Fuel levels toggle back and forth between just "a little rich" and just "a little lean"

             Idle                                                  Partial Open Throttle

The toggling speed of the meters are a good approximation of my 84 Porsche 944. You will notice that the toggling is much slower when the car is idling, faster at part throttle. At wide open throttle (WOT), the O2 sensor is not used by the DME, there will be no toggling and the meter will show rich.

Description - How the DME monitors oxygen levels in exhaust gas

This Motronic Lambda system takes the raw Lambda (l) sensor input and converts this signal so that a computer can understand it. Some computer programming and flow charting will be discussed. Oxygen sensor and O2 sensor are other commonly used names for the Lambda sensor.

The Lambda is a minor system in the Motronic. It is an option, an add-on. The first of the European & ROW Motronic equipped cars had no Lambda capability. It is not a critical system. It is a smog device and does not enhance engines power. It does just the opposite. What the Lambda system does is fine tune the fuel to maintain an ideal air to fuel ratio of 14.7:1. The Motronic ECU meters out fuel according to preprogrammed fuel maps. The Lambda system then makes minor fuel level changes with feedback from  in the exhaust gases (oxygen levels).

<---  Voltage Divider Circuitry

BOSCH, the designers of the Motronic fuel injection systems, defines the following levels:

V_Lean = +0.45V
V_Reference =  +0.475V
V_{Rich =} +0.50V

A simplified Lambda circuit for an early Motronic ML1.2 is shown below. At left is the voltage divider section made up of four precision resistors between +5 Volts and ground. This (V_l) increases and passes though each of the threshold voltages, the output of one of the comparators will changes.

Strategically between V_Rich and V_Lean, is a midway point called V_Reference. This leaves a ±25mV wide window between lean and rich. If the Lambda sensor is not connected, the input to the comparators will remain at V_Reference. If the Lambda sensor is present, the comparator inputs will toggle back and forth between 0V and 1V.

The Lambda System Schematic

This schematic has been simplified. The Lambda sensor connects to R801, and with C801, make up a low pass filter. Noise frequencies above 1Hz are filtered out. Through R808, this point is connected to V_Reference. R802, R803, R804, and R805 make up the voltage divider as previously described. Each comparator is wired to trip at a different threshold voltage. The comparator outputs go to the 1802 processor.

EF3 and EF4 are hardware interrupts. They set a flag that the processor routinely monitors. Once alerted to the flag, the processor investigates the cause. Opposite to what is commonly thought, the processor does not constantly check the Lambda's status. The Lambda circuitry tells the processor when there is a change in status and then the processor gets involved.

The outputs of the two comparators change as the inputs transition from rich to lean and back again. The following table shows the binary equivalent of the Lambda output voltage. This is how an analog input V_l is converted to a two bit binary number.

Lambda Voltage vs. Time

This is an ideal representation of the Lambda out voltage vs. time. This is a engine at idle. At part throttle, the repetition rate is about double. T_D is dead time. During dead time, all fuel level adjusting activities stop. Dead time is about double for an idling engine than at partial open throttle. It is less demanding at idle.

This is a less ideal diagram but more realistic. The V_l oscillations, shown here between the two threshold voltages, is from "cylinder scatter". Differences in each of the four cylinders can cause the Lambda voltage to jump about and even go in the reverse direction that the Motronic is trying to correct to.

Starting with a slightly lean condition, the output is 11_binary. The fuel is increased while changes in the output is monitored. As the output changes to 10_binary, we enter the 50mV neutral area and continue incrementing the fuel level. When the output changes too 00_binary, the fuel incrementing stops. There is a wait period. After the wait period is passed, the fuel level is decreased incrementally from 00_binary, passing through 10_binary, and 11_binary and again it stops for a wait period. This cycle continues endlessly.

Lambda Two Level Control Process

The basis ML1.1 system, the early Euro 944, has no Lambda system. The air flow sensor and speed sensor are used to determine fuel flow from a map that is stored in system memory.

Next to it is the ML1.2 Lambda system flow chart, the early USA 944, which has a Lambda sensor (O2 sensor). The fuel level is a two level control process, map and Lambda control. The Lambda system also uses the air flow sensor and speed sensor to determine fuel flow from a map. The Lambda control makes positive or negative changes to the fuel level in order to fine tune the fuel output. The range of the Lambda control is quite limited which requires the fuel map to contain values very close to Lambda=1 (air/fuel ratio 14.7). Comparing the maps of a non-Lambda (Euro 944) and Lambda system, the Lambda maps will appear flat and unimpressive. The power is in the non-Lambda map.

The fuel map is always used with a Lambda system. One common but false saying is that the fuel map is no longer used when the car is equipped with an Lambda sensor. This is far from the truth. Where does the original fuel value come from? Is it a guess? No. Any change in load or engine rpm, the DME must go to the fuel map to get its initial value. Then the fine tuning begins. The Lambda sensor subroutines take charge and the fuel is continuously toggled back and forth. If the Lambda sensor is disconnected, the Motronic gets it fuel levels from the maps. It never just "goes rich to protect the engine" which is another famous and false saying. When the Lambda is disconnected, it uses the fuel map.

For simplicity sake, the contribution of temperature has been omitted from the discussion. Fuel atomizes poorly at low temperatures requiring some mixture enrichment.