hevac.

gentlemen, does anybody know for sure, that if you replace a hevac ecu does it need 2 be initialised by testbook 2 work properly. swopped mine recently and now t a c dont work, all else is perfect, checked everything through all connectors good , but t hevac aint sending power through t black and yellow 2 switch on t compressor, if i piggy back the black and yellow with +12v the compressor engages, been through all t forums without a conclusion, according 2 rave it needs initialisation, my not so local stealer says no, any ideas. thanks - 4 here now.

ive changed mine and its fine

followed t problem a little further, removed radio 2 gain acess 2 t hevac wiring, removed white plug and put +12 v 2 the black and yellow wire and guess what t compressor magnet engages, so that means t pressure switch is ok, its seems the ac ecu must be playing up so i changed it for another one but still good, could t 2 be faulty, any ideas.

not sure on the wiring without looking up and giving myself heed ache. but, what i wonder is:

why isnt the hevac sending 12v, is/are there any other conditions that need to be met, i.e temp settings/switch position? aircon system pressure?

i doubt two hevac units will be faulty, can you test one known working one from another p38?

or yours on a known good car???

just sending 12v down wires proves very little apart from the contiuity of the cables. and it could be dangerous to the rest of the electrofeckery on the car!!!

i would investigate the switch/valve unit (cant remember what its called), on the pass side of rad tucked way down near the bumper joined onto the aircon pipes

t 3 way pressure switch, have that checked, i dont think its t problem, you need 2 get power down 2 it before it can work. even if i remove t plug and by pass it by joining t green white and t wlg wire, then turn on t car let t system check , start t car let it runa still no ac. im sure its t ecu but share your concern could 2 be faulty, but rave says you must initialise t hevac with t becm so i dont know,

if the unit you have fitted has faults it will keep them logged untill it has them cleared buy testbook or another tool

mine didnt keep any faults, well none that stopped it working. i know they usually do a self test when they start up!

Mine has an open book symbol on mine, but believe that maybe due to no aircon belt connected

let me know how you get on as i may change mine from another vehicle as some of the temp leds don't light on certain digits

ya , will let you know whats t outcome, but it may be the end of next wk,miles from a diagnostic.

Land Rover designed a new wiring loom to the AC pump to get round an issue that you seem to be having. It would seem on some vehicles the original is not up to the job so you dont get sufficient voltage to the clutch, a replacement adds a feed direct from, I believe, relay 10 in the main fuse box. May be worth some investigation. The ECUs are a direct replacement, that dont need syncing. Also try removing the nut on the front of the compressor pulley, remove washer, replace nut. This takes up the wear in the clutch and helps it engage.

Keep us posted.

thanks rewmer. iknow about t wiring loom but i thought that only applied to the t early ones, mines a 99. i even checked t fuse and t relay box, relay # 10 is not present but the connections are there , so i put in a relay and t corosponding maxi fuse and still nothing. i have also checked out t clutch issue, but im not getting any voltage at all down t b y wire, and according 2 rave initialisation is need after replacement. thanks for t reply, will let you know how i get on. by t way t ac still wont come on but t test book light is gone now also, just back from a 7 mile trip after disconnecting t battery for an hour 2 see would it reset anything, it did not.

just to let you know, have done some more fault finding, and still nothing, checked all t sensors manually and all seem fine, ambidient reading t outside temp correct, eva temp has continuity, solar sensor corrosponds 2 light and dark , heater matrix temp is new but reads t same as t old one and t internal air sensor s fan is spinning, still checking but it looks like i need the plug in.