Heat Output Test Information And Tools Required
Special tools and workshop equipment required
- Vehicle Diagnostic Tester
- A commercially available thermometer (for temperature measurement; if necessary, a thermometer with two measuring probes for simultaneous measurement, for example, for temperature on the right and left)
This service information only outlines the testing procedure. Perform the detailed function test for the heating as described in the Guided Fault Finding using the Vehicle Diagnostic Tester in the "Guided Fault Finding " function.
If the coolant system is not bled completely after being filled, then air can get into the heater core for the heater. This will reduce the heating output. Noises may result or the customer may complain that there are different air temperatures coming out of the vents.
Corrective Measure for Poor Heating Output or Noises from Coolant Circuit:
- For vehicles without a high-voltage system, perform a test drive at high engine speed (at least 10 minutes, engine speed above 2500 RPM), while selecting a low gear to prevent excessive vehicle speed.
- If the customer complains of poor heating output at certain engine speeds, check the incorporation of the heater core for the heater in the engine coolant circuit. Refer to CONNECTION DIAGRAM - COOLANT HOSES (Connection Diagram for Coolant Hoses).
- If the customer complains of poor heating output on a vehicle with a high-voltage system, check the incorporation of the heater core for the heater in the coolant circuit as well as the activation and the function of the different shut-off valves, coolant pumps and the High-Voltage Heater (PTC) -Z115- before performing the heating output test using the Vehicle Diagnostic Tester in the "Guided Fault Finding " (for the A/C system and the engine control module). Refer to High-Voltage Heater (PTC) -Z115-, INCORPORATION IN COOLANT CIRCUIT and CONNECTION DIAGRAM - COOLANT HOSES (Connection Diagram for Coolant Hoses).
For vehicles without an A/C system with a Start-Stop System, the stop function is prevented depending on the setting on the Heater Control Module -J65- control head (for example, in heating mode). As soon as there is a request for heating, the engine is started.
In vehicles with a manually regulated A/C system and a Start-Stop System, the stop function is prevented depending on the setting on the A/C Control Module -J301- control head (for example, in heating mode). As soon as there is a request for heating, the engine is started. In cooling mode, the A/C Control Module -J301- does not limit the stop function.
If the A/C Humidity Sensor -G260- is not installed on vehicles with a A/C Control Module -J301-, a permissible stop time is currently calculated by characteristic curves which are influenced by the measured outside air temperature, the "recirculated-air mode" function and operating status of the windshield wipers. The calculated time must be greater than 20 seconds so that the stop function is permitted by the A/C Control Module -J301-. The stop time can be shortened at a lower exterior temperature, when the recirculating air mode is active and/or when it is raining to prevent the windshield and door windows from fogging up. If the calculated time for the Stop function is less than approximately 20 seconds, it is prevented.
In vehicles with an automatically regulated A/C system and a Start-Stop System, the stop function is prevented depending on the setting on the Climatronic Control Module -J255- control head. If, for example, "defrost" mode is selected, the Stop function is not possible or is canceled and the engine is started as soon as this mode is selected. The same applies in the case of heating and cooling. The difference between the set specified and actual temperature exceeds a certain value. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding " Function.
Depending on the different conditions, the coolant which flows through the heater core for the heater in the A/C unit is warmed either by the engine or by the High-Voltage Heater (PTC) -Z115- on vehicles with a high-voltage system. Refer to High-Voltage Heater (PTC) -Z115-, INCORPORATION IN COOLANT CIRCUIT , (Connection Diagram for Coolant Hoses) and use the Vehicle Diagnostic Tester in the "Guided Fault Finding " function (for the A/C system and the engine control module).
- Depending on the engine version, an After-Run Coolant Pump -V51- may be installed in the coolant circuit to support the engine coolant pump on vehicles with a high-voltage system. Refer to CONNECTION DIAGRAM - COOLANT HOSES (Connection Diagram for Coolant Hoses) and Wiring Diagram. The After-Run Coolant Pump -V51- can be actuated in "stop mode" (engine stopped) to maintain the coolant flow rate through the heater core for the heater on vehicles with a Start/Stop System.
- Depending on the engine version, a Coolant Shut-Off Valve -N82- may be installed in the coolant circuit on vehicles without a high-voltage system. Refer to CONNECTION DIAGRAM - COOLANT HOSES (Connection Diagram for Coolant Hoses) and Wiring Diagram. The Coolant Shut-Off Valve -N82- is then activated by the Engine Control Module -J623-, for example, when heating output is not requested by the Heater Control Module -J65- /A/C Control Module -J301- /Climatronic Control Module -J255- and the engine is still cold. Use the Vehicle Diagnostic Tester in the "Guided Fault Finding " function (for the engine control module). When the Coolant Shut-Off Valve -N82- is activated, no warmed up coolant can flow through the heater core for the heater.
- On vehicles with a high-voltage system different shut-off valves, coolant pumps and a High-Voltage Heater (PTC) -Z115- are installed in the refrigerant circuit. These components make sure that also in electric driving mode the vehicle interior (with the engine switched off) can be heated. Refer to High-Voltage Heater (PTC) -Z115-, INCORPORATION IN COOLANT CIRCUIT .