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Thread: HEVAC headaches

  1. #1
    Join Date
    May 2008
    Location
    Camp Hill, Brisbane
    Posts
    168

    HEVAC headaches

    It all started with the dash removal project to do the O rings and fix LH blend motor which was generating check book symbol on HEVAC display. Blend motor had burned commutator and after a clean-up and re-assemble was running OK. O rings went smoothly. Check book still present, but Nanocom shows LH blower regulation fault, then over current fault. I removed, cleaned and desoldered some of the blower motor regulator PCB connections. Transistors and relay checked out OK.
    Still get check book faults which wander from distribution motor faults to recirc motor faults back to blend motor stall faults. I don't believe these are genuine, as I can see the LH blend motor do its self-check travel on start-up when the passenger side airbag is out and there was never a problem with any of the others prior. HEVAC does this traverse test on the motors at start-up error free, then throws the check book up each time.
    Have tried recalibration of blend and distribution motors with Nanocom but there is no message to say it was successful (or failed!).
    BTW, I 'clear faults' then remove key and restart and then 'read faults'. I presume I am clearing the logged faults properly as I get a new one most times!
    Any hints or advice appreciated.
    Cheers,
    Steve '95 HSE

  2. #2
    Join Date
    Jan 2016
    Location
    Netherlands
    Posts
    120
    I partially had the same problem you did. I had a faulty blend motor, like we all do tend to get eventually, and I replaced a defective one with a spare from a wrecker. Of course, within a few months the problems returned so I replaced them all with new ones. Unfortunatly the potentiometers were out of range a bit and that causes all sorts of havoc on the HEVAC unit. I replaced the HEVAC unit with a spare, replaced the blend motors with 3 different sets I had laying around until finally it worked.

    All I can figure is that the HEVAC is picky and has a bit more memory then we would like, even if you clear the error codes. If the HEVAC detects for only a fraction of a second that the potentiometer input is above (I believe) 125% and below 0% it simply shuts down that blend motor, even manual override no longer works. In orde to prevent this from happening you should try and set them by hand with a power source directly on the wires of the motor (opening them up as a last resort) and mounting the motor inside the heater block so as to prevent over running.

    Come to think of it, perhaps they named the hevac wrong and it was supposed to be called havoc? :P

    Cheers and good luck!

    -P

  3. #3
    Join Date
    May 2008
    Location
    Camp Hill, Brisbane
    Posts
    168
    Quote Originally Posted by prelude View Post
    . In orde to prevent this from happening you should try and set them by hand with a power source directly on the wires of the motor (opening them up as a last resort) and mounting the motor inside the heater block so as to prevent over running.
    -P
    Thanks!
    I can drive them with an external power source, but what adjustment can I make without opening them up and re-positioning the toothed gears?
    Is there a documented 'ideal' voltage or resistance range for the feedback pots?
    Cheers,
    Steve

  4. #4
    Join Date
    Jan 2016
    Location
    Netherlands
    Posts
    120
    Hi Steve,

    When mounted on their respective positions the motors are limited in their travel due to the end points of the flaps. Letting them run whilst not mounted can give the motors a bit more travel and you can get overshoots. The chinese copies (cheap alternatives) have a bit of a tolerance problem at times has been my experience. Anyway, the pot meters are 10K ohms iirc and 0% is pretty close to 0 ohms, depending on their calibration and 100% is about 9K ohms+ I can see if I can figure that out for you with a spare unit I have laying around. I had to open one of those chines buggers up and fiddle a bit with the potmeter, it was mounted slightly out of whack...

    Mind you that overshoots can occur on dirty or worn pot meters, just a fraction of a second of no contact on the pot and you get a very high reading, immediatly kicking the HEVAC into maint-book-mode.

    Powering them externally whilst reading the position value with something like a faultmate (be carefull not to put power on the motor whilst the cable is plugged into the hevac!) and putting them half way should provide your system with the "perfect" starting position for calibration. it will usually first travel up and then down resetting the high point first and then resetting zero. Since this last point gives the most trouble, at least it did in my case, making sure the motor doe not cross that point until the last, gave me my break

    also, it has been said over and over, but recent trouble with my cruise control reminded me once more, our cars REALLY love their voltage levels to be near exact all the time! if you read 13volts on your faultmate, bets are that a lot of components just won't like it and are more prone to error. If you have one of those power supplies used by CB and HAM operators, a beefy thing that can do a regulated 13.8v and stick that on your battery terminals you may be able to satisfy the BECM without having to run the engine all the time.

    Hope this helps

    -P

  5. #5
    Join Date
    May 2008
    Location
    Camp Hill, Brisbane
    Posts
    168
    Quote Originally Posted by prelude View Post
    Hope this helps
    -P
    Yep, will probably dust off an old analog ohmmeter now, as I just looked at the pots on the DMM and it all 'seemed' ok.
    Thanks.
    Steve

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