Modulated Displacement (MD)

Modulated displacement is an electromechanical system which deactivates certain engine cylinders to reduce fuel consumption. The MD system consists of ECM, Modulated Displacement Amplifier (MDA) and cylinder selector solenoids.

The ECM indirectly controls cylinder selection through the MDA. The MDA switches cylinder selector solenoids on and off. When activated, cylinder solenoids cause mechanical valve selector bodies to simultaneously close intake and exhaust valves of selected cylinders. This is accomplished by changing rotation point of rocker arm for that particular cylinder while remainder of valve train operates normally.

The MDA is connected between ECM and cylinder selector solenoids to supply additional power and to perform switching function. Cylinder select solenoids require initial pull in current of 4 amps, to be activated and a holding current of 1 amp. During 6-cylinder operation, MDA deactivates cylinders No. 1 and No. 4; during 4-cylinder operation, MDA deactivates cylinders No. 1, No. 4, No. 6 and No. 7.

The ECM monitors MDA to determine if cylinder selection is actually occurring. In 4-cylinder operation, MDA supplies 0 volt feedback signal to ECM; in all other modes, MDA supplies 12 volt feedback signal. If feedback signal is not correct, ECM sets "CHECK ENGINE" light and code "25" is stored in memory. The following general operating conditions must be met before ECM will allow modulation:

• Coolant temperature above 148°F.
• Transmission in 3rd gear.
• Vehicle speed above 24 MPH.
• Engine speed below 2600 RPM.
• Code "25" not set (MD malfunction).