924 Tubo Enrichment Control


Watch polarity on electric plug connection (see sketch for positive and negative connections).

Wrong connections would destroy the temperature switch and also the control pressure regulator.

Diagram for Control Pressure Regulator

Control pressure "cold" (matching outside temperature)

Part No.: 477.133.403 Bosch Code: 0438 140 088

Auxiliary Air Regulator

The auxiliary air regulator is new. Part No.: 477.133.455 Bosch Code: 0280 140 104

Oxygen Sensor with Acceleration Enrichment

Switching Point


Acceleration enrichment

Acceleration enrichment and idle stabilization

Full throttle enrichment

When the throttle is opened with a cold engine and cold oxygen sensor an accelerating enrichment occures by changing the dutycycle of the frequency valve. This requires an additional microswitch (b) on the throttle housing and a new control unit.

3 microswitches are now mounted on the throttle housing, which have the following switching points and functions.

The control unit for acceleration enrichment is located next to the oxygen sensor control unit.. It also performs the function of the former speed relay (speed-controlled enrichment above 3500 rpm).

Oxygen Sensor with Acceleration Enrichment — Operation

With cold engine (temperature below 60 °C *) and cold oxygen sensor (temperature below 250 °C) the duty cycle will be switched from 50 % to 75 % when the throttle is opened, if angle positions 1° or 8° are passed. This produces an acceleration enrichment of 2.5 seconds each time. If the opening time of the throttle (from 0° to 8°) is less than 2.5 seconds, there will be enrichment only once. There will be no acceleration enrichment above 35 °C/60 °C.

* Temperature switch (installed in coolant return hose near cylinder no. 4) has a switching point of 35 °C or 60 °C.

< less than / > more than

Engine Temp.


Throttle Angle


When Throttle opened past 1° or 8° position

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