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MAF output voltage modulation by the EGR duty-cycle

The voltage across c_2 is equal to Uc2=Battery voltage*EGR_duty/2. So the collector current of tp_4 is equal to (Uc2-Ube)/r_11. Ube is the base emitter voltage of tp_4. The collector current of tn_3 is equal to this value. So the output voltage Uecu=Umaf-(Uc2-Ube)/r_11*r_6.

With some numbers: At idle with closed EGR Umaf=2.2V (for a Bosch), EGR duty is 80%, Battery voltage is 14V => Uecu=2.0V (so the MAF voltage is lowered by 0.2V due to EGR action)

Can I adjust the circuit

Yes. Change the value of r_11 (only!) in small steps to increase (lower r_11 value) or decrease (higher r_11 value) the modulation of the MAF voltage. Do not go below 6.8k ohm or above 100k ohm. Below 6.8k ohm the modulation is very large, above 100k ohm there's hardly any modulation left.


The transistors drawn are bc548b and bc557b, but you can also use 2n2222 and 2n2907 type transistors or similar small signal types. The resistors are all 1/4W types. C_2 is a 22uF/16V electronic capacitor, C_1 is a ceramic capacitor. Total cost of parts will only be a few dollar.

If you like to use it, make sure you place it in a water tight housing. Otherwise corrosion will occur. A 35mm film container will do. Stick the wires through small holes and seal it with silicon.

Intellectual property

You are free to use the information presented here. I do not ask any money for it, but also do not accept any liability. You are not allowed to sell the schematics or printed circuit boards with a profit. If you find the circuit valuable and are going to use it, I would be happy to hear your experience with it.

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