Hot Wire Sensor
The hot wire sensor is installed between the air cleaner housing and throttle housing. This hot wire air sensor has a 6-pin plug connection for electric power supply and signal output. An adjusting screw on the air sensor housing is connected to a potentiometer of the air sensor electronics and is used to adjust the CO. The air sensor is inserted in the air guide housing and sealed off on the air cleaner lower section with a rubber sleeve.
The air sensor unit consists of a cylindrical housing. The air openings of each opening have a protective grille to guard the measuring device against damage. In the flow direction there is a temperature compensation resistor, hot wire element (Platinum wire) and precision resistor in a cylindrical measuring port. The necessary electronic regulating circuit is contained in a cast on regulator housing and has a plug connection for the wire harness plug. Potentiometer for adjustment of CO is also located in the regulator housing.
Compensating resistor (RK), hot wire (RH) and measuring resistor (RM) are wired together with a balancing resistor (R(2)) to make up a bridge circuit. The bridge is balanced with resistor (R(2)) which takes on a value which is equal to a temperature difference of approximately 212°F (100°C). Hot wire temperature is maintained regardless of the air temperature.
Regulation is accomplished with a variable gain amplifier, which compares the voltage at inlet and outlet of the hot wire and makes corrections when necessary. Since the heating current (IH) flows through measuring resistor (RM), the current change (air mass change) can be taken via this resistor.
Deposits which could influence the resistance and/or measuring results could settle on the hot wire located in the intake air stream. For this reason each time the engine is stopped and after a delay of approximately 4 seconds, the power transistor charges the bridge circuit for approximately one second. The hot wire is heated to a temperature of approximately 1400°F (1000°C) and is burnt free of deposits. Electric power is supplied via main relay (XXV) and the CEB.
The 4 second delay is necessary so that if the engine is started immediately after stopping, fresh air will not be fed over the hot wire. This could reduce the service life of the hot wire. Should the hot wire fail, the LH-Jetronic system has an emergency operation mode. Injection time is lengthened for a richer mixture by the electronic controls.