Motronic box adjustment screw (B230ET)

On top of the Volvo 740 Turbo's Motronic control unit, under a protective cap, there is often a small blue adjustment screw. With this screw, fine tuning of fuel supply and ignition advance can be made without reprogramming.

The adjustment screw typically has 7–8 different positions, which affect fuel ratio and ignition advance as follows: Position Effect on fuel Effect on ignition 1 Standard (Standard) No change 2 +2–4 % richer mixture No change 3 +4–7 % richer mixture No change 4 -2–4 % leaner mixture No change 5 No change -2–4 degrees later advance 6 +2–4 % richer mixture -2–4 degrees later advance 7 +4–7 % richer mixture -2–4 degrees later advance

The screw is located on the side or top of the control unit (box) under a small plastic protective cap. If the cap is in place, it should be carefully removed. Tip: It is recommended to open the control unit's cover so you can see the screw position numbers clearly and ensure you are turning in the right direction.

Not necessarily. Not all Motronic units have this adjustment installed, or it may be located elsewhere. If your box does not have a numbered scale, screw adjustment is risky without a wideband lambda or timing light.

The change in ignition timing is typically expressed in degrees (TDC), not percentages. Later advance (position 5, 6, and 7) is useful if the engine suffers from knock, for example from poor fuel quality or higher boost pressures.

Richening (position 2 or 3): Useful if the car runs lean, for example with a freer-flowing intake manifold or exhaust. Retarding the advance (position 5, 6, or 7): Used to prevent knock. Note that retarding the advance can reduce engine power and increase exhaust temperatures.