On the 1988 MC18 (first year MC18 model) there are three control units.
1. CDI Control Unit - This is the larger unit on the left side of the sub frame.
2. Sub Control Unit - This is the smaller unit on the right side of the sub frame.
3. RC Valve Control Unit - This is in the center of the rear sub frame.

When turning on the ignition and increasing the engine RPM to around 2K the servo pulley should move to the HI position. Increasng the engine RPM to around 3K, the servo pulley should move to LO.

The MC16/18/21 service manual goes into further trouble shooting details like checking the servo motor as well as inspecting the wiring harness.