compressor clutch or the whole system ;-)
But I might be on the completely wrong track for romanrobs problem ...

Thanks KCR you reminded me that I had one of these harnesses in the garage, so i fitted it last night. After a few ignition cycles without the clutch working then it just randomly engaged - once, and got the evaporator down to 5 degrees - but i can't replicate that... I will do a bit more digging, I smell the end game

So in the end I gave up - there is some condition not satisfied at the ECM, and I've no idea what... so in the RH footwell i earthed the grant line going back to the HEVAC. The clutch still goes on and off with the a/c button for sure - i just need to check that over-riding the Grant feature has not also bypassed the temperature check that the HEVAC ECU makes on the evaporator