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'85 QV - Heater Rehostat not working. Constant power to heater valve. What's next?

Discussion in 'Mondial' started by Albert Penello, Feb 15, 2020.

  1. Albert Penello

    Albert Penello Karting

    Jul 21, 2019
    133
    Woodinviille, WA
    Full Name:
    Albert J Penello, Jr.
    So I have no heat in my '85 QV Cabrio.

    Being familiar with older Mercedes, I pulled the heater valve expecting the rubber plunger to be damaged. Good news (I guess) is that the valve is in great shape.

    I tested the rheostat as listed in this post (https://www.ferrarichat.com/forum/threads/mondial-heater-not-staying-hot-please-advise.84386/) and am getting constant power regardless of the position of the heater knob. Sweeping from cold-to-hot has no change in the light.

    So it seems the problem is electrical and I'm not sure where to look next - or how to best test the rheostat to make sure it's functioning.

    I do have fully working gauge lights and the fan speed works fine. It's only the actual heater that's not working.

    Any ideas what to do next?
     
  2. Wade

    Wade Two Time F1 World Champ
    Rossa Subscribed Owner

    Mar 31, 2006
    28,795
    East Central, FL
    Full Name:
    Wade O.
    There's a bit of automation in the Mondial's HVAC system via the cabin temp sensor on/in the left side of the dash. What's the ambient temperature while you're testing?
     
  3. BillyD

    BillyD Formula 3
    Silver Subscribed

    Feb 28, 2004
    1,485
    Pacific Northwest
    Full Name:
    Bill
    Hey this is an easy fix. You live in the cold NW, the top is broken and there’s no sunshine so just leave the wires off the valve and you’ll get full heat. For the 2 days it’s warm up here take the Mercedes. :)
     
    Albert Penello likes this.
  4. Albert Penello

    Albert Penello Karting

    Jul 21, 2019
    133
    Woodinviille, WA
    Full Name:
    Albert J Penello, Jr.
    It was 58 deg in my shop when testing.
     
  5. Albert Penello

    Albert Penello Karting

    Jul 21, 2019
    133
    Woodinviille, WA
    Full Name:
    Albert J Penello, Jr.
    That's the current option but I like to get everything working!
     
  6. afterburner

    afterburner Formula 3
    Silver Subscribed

    Jun 20, 2008
    2,123
    Hong Kong
    With the car "cold" (not driven recently) you should be getting the above resistance readings from the 3 sensors. Wade is spot on - should these sensors tell the ecu it's already warmer than that, it won't turn the heater on.

    PS: the evaporator temp sensor doesn't enter the equation for your issue but you might want to test it anyway.
     
  7. Albert Penello

    Albert Penello Karting

    Jul 21, 2019
    133
    Woodinviille, WA
    Full Name:
    Albert J Penello, Jr.
    Afterburner thanks very much I have all this stuff apart right now tracing down other electrical problems.

    I'm new to Ferrari's would you mind a tad more instruction what I'm looking for?

    It sounds like I need to read the cold resistance of the 12c - 14c ranges for the Heater Temp and in Car Temp sensors at a minimum. Seems like it can be done at this plug?

    Where are the actual sensors to test and what are the right pins? Just a tad more direction would be helpful thanks SO much!
     
  8. Wade

    Wade Two Time F1 World Champ
    Rossa Subscribed Owner

    Mar 31, 2006
    28,795
    East Central, FL
    Full Name:
    Wade O.
  9. BillyD

    BillyD Formula 3
    Silver Subscribed

    Feb 28, 2004
    1,485
    Pacific Northwest
    Full Name:
    Bill
    Albert,
    Did you figure it out?
    Looks like page H35 and H36 of repair manual has the details for sensor diagnosis.
     
  10. afterburner

    afterburner Formula 3
    Silver Subscribed

    Jun 20, 2008
    2,123
    Hong Kong
    Alberto, the best way to check the sensors is to disconnect them and measure their resistance directly, thus eliminating any wiring issue. The are all accessible (somehow).
    cabin temp: on the left side behind the small grill on the lower dash panel. Two plugs, one for the suction fan, one for the sensor, accessible from below the dash
    HVAC exit air: in front of the outlet, right behind the dash, accessible from below the dash.
    evaporator temp: remove the grills in front of the windshield and remove the intake water shield.
    Alternatively, you can check them in the plug behind the pax footrest panel first. If all turns out well, no need to bother with the behind-the-dash fiddling.
     

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