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Thread: HEVAC - Hmmm

  1. #1
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    HEVAC - Hmmm

    So I had the book symbol on when I got the car. Pulled the filters out and gave the thing a good clean, new filters, new relays and book symbol gone, never seen it again. Ordered some waterproof 40A relays to move the blowers out of the fusebox and will be doing that soon, just need to find a diagram of the electrical part of the fusebox.

    The problem is what I believe the blend motors. On any temp setting other than LO I get full heat, on LO I get full cold with the AC on or just outside air with AC off. There is no middle ground. I can select air going to the floor or face etc but I don't think it's 100%, at full speed (sounds like a tornado) I think not all the air comes out of the vents. So my question is this. Are the blend motors out of sync? if so can they be brought back in sync? there is no bookmark symbol so not sure if there should be one if they are out of sync. If out of sync how do you align them again? Can it be one with a diag tool?

  2. #2
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    No solution for you I think, sorry, but some insight:

    blend motors are basicly servo's built into a gearbox with a variable resistor inserted into the housing. When the blend motor changes position, the gears also move the variable resistor to a different position. By putting a voltage on that resistor the HEVAC can measure the current/voltage and that way determines the position of the bled flap. The blend motors can be recalibrated by using one of the many P38 tools (I have a faultmate extreme for example).

    Having said that, it seems unlikely that the HEVAC would not throw a maintenance book if the servo's were out of whack. The reason for this is even if the variable resistor would read differently or somehow was slower than the rest of the cogs, changing the HEVAC from LO to HI would force the servo's to run through the entire range from open to closed blend flaps. If the resistor were to lag behind, you would never reach the "high" end and the servo would jam, the HEVAC would detect a high current (stall) and throw an error.

    I have recently had a similar experience though, it seems that the bled flaps never close properly and with a hot engine on a warm day, the air coming out of the vents seems pretty warm but not hot.

    Anyway, I know of no situation in which a fault as you describe would occur, without throwing an error.

    Cheers,
    -P

  3. #3
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    Mine has some similar symptoms (glad I don't have a lisp!). I have a full set of blend motors to fit, once the motivation hits. Apparently the plastic gears can strip once the blend flaps get stiff with age (I know that feeling!). I'm going to fix the display screen, and do the heater o-rings while I'm at it.
    -----
    You have the right to remain silent. Anything you say will be misquoted, then used against you.
    -----

    1999 Disco TD5 ("Bluey")
    1996 Disco 300 TDi ("Slo-Mo")
    1995 P38A 4.6 HSE ("The Limo")
    1966 No 5 Trailer (ARN 173 075) soon to be camper
    -----

  4. #4
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    ok so there are how many blend motors?

    so to re-define the current stat

    AC on with both controls (passenger and driver) to LO will give me full cold aircon
    AC off with both controls (passenger and driver) to LO will give me full cold with no aircon
    AC off with both controls (passenger and driver) to anything other than LO will give me full heat
    AC on with both controls (passenger and driver) to anything other than LO ... have not tried

    I guess its dash out and replace the blend motors and seals while at it? how many motors and is there an alternative?
    with the seals can I use push lock fittings? I have a tig welder if that helps, why were seals used in the first place? is to to give it movement or?

  5. #5
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    There are two blend motors, one on each side (passenger/driver). There is one motor that selects the outputs for both driver and passenger. Lastly there are two motors that select between inside and outside air (recirculation) on both blower motors. The last two hardly ever get used and hardly ever break in my experience, but in hot climates I guess they get used more often. They are quite a bit easier to get to as well The hevac motors come in a set of 3 that terminate into one plug

    There really is no alternative. Some people hack away the internal ducts (those that go to the side windows) to get to the blend motors without having to take the dash out, I have found that to be an ugly hack. Removing the dash is a bit more work admittedly...

    The seals are used because you push aluminium tubes into a plastic housing and you need to seal that with rubber. Movement is hardly and issue but you cannot weld it. There is a hack using an audi A6 heater core and a view pieces of rubber hose with clamps. I am no fan of this solution since you introduce a new metal to the cooling system and the softest metal always get's eaten up first due to electrolosys. (they are made of copper). Also, you need to slightly cut into the HEVAC housing but that's minimal iirc.

    It took my P38 nearly 20 years to develop problems with the HEVAC and I reckon that replacing it all this once will set me up for at least another 10 so I am not worried. Just don't cut corners and try and do it on the cheap and you'll be fine

    Good luck mate!

    -P

  6. #6
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    thanks, I guess I will order some O rings and two new blend motors and do it right, I design large scale computing environments for a living (think 30.000 hosts) so i'm not used to cutting corners.

    is it worth in the mean time getting a U pipe with some clamps in the spares box in case I have to bypass the heater box on the move? or does it give you a fair bit of warning before it lets go?

    also is it definitely the blend motors that is faulty in my case? is there a way to test them? are they like stepper motors with a potentiometer for reference? if so I could whip u an arduino board and test them

  7. #7
    TheTree is offline ChatterBox Silver Subscriber
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    Quote Originally Posted by DavidP38 View Post
    thanks, I guess I will order some O rings and two new blend motors and do it right, I design large scale computing environments for a living (think 30.000 hosts) so i'm not used to cutting corners.

    is it worth in the mean time getting a U pipe with some clamps in the spares box in case I have to bypass the heater box on the move? or does it give you a fair bit of warning before it lets go?

    also is it definitely the blend motors that is faulty in my case? is there a way to test them? are they like stepper motors with a potentiometer for reference? if so I could whip u an arduino board and test them
    The other option, which many of us have done, is to use an Audi heater core and do away with the O ring BS altogether. Some people carry a U pipe and clamps for that purpose.

    The motors are servos with a pot ... you can read more here

    Blend Motor Diagnosis and Replacement

    and here

    Range Rover P38 Maintenance repair improvements and tips learned by experience by ownership.

    I am sure you could whip up an ardunio board to test them

    BTW there are 3 "blend motors" and 2 "reticulation or recirculation motors" one on each blower motor. Any one of these five can give you the infamous "chequebook symbol"

    Regards
    Steve
    MY96 4.6 HSE P38. Niagara Grey; "The Phoenix"
    MY99 4.6 HSE P38. LPG, Blenheim Sliver; "Voldemort" burnt to a cinder
    MY79 RR Classic, Gone, MY57 Series 1 SWB, Long gone
    P38 write ups and info on my website http://p38.hts.com.au/

  8. #8
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    Quote Originally Posted by DavidP38 View Post

    is it worth in the mean time getting a U pipe with some clamps in the spares box in case I have to bypass the heater box on the move? or does it give you a fair bit of warning before it lets go?
    Yes, I have used this for some time before I got to the o ring repair. As Prelude said, it is best to remove the dash for any blend or distribution motor work and if you are doing the o rings it is the best option I'm afraid.

    Quote Originally Posted by DavidP38 View Post
    also is it definitely the blend motors that is faulty in my case? is there a way to test them? are they like stepper motors with a potentiometer for reference? if so I could whip u an arduino board and test them
    Sounds like faulty potentiometers which are not allowing mid-range excursions to occur. Usually, this would result in a HEVAC fault and the system would shut down the offending motor, but (almost) anything is possible with this system. The motors can be driven with a 6 volt source and the pots removed and tested with an analog ohm meter. They will also fault the HEVAC if they present an over-current spike which can happen with older motors in particular. When you couple this with my faults in the recirc and blower motors, I decided to start rebuilding the whole system. It is a work-in-progress ATM!
    If you wanted an Arduino to drive the Valeo motors you would need to both drive the motors and sense with an ADC channel the potentiometer position and implement the feedback loop i.e. write code to emulate what HEVAC does anyway. Since they are over-priced, obsolete rubbish, I have taken a different approach.
    I have removed blend and distribution motors altogether and put RC servos in their place and have run them on a breadboarded Arduino. This gets you out of the coding problem by using PPM servo libraries already in the IDE. It all still needs to be integrated with the A/C sensors and packaged up, which has been big job especially when the radiator, brake booster, and now camshaft sensor intervene with their own demands!
    HTH
    Steve
    '95 HSE

  9. #9
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    ta, i guess I will have to pull the dash out and have a better look

    i think he system LR made is not bad at all, it's actually genius for the time but doing some more reading you realised they used cheap parts (not the engineers fault, think bean counters) and that is the problem. I have seen photos where whey used cheap potentiometers and cheap plastic which warps (flaps) with time and makes servo motors fail.

    I think when time permits I'm going to pull dash out and have a look at the flaps and see where it binds, replace old potentiometers and re-grease everything, it's a 95 year model so for over 20 years is been a pretty good run!

  10. #10
    TheTree is offline ChatterBox Silver Subscriber
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    Quote Originally Posted by DavidP38 View Post
    ta, i guess I will have to pull the dash out and have a better look

    i think he system LR made is not bad at all, it's actually genius for the time but doing some more reading you realised they used cheap parts (not the engineers fault, think bean counters) and that is the problem. I have seen photos where whey used cheap potentiometers and cheap plastic which warps (flaps) with time and makes servo motors fail.

    I think when time permits I'm going to pull dash out and have a look at the flaps and see where it binds, replace old potentiometers and re-grease everything, it's a 95 year model so for over 20 years is been a pretty good run!
    Hi

    I found that the flaps in mine did not bind, in fact they had about 10mm of flexible rubber around all the edges. The problem was the old grease on the shafts had gone hard, judicious use of Inox and a toothbrush got rid of much of the old grease, I then used a short squirt of lithium grease and the flaps moved very freely once again

    Steve
    MY96 4.6 HSE P38. Niagara Grey; "The Phoenix"
    MY99 4.6 HSE P38. LPG, Blenheim Sliver; "Voldemort" burnt to a cinder
    MY79 RR Classic, Gone, MY57 Series 1 SWB, Long gone
    P38 write ups and info on my website http://p38.hts.com.au/

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