Heat Output Test Information And Tools Required
Special tools and workshop equipment required
- Vehicle Diagnostic Tester
- A Standard 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 test for the "Heating Output of Heater/A/C System, Checking" function as described in the Guided Fault Finding using the Vehicle Diagnostic Tester in the"Guided Fault Finding" function.
If the coolant circuit is not bled completely after being filled, then air can get into the heater core for the heater in the heater and A/C unit. 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:
-- Perform a road test at an increased engine speed (at least 10 minutes, engine speed above 2500/min). While doing so, select a lower gear/position to prevent excessive vehicle speed and set the heating and A/C system to maximum heating output.
-- 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 COOLANT SYSTEM/COOLANT [DAZA] / COOLANT SYSTEM/COOLANT [CNTC, CYFB] (Connection Diagram for Coolant Hoses).
For vehicles without an A/C system or with a manual climate control system ("Basic" A/C system) and Start/Stop function, the Stop function is prevented (for example, in heating mode) depending on the setting on the regulators and buttons (Front A/C Display Control Head 2 -E775-, Front A/C Display Control Head 3 -E776-, Front A/C Display Control Head 4 -E777- and Fresh Air and Recirculating Air Mode Button -E781-). As soon as there is a request for heating, the engine is started. In cooling mode, the Climatronic Control Module -J255- does not limit the Stop function.
If there is no Humidity Sensor -G355- installed on vehicles without an A/C system or with a manual climate control system ("Basic" A/C system version), a permissible stop time is currently calculated by characteristic curves that are influenced by the measured exterior temperature, the "recirculating air mode" function and operating condition of the windshield wipers. The calculated time must be greater than approximately 20 seconds so that the Stop function is permitted by the Climatronic Control Module -J255-. 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.
For vehicles with an automatic climate control system ("Convenience" A/C system version) and Start/Stop function, the Stop function is prevented depending on the setting on the regulators and buttons (Front A/C Display Control Head 2 -E775-, Front A/C Display Control Head 3 -E776-, Front A/C Display Control Head 4 -E777- and Defrost Button -E782-). 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 also applies if the difference between the selected specified temperature and the measured actual temperature exceeds a certain value in heating and cooling mode. Use the Vehicle Diagnostic Tester in the"Guided Fault Finding" function.
- Depending on the engine version, an After-Run Coolant Pump -V51- may be installed in the coolant circuit to support the engine coolant pump. Refer to COOLANT SYSTEM/COOLANT [DAZA] / COOLANT SYSTEM/COOLANT [CNTC, CYFB] (Connection Diagram for Coolant Hoses) and the appropriate 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. Refer to COOLANT SYSTEM/COOLANT [DAZA] / COOLANT SYSTEM/COOLANT [CNTC, CYFB] (Connection Diagram for Coolant Hoses) and the appropriate 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 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.