Pre Facelift D2 Retro Fitted Heated Front Windscreen

[ikillcopiers]

Hi all,

On the advice of Notorious, who's really helpful, I'm posting here in the forums on this query.

I have a Pre Facelift Audi S8 D2, in which I am about to fit the front heated windscreen from the Facelift model.

We have ordered the screen from Audi, and I have the heater module/ecu/amplfier from A8Parts.

Notorious had a lot of useful information on running the wires up to the screen, and how he connected in to his car.

However, one little niggle I have is how to activate the heater module.

There are two wires that connect in to the HVAC in order to activate it, before I start experimenting does anyone have any experience as to how these are used to activate the heater module?
Is it just a case of a simple switch to ground it out, or does it need 12v, or does it use a signal from the HVAC?

I was hoping to wire a switch in my dash similar to my rear heated window switch, in order to activate it manually.

Does anyone have any experience with such a setup, or the operation of the heater module?

Thanks!

[David's8]

It comes on either manually by selecting the appropriate screen heating button (which you wont have I guess) on the CC module or automatically when the outside temp is <5degC and the CC is in Auto mode. But how the switching is achieved I am not sure.

[notorious]

Hi David, on my FL it never comes on automatically. Only when I press the windscreen button and when outside temp is below +5C.
Perhaps I forgotten to code something in the HVAC module after I retrofitted the system into my car.
Or maybe because I keep the car in heated garage, so there was no situation for HVAC to switch it on automatically.

[notorious]

Information from ELSA for facelift cars:

Functional description of heated windscreen
* The heated windscreen control unit -J505 is actuated by the operating and display unit -E87 (=>Read measured value block, Display group 019)
* After every engine cold-start if the ambient temperature is less than + 5°C and "Auto" mode is set on the operating and display unit -E87. The temperature at the -E87 must be set so that the air conditioner is in heating mode, and neither of the two vent temperature senders (-G150, -G151) should measure more than + 35°C. The heating period lasts up to 4 minutes, depending on the ambient temperature.
* When "Defrost" mode is selected on the -E87 if the ambient temperature is less than + 5° C. The heating period lasts between 2 and 4 minutes, depending on the ambient temperature. The indicator lamp in the "Defrost" button flashes while the heated windscreen is switched on. The heated windscreen is only actuated when the engine is running.

The heated windscreen control unit -J505 -B- switches on the windscreen heating if the following conditions are met.
* The operating and display unit -E87 activates the -J505.
* The voltage at the control unit -J505 is greater than 12.70 V.
* The control unit -J505 is not overheated.

The electrical resistance of the metal foil in the heated windscreen -Z2 is approx. 2 ohms. To enable the heated windscreen to attain the maximum possible heating power of approx. 1000 W (Watts), a voltage greater than the vehicle electrical system voltage is required. The heated windscreen control unit -J505 converts the vehicle voltage into a variable voltage which, depending on the vehicle voltage, is between 25 and 48 V.

To prevent the vehicle electrical system from being overloaded, the -J505 regulates the heating power of the heated windscreen. At a vehicle voltage of 12.70 V the heating power is approx. 200 W, and it is regulated steplessly up to 1000 W at 13.10 V. To prevent the vehicle voltage from collapsing when the heated windscreen is switched on, the -J505 permits the specified power output to rise by a maximum of 200 W per second.

During activation of the heated windscreen the voltage at the fresh air blower -V2 is limited by the operating and display unit -E87 (to approx. 4V in "Auto" mode and to approx. 7V in "Defrost" mode).

Disabling heated windscreen
* If a crack develops in the windscreen as a result of some external impact such as a stone chip, the heated windscreen is disabled to prevent the crack from growing.
* The operating and display unit -E87 does not activate the heated windscreen control unit -J505 in "Econ" mode.
* So as not to restrict the functioning of the air conditioner, the heated windscreen control unit -J505 must be disabled until a damaged heated windscreen -Z2 is replaced. To do so, unplug connector -A- from the control unit -B-.

[notorious]

On FL cars there is only one wire (yellow/black colour wire) that connects the heated windscreen with HVAC module (the one that you disconnect to disable heated windscreen functionality from above ELSA article).

[mattylondon]

Originally Posted by notorious

I didn't find information in ELSA about communication 'protocol' on that wire between HVAC module and the heater module in the boot.

It's a bi-directional wire with a wave form of the heating demand. Be great to get an oscilloscope on there just out of interest.