Burn-Off

HEATED-WIRE BURN-OFF
Deposits which influence the measurement may form on the heated wire in the intake-air flow. For this reason, each time the engine has been switched off, after a delay of approx. 4 seconds power transistor T is energized at a voltage of 5 V for a period of 1 second. Transistor T applies the service voltage UB = 12 V to the heated wire RH.
The delay is necessary to ensure that wire burn-off only occurs when the engine is at a standstill.

ELECTRIC CIRCUIT DURING BURN-OFF
When the ignition is switched on, the DME control unit receives a positive signal at terminal 27 and grounds the DME relay, terminal 85, via terminal 36. The DME relay closes and now supplies a positive signal to terminal 37 of the DME control unit. The injection valves, air flow sensor at terminal 5, tank breather and idle speed positioner also receive a positive signal via DME relay terminal 87. In order to trigger off the burn-off process for the heated wire, the following operating conditions must apply:

^ Engine was previously operating at an engine speed of above 2000 rpm.
^ Engine temperature above 60° C.
^ Ignition is switched off (DME control unit terminal 27 is de-energized).

A microprocessor integrated into the DME control unit maintains the ground signal for approx. 4 seconds at DME relay terminal 85 and therefore also the voltage, e.g. of the air flow sensor, via terminal 5.
Approximately 4 seconds after the ignition has been switched off, the DME control unit emits a signal to the air flow sensor, terminal 4, via terminal 25, and therefore to the transistor in the air flow sensor's electronic system. The transistor completes the circuit and energizes the heated wire for approx. 1 second with the voltage of 12 V at terminal 5 of the air flow sensor, as a result of which the wire is heated up to approx. 1000° C. After a total of 4 seconds, the ground signal is switched off by the microprocessor. The DME relay opens and the heated wire is de-energized