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

I have tagged onto this posting as I have a very similar problem to romanrob post 7.

I fitted the loom as in TB 0010 from my now spares 1998 model.

I knew there wasn’t a full charge of gas in the system from attempts to get working a/c a couple of months ago, before the loom was fitted so I just put a jumper lead across
Trinary switch socket 1 black/yellow and 2 white/light green.

I had seen in the post 9 that it was on the 3rd starting the compressor clutch engaged … well so did mine, so downed tools for the day.
The next morning started engine …. No clutch engagement.
I thought it might be because not enough gas on the system so booked it in at my auto electrician.

He evacuated the system and fitted the new receiver drier and pressure switch 2 single switch that I supplied and re gassed it. Only to tell me the compressor clutch was not engaging so I had an electrical fault.

I have been trying to get my old fashioned brain to sort it out.
If I jumper across relay 10 socket 30 and 87 the clutch pulls in and cold (not freezing) air starts to come out of the dashboard vents. But there is no power at 86 to pull the relay contacts in.

Very late Friday afternoon I noticed a sticker in the engine area saying 700 grams was put in the system. Just looking at info on the net last night I realized that is not a full charge, the manual advised me it should be 1250 gms.

Is there a chance that with a full charge of gas the feed to relay 10 would be there to pull the compressor clutch in ?? My electrician does’t work weekends)

Nanocom lists the only fault is 6 RH blend motor current fault.
It also says YES to the grant.

Again any ideas would be most appreciated
Andrew

Yes to the grant means that the HEVAC is trying to engage the clutch but with only 700 grams of R134a in there, the pressure switch will be open circuit so the clutch won't engage. It really does need a full charge, that is why you have nothing at pin 86 and when you force it in with the jumper you are only getting coldish air from the vents.

While on the subject of AC, I filled a diesel P38 yesterday which only takes 1100 grams. The owner has had the car for almost 10 years and never had working AC but was complaining that his kids weren't happy being driven around in a mobile greenhouse over the summer. I pressure tested it for him and the condenser was leaking. He replaced that and I put 10 bar of Nitrogen into it and unplugged the compressor so it didn't try and engage. 3 weeks later it still had 10 bar of Nitrogen, so we declared it good. With a full charge of refrigerant, the Nano reported an evaporator temperature of 4-6 C but air from the vents was reading around 10C. Another good reason for fixing the leaks in the ducting..... But of course now he has another problem, condensate pouring into the passenger footwell.