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P38 front windscreen problem.


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#1 j55ptk

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Posted 20 December 2009 - 02:41 PM

Hello again, it was minus 4 celcius this morning and I have discovered my front windscreen doesnt defrost. The screen has power to both sides of the screen and my test light lights up when linked between the screen earth wires and earth. I have cleaned the earth point and still the screen wont defrost. Does anyone have any ideas? Tim.

#2 David Sparkes

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Posted 20 December 2009 - 04:06 PM

I assume you had the engine running when you did the tests.
CIRCUIT OPERATION
This operates only with the engine running, which is determined by the HEVAC panel. In the off state, the signal is at 12 volts. When the Heated Front Windscreen is activated, the signal is switched to 0 volts. This provides the ground required for relay 5 to be pulled in, as there is an ignition feed on the other side of the coil provided by relay 15. This relay energises one half of the heated screen elements.
Once relay 5 has been energised, a 12 volt signal is provided to the coil of relay 4. The other side of the coil is connected directly to ground, thereby energising the relay and switching on the other half of the elements.
There is also a feed spliced from the harness which provides the petrol Engine Control Module with a signal that tells the unit there will be a large electrical load turned on/off.


.... my test light lights up when linked between the screen earth wires and earth. ...

If the wires were connected to the screen at the time, the test light lighting is wrong, as it shows a voltage difference exists between earth and screen earth connection.

I have cleaned the earth point and still the screen wont defrost. Does anyone have any ideas? Tim.

Run a temporary earth between another earth point (like the battery negative terminal) and at least one of the screen earth points. Now switch the screen on.

#3 vougese39

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Posted 29 December 2009 - 07:26 AM

you could have a failed screen which is the more coman problem have the hevac ecu fault tested if it has a fault with the system it can also be forced to work and show a fault too
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