Coolant temperature is shown in °C. If applicable, compare with display instrument in vehicle. If value does not correspond check heater temperature sensor (at 25 °C 990–1010 Ω, at 80 °C resistance increases to approx. 1500 Ω).
After heater unit repair check following function processes:
1. During warm up phase (up to approx. 80 °C) heater unit runs on full load.
2. From 80 °C control unit switches heater to partial load (metering pump impulse slows down (once per second) and speed of combustion gas blower decreases).
a) With a lower heat requirement coolant temperature rises further (coolant circulation set to cab heating and vehicle engine running).
From approx. 90 °C control unit switches heater to stand-by (heater unit switched off and combustion air blower switched to run-on. Actual value 02: flame monitor becomes "dark"), temperature is therefore further monitored by control unit.
If temperature falls to approx. 75 °C control unit switches back to partial load.
b) With a higher heat requirement coolant temperature falls (coolant circulation set to engine heating). From approx.
70 °C control unit switches heater back to full load (actual value 02: flame monitor constantly "bright").