420SEL & 560 Series

Idle speed is controlled by idle speed control unit. Control unit is located behind the glove box on 560SL models. On 420SEL and other 560 series, control unit is located on firewall inside the passenger compartment.

Idle adjustments can only be obtained by connecting terminal "C" (terminal No. 5 on control unit connector) to ground. This will raise idle speed by 50 RPM. This procedure can also be used on 1986-87 models, except on 420SEL where the idle speed control unit must first be replaced with the 1988 version, as the previous control units do not have connecting terminal "C".