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

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

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....

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

Is there a printer in the load space : o)

😀 I need to re-chassis a 90 Defender, in case you fancy a commission. Body comes off in one piece

I had to do mine when I first got the car. They definitely don't need welding. IIRC I think they are just a bolt-on job - i think the 4 horizontal bolts you see are what holds the crush can on - definite angle grinder job, that one

Even when you know the technique, you're pretty much guaranteed to break something - so buy some spare plastic fitting rings at least

If you have contact cleaner in yiur tool kit...i'd remove the xyz switch and find the little breather Richard mentions, and squirt some contact cleaner in.. And then let it drain/ breathe
Later models of the switch dont have the hole. While youre there take down the zf part no from side of the switch, so you can buy a back up off ebay. If you're stuck i have a spare one with no breather

Mine are tinted...maybe 10 or 20pc

No accounting for taste...probably worth a few quid in parts though

I think the change in note is just when ecm grants the a/c. After grant, if the clutch has not engaged within about two minutes the service book symbol will come on, and at that precise moment you will get another change in tone. So i'd say your prob is between hevac and compressor, or the compressor clutch itself.
Check fuses and relays (esp fuse 42 10A, which is the battery feed to the ecu, without that one you' re going nowhere)
Put a 9v battery across the spades of the compressor...does clutch engage?
Engine off and 9v connected to spades... using a ferrous spanner, does the central spindle/nut on the clutch magnetise? If it does then check the gap on the clutch with a feeler gauge.

If the clutch IS working then pull the hevac ecu and test the BS line on the 16 B connector for ac on. When you turn to Ignition II it will start at 12v, and go to near zero when the ac is requested. If you toggle the ac switch you can watch this go high (off) and low (on).
Test the BG line on the 16 B connector for ecm grant. It will start at 5volts and should drop to zero ( grant) within 30s or so of start up. If not, try raising the revs to about 1500.
And lastly take your 9v battery and put the +ve to the BY pin (no 7 on the 8 W connector) and the negative to a good earth eg back of the cigar lighter...clutch should lock... If not then do a resistance check from pin7 to the +ve at the ac round connector, it should not be more than 10 ohms or so.
If resistance is high, then is your ac charged, if yes then you need to go to the rh kick panel and spray the hell out of positions 1,7,12 with contact cleaner, and if that still doesnt work then test the resistance from pin7 ( lower) to the +ve at the round compressor connector...if you get high resistance there then you prob want to pull the bumper and bridge the trinary switch and/or check the resistance across it. Shd be a few ohms only when closed. If the switch is toast then you can change it without refilling the ac.
Hope it helps

Super reference KCR, thanks for that.

I dug out the HEVAC unit the other day and it turns out I have JFC102550 which as far as I can make out for my car (a 1998, chassis 395xxx) was the "wrong" / later model (it's been there since I bought it 3 years ago). I'm thinking through the implications of that before I make my next move. At the ECU I can see Request (Low/0.7V on a 12v line) and the incoming Grant (Low on a 5v line) but after a couple of minutes the HEVAC gets bored, service symbol comes on, and both lines go high. So far I have not been able to detect any voltage on the black/ yellow pin at the HEVAC ECU. I pulled apart the ECU the other day and there was no visible signs of corrosion or damage. So I'm just trying to work out if something has failed in the ECU, or there's some other condition not satisfied.
Having cleared the distribution flap jam the other day, HEVAC now reports a short to front windscreen (despite me having pulled both the fuses and relays)...I can go chase that next... but I assume that is not linked to a/c not working.

I wonder if either the fusebox or HEVAC ECU are messing with me?

Cleared the distribution doors error (was a stiff mechanism) - now back to just "a/c compressor circuit open" which is not true - as I measured the resistance manually as I mentioned above. I can read on nano that GEMS ECU is receiving an ac request, until after a few minutes of engine running and set at LO, and then it shows the book symbol.

I wonder if this has to do with the ECU determining that the resistance of the circuit is somehow out of spec. I'm still holding out for an a/c systems document, if anyone has ever seen one. I have an EAS one, that is more detailed than rave, i thought there might be a hevac one too

Tnx Richard. It's not the most elegant fix in the world, but it definitely works...amazing

I'm looking for a good way to free up my distrubtion flaps, and have found reference to putting a self tapping screw in the heater box "somewhere", but does anyone have a ref or pic? Seems the definitive guide was on rr.net once upon a time, all i get is dead links, but maybe someone here has actually done it?

If you're not in a rush, ask 9 and get 6-8. Nice looking car...

I measured the resistance from pin 7 at the RH kick panel through to the positive at the round plug for the a/c compressor - which is negligible (it was 28 ohms before i jetted pins1/7/12 with contact cleaner). I no longer get a ghost 12v signal at the condenser (which was there at ignition off, too),. After I cleaned the RH connector the clutch even kicked in for a couple of brief moments, so i think my issue is that one or more conditions for grant and request via pins 1 and 12 are not being satisfied.

I'll keep looking tomorrow, but does anyone have a detailed systems document for the HEVAC ECU - with a list of the conditions required for a/c grant, and request Rave is a bit vague, so I'm looking for the next level of detail My sensors all look to be reading fine. Temp is at Lo. Engine on

In the last couple of weeks (after i tried to calibrate the blend motors, in fact) my distribution flaps have frozen at 83%. I can't really be bothered to free them up at this point, unless we think that that would be enough for the ECU not to apply a/c? So it does have a service book symbol, which registers during the blend flap test cycle at start-up - so most likely the distribution flap being frozenas opposed to anything else.

I just saw today that the GEMS ecu reports that the AMFR correction is at the maximum positive value, so MAF may need a clean, but I assume that's unconnected

Happy days (the first drive, that is...)

pics?

WIfe just got a Tesla (I haven't even been in it yet... too busy fixing the P38) but she tells me that you subscribe to a provider, and pay that way