Auxiliary Heater Heating Element -Z35- Design And Function

WARNING: This page is about a different variant/trim than selected.

The Auxiliary Heater Heating Element -Z35- is comprised of several layers of resistance elements -E and F-. These can achieve a heating output of approximately 1000 W via all three -A through C-. These resistance elements have a positive temperature coefficient (the resistance increases with the temperature and current pick and the heating output decrease).

Fig 1: Auxiliary Heater Heating Element Is Comprised Of Several Layers Of Resistance Elements -E- And -F-
Courtesy of AUDI OF AMERICA, LLC
The resistance heating elements (-E and F-) are activated via the cables -A through C- ; the current flows through the ground connection -D-.
The Auxiliary Heater Heating Element -Z35- is designed so that a current greater the 35 A can flow via the contacts -A through C- when switched on. Immediately after switching on, the resistance on the resistance heating elements -E and F- increases and the current goes down.
To reach the maximum heating output of approximately 1000 W (3 x 333 W), the resistance heating elements temperature must be low and the Generator -C- must deliver enough current.
The wire -A- provides the voltage to the resistance heating elements -F- via the Low Heat Output Relay -J359-. Refer to the appropriate Wiring Diagram.
The wires -B and C- provide the voltage to the resistance heating elements -E- via the High Heat Output Relay -J360-. Refer to the appropriate Wiring Diagram.
The resistance of the heating elements depends on the temperature; The Resistance Is Approximately 0.2 To 5 Ω ; measured between the connections -A through C- (in the connector) and the ground connection -D-

NOTE:

To Measure The Resistances And To Check The Activation Of The Auxiliary Heater Heating Element -Z35-, disconnect the connector from the A/C unit. Refer to Auxiliary Heater Heating Element -Z35-, REMOVING AND INSTALLING .

Since resistances smaller than 10 Ω can no longer be measured with sufficient accuracy using workshop tools, a short circuit in the Auxiliary Heater Heating Element -Z35- cannot be determined.

Activate the Low Heat Output Relay -J359- and the High Heat Output Relay -J360- to check the function, for example, "output diagnostic test mode" for the respective engine control module. When doing so, measure the voltage between the contacts in the mating connector to the connector on the Auxiliary Heater Heating Element -Z35- and the ground using the Vehicle Diagnostic Tester in the "Guided Fault Finding " function.

The engine control module/Auxiliary Air Heater Control Module -J604- activates the Auxiliary Heater Heating Element -Z35- in 3 stages. Contacts -A through C- are supplied with voltage as follows. Refer to the wiring diagram for the respective wiring. Refer to the appropriate Wiring Diagram.

-- For stage "1", the voltage from the Low Heat Output Relay -J359- is switched to the contact -A-

-- For stage "2", stage "1" is switched off and the voltage is switched to contacts -B and C- via the High Heat Output Relay -J360-.

-- For stage "3", stage "2" and stage "1" are switched on using the Vehicle Diagnostic Tester in the "Guided Fault Finding " function.

NOTE:

The Respective Control Module Constantly Monitors The Generator -C- Load. The Next Stage Is Selected If The Load Remains Under A Certain Limit. If The Load Exceeds A Specified Limit, Then The Stage Is Switched Down.

Switch-On Criteria for Electric Auxiliary Heater Activation

NOTE:

These Switch-On Conditions In The Climatronic Control Module -J255- A/C Display And Control Head Must Be Met So That The Request Can Be Sent To The Respective Engine Control Module.

-- The Climatronic Control Module -J255- A/C Display And Control Head Sends A Request To Switch On The Auxiliary Heater To The Engine Control Module As Long As The Following Requirements Are Fulfilled:

Engine is running for a minimum of 8 seconds and engine speed is greater than 500 RPM.
Engine temperature is less than 75 °C (167 °F).
The calculated outside air temperature is lower than 8 °C (46°F).
The electrical system voltage is greater than 12.2 volts and no request that forbids the switch-on is sent by Vehicle Electrical System Control Module -J519-.
There is no malfunction stored in the Climatronic Control Module -J255- A/C display and control head.
No DTC is stored in engine control module and the Generator -C- load is less than 30 to 77% (depending on engine speed).
Based on the setting and the measured temperatures, the Climatronic Control Module -J255- A/C display and control head has calculated that additional heat output is necessary to reach the desired passenger compartment temperature.
The Climatronic Control Module -J255- A/C display and control head has calculated that more than 90% of the air will be guided through the A/C heater core.

NOTE:

The Engine Control Module/Auxiliary Air Heater Control Module -J604- Activates The Auxiliary Heater Heating Element -Z35- (3-Stage) Via Both Auxiliary Heating Relays (Low Heat Output Relay -J359- And High Heat Output Relay -J360-), So That The Load On The Generator -C- Does Not Exceed 95% Using The Vehicle Diagnostic Tester In The "Guided Fault Finding " Function (For The Engine Control Module On Vehicles With A Diesel Engine Or The A/C System Control Module On Vehicles With A Gasoline Engine) And To The Wiring Diagram. Refer to the appropriate Wiring Diagram. Thus the next stage only is switched on if the Generator -C- load is less than 30 to 77%, depending on engine RPM.

Switch-Off Criteria for Electric Auxiliary Heater Activation

NOTE:

Activation Is Switched Off As Soon As A Switch-On Condition Is No Longer Met In The Climatronic Control Module -J255- A/C Display And Control Head, Or In The Respective Engine Control Module Or One Of The Switch-Off Criteria Has Been Detected.

-- The Climatronic Control Module -J255- A/C display and control head switches off the request to switch on the electrical auxiliary heater.

When one of the switch-on requirements is no longer met.
The calculated outside air temperature is higher than 11 °C (52 °F).
The Generator -C- load is greater than 95%.
The Climatronic Control Module -J255- A/C display and control head has calculated that less than 60% of the air will be guided through the A/C heater core (position of temperature doors)