Engine Compartment Fan: Description and Operation

1981 Engine-compartment blower

The engine-compartment blower on the engine compartment lid is activated in two stages.
Stage 1 (permanent operation) is activated by the fuel pump relay as well as a changer relay and a shutoff relay. Behind the fuel pump relay, a resistance lead (limiting the power to approx. 10 watts) leads to the engine-compartment blower via the shutoff relay.
In stage 2, the engine-compartment blower is activated directly by the Motronic control module via the changer relay and the shutoff relay (power approx. 33W) in dependence of coolant temperature, engine temperature and a ratio between intake and ambient air temperature. Stage 2 is also activated during sporty acceleration when detected by the Motronic control module.
Once temperatures fall below the respective temperature parameters, the engine-compartment blower is set back to stage 1.

If the engine compartment lid is opened while the engine-compartment blower is active, the shutoff relay deactivates the engine-compartment blower. After the engine is switched off, the engine compartment temperature is monitored for approx. 30 minutes. During this period, and depending on temperature, the engine-compartment blower may continue to run or start to run.

If no reduction of temperature is detected after 5 minutes of the engine-compartment blower running in stage 2, the temperature warning light of the coolant temperature display in the instrument cluster starts flashing. Additionally, a "Failure - engine-compartment blower" warning is displayed in the on-board computer.