HEVAC ECU checks the coolant temp sensor after 5 minutes on
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Thread: HEVAC ECU checks the coolant temp sensor after 5 minutes on

  1. #1
    FRESHMAN ROVER
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    March 16th, 2016
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    Melbourne, Victoria, Australia
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    HEVAC ECU checks the coolant temp sensor after 5 minutes on

    Greetings from a wintery Melbourne, Australia.

    It's getting cold so it's time to get the heating working properly. My issue was that the car would go both to HI and LO on the temp controller but not hold correct temp in between. All checks done on the HEVAC test page and all seemed well. Blowers, fuse box and blend motors all OK. I fixed them all up a year or so ago. Temp distribution motors working OK. All flaps in the heater box working and free but the book symbol on.

    So I did a test: how long does it take for the book to light up? I ran four tests and timed it with a stopwatch: exactly five minutes each time, regardless of whether or not I moved any of the temp settings or adjusted the distribution motor. I figured the system is doing a test, but what's it checking? The coolant temp of course; five minutes is enough to get the coolant up to temp from cold.

    Bought a replacement from my wrecker Matar Motors in Melbourne (Derek; great guy for second hand parts) and now I have NO book symbol for the first time since owning the car. Yes I know. Small things amuse small minds. But it's fun to have the car working.

    Funny how I could find no other reference to the five minute self test.

    Let's look at the options:

    1) Drive the car with it not functioning correctly. Yuck! How low-rent and distasteful. These cars are grand. Get them going correctly with the help of RR.net.

    2) Sell the car and buy another car. Well if I wanted another car I would buy another car. Simple as that. I like this one, plus I have a late model Mercedes SL500. I believe automotive perfection is obtained with one serious four wheel drive and one serious sports car/cruiser. How could that situation be improved? Not possible.

    3) FIX the car properly with the help of RR.net and doing proper diagnosis by elimination or with the right tools (still thinking about buying a Nanocom. They're expensive and second hand parts are cheap).

    *****************

    What is the temperature sensor spike that slides into the heater box on the bottom; It's about six inches long. I couldn't find any reference to it. What does it do and do they fail?

    Tom
    Vehicle is a 2000 Vogue.

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  3. #2
    LEGACY VENDOR marty_nz's Avatar
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    Re: HEVAC ECU checks the coolant temp sensor after 5 minutes

    It must be a polling time for it to check for a reasonable change in the temperature to the heater core before it flags it as faulty.... somewhat clever!

    The temp sensor that goes into the heater box is the evaporator temp sensor for the AC system. It reports back to the HEVAC to the temperature of the evaporator unit when the AC is running. The HEVAC uses this information to cycle the AC compressor. From memory when the evap temp gets down to about 4 deg C the HEVAC switches off the compressor to prevent the evaporator from getting too cold and icing up. Once it reaches about 7 deg C, the HEVAC switches the AC compressor back on to cool again. It will constantly cycle like that whilst the AC is being used.
    ------------------------------------------------
    '01 4.6 Vogue Black/Lightstone "Snog" -- Bought Jan 2012
    '99 4.6 HSE Cobar Blue/Lightstone -- Sold March 2009
    '95 4.6 HSE Epsom Green/Tan -- Traded June 2008
    http://p38webshop.co.uk

  4. #3
    FRESHMAN ROVER
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    Re: HEVAC ECU checks the coolant temp sensor after 5 minutes

    Thanks Marty

    I knew you'd know about the AC sensor but surely I can't be the only one here who knew about the five minute test on the coolant temp sensor! Strange how I could not turn up any reference to it. That sensor was bad: it was open circuit. A good one should vary between about 10K down to 1K or so on the quick test I did on it before I put it back; it's just a thermistor in a potted epoxy pack.

    ************************************************** ************************************************** ********

    I suspect now my blend motors need to be recalibrated. The book symbol is gone but both sides now won't get hot - with the temp setting on 28 it's blowing only coolish/slightly warm air. Moving both temp setting to HI gets very hot air but I can't set a temp in between.

    Do I need to buy a diagnostic tool now to calibrate the blenders or can this be set without it? Surely it has something to do with calibrating the flaps with reference to the resistance of the potentiometers in the blend motors.

    Tom
    Vehicle is a 2000 Vogue.

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  6. #4
    LEGACY VENDOR marty_nz's Avatar
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    Re: HEVAC ECU checks the coolant temp sensor after 5 minutes

    To be honest, I didn't know about the 5 minute check for the HEVAC. But then, if someone comes to me with a HEVAC issue, the first thing I do is plug my Nanocom in and have a look at the faults and all the live data, so I can see if there is a sensor which is giving erroneous readings to start with!

    I did a measure on my good spare heater core temp sensor (which is on my HEVAC test loom actually) and whilst at least a couple of the other sensors are 10K NTC thermistors, it appears that the heater core sensor is actually a 20K, possibly 22K NTC thermistor. So thus showing 20/22K at 25deg C. I'm guessing they went for that value on the heater core as it has to generally read higher temperatures and is a bit more accurate - but who knows!

    Regarding the calibration - yes you need a Nanocom or similar LR specific diagnostics to go into the HEVAC and force it to recalibrate the blend motors. I believe it also automatically recalibrates them every 500 power cycles too - but that could take awhile!

    I don't know quite how the logic in the HEVAC works regarding heating/cooling to the desired temperature - but it uses all 4 temp sensors and the solar sensor to make it's decision. I think it will check the cabin temp sensor to see what it's currently at, and then also the ambient outside air temperature to see whether it needs to warm/cool the incoming air to reach the target temp. Then it would use the evaporator and heater core temp sensors to see how warm/cool the air is to decide how far to drive the flaps in either direction hot/cold to mix the air appropriately. Whereas the 'HI and LO' settings will just move the flaps to the extreme hot/cold with no temperature target.

    If the system is in fully 'AUTO' mode, then the HEVAC will also control the blower motor speed aswell... quite clever, but maybe also a bit over engineered in some respects!
    ------------------------------------------------
    '01 4.6 Vogue Black/Lightstone "Snog" -- Bought Jan 2012
    '99 4.6 HSE Cobar Blue/Lightstone -- Sold March 2009
    '95 4.6 HSE Epsom Green/Tan -- Traded June 2008
    http://p38webshop.co.uk

  7. #5
    JACK'S GRANDAD leftlanetruckin's Avatar
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    Re: HEVAC ECU checks the coolant temp sensor after 5 minutes

    I can attest to Marty knowing his HVAC stuff, even over the Atlantic!
    Between 2 or 3 of us, got the LSx P38 AC working great agiain...After jumping 2 wires.

    Martin
    '98 P38 HSE, with 385hp/385ft lbs of LSx under the hood. Fully functional HVAC, EAS, etc.
    '11 L322 SC
    '04 DiscoII - wheeling rig
    '02 P38, '79 924 for the kids.
    Custom fabrication, engine rebuilds and knowledge that comes from experience and not Google, always available for a reasonable cost.

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