Heat won’t turn off

DaveH, this is your first post and you have even introduced us to your car, model, and year. Love to help, but I don't even know your model year.

 

I'll guess it's a 355 in which case it either motor which you say you tested and is good or the HVAC computer which is know to fail in that mode. There is a remote possibility that it is the outside air temp sensor in the driver's side mirror. Since you tested the motor I suggest you close the valve by applying 12V of the correct polarity and it should stay closed. I've had the same problem. To be sure you really need to have a shop with an SD1 run a diagnosis.

Here is a thread on the 355 problem.

https://www.ferrarichat.com/forum/threads/f355-hvac-ecu.526186/

 

Put a shut off valve in mine. Ice cold a/c .
In winter turn the valve.
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The 355 HVAC controller goes off wandering and needs to be reset. For resetting, you shut off the power and turn it back on. Sometimes that works. Sometimes you need to push a sequence of buttons, and no one knows exactly that the sequence is. For the brute force solution when you only want AC, you do what F355bob did, or you bypass the heater from the coolant circuit. Then you only get cold AC.

 

Great advice. LOL.

 

I've gone thru two ECUs checked the outside air sensor in the mirror, the control panel and still can't get it off defrost so i gave up and put in the valve. I think most stock valves leak a little hot water diluting the cold air. With the valve closed I get really cold air. In the colder season I open the valve to get warm air
Not exactly high tech but works well.

 

That would be a different problem if you can't get it off defrost. In my case, and the situation linked, the HVAC control works correctly. It just that the hot water valve would not close. So, for example, driving around on a nice day with recirculation flap open, I got hot air coming in. Fortunately I could close the recirculation flap to stop the heat until I was able to close the valve. Have you checked the motor under the dash which controls air flow distribution?

 

Yes checked it. My water valve does not work nor does the air diverter. Checked all motors. Have not checked inside cabin temp sensor located at drivers left knee
That part of the dash needs to be removed to get to it so never got around to check it. I suppose if that sensor failed
It would default to warm air and air out of the defrost.

 

If you ever get around to checking it, here is a table of what the resistance of the various sensors should be vs temperature. Image Unavailable, Please Login

 

Great. Thanks

 

The valve also has a built in potentiometer which tells the AC ECU the position of the valve. If the pot fails, the ECU will stop operating the valve motor. The middle 3 pins are to the potentiometer which can be used to test it.

 

Perhaps I'm missing something, but could you elaborate on which of the three inner pins to test? Three pins give three possible pairs of pins, right (2-3, 3-4, 2-4)?

 

Sorry, I should have given more details. The three middle pins in the motor socket, 2-3-4 (looking from any side) are the usual three points of a potentiometer - a fixed value resistor between 2 and 4 with a moving resistance point (3) in between. As the potentiometer is rotated in one direction, the pin 3 will, resistance wise, move away from pin 2 and get closer to pin 4 (and vice versa). To test the potentiometer, first connect an ohmmeter between 2 ad 4 - you should get a reading of 5.5 Kohm, irrespective of the position of the valve. Then, connect an ohmmeter between 3 and 2 (or 3 and 4) and, using a 12V supply on the pins 1 and 5 (one polarity opens the valve, the opposite polarity closes it), run the motor between closed and open positions and watch the resistance change (up or down depending on the valve/motor direction of rotation); the resistance change should be smooth (between about 0.5 to about 5 Kohm) without any momentary open circuits. It is better to use an analogue ohmmeter for this test as you can easily see if the needle is suddenly jumping to open circuit at some points.

One thing to be careful about - when running the valve motor using a battery on the pins 1 and 5, you should be careful not to hit the stops by the rotating bar positioned between the motor and the valve as it will stall the motor (see thread "Hot wiring" the 348 heater valve?").

 

Thanks, m.stojanovic, that is really helpful.