Checking the idle shut-off valves (Mercedes-Benz S-Class W126)

Two idle shut-off valves close the idle channels when the ignition is switched off, thus preventing further engine operation. The valves remain closed until voltage is supplied to them again after the ignition is switched on. If the valve is faulty or there are voltage supply faults, the engine starts poorly and idles unstably. If voltage is not supplied to both valves when the ignition is switched on, the engine will not start. The idle shut-off valves are screwed into the side of the carburetor throttle body.



Turn on the ignition.

Remove the plugs (253) one by one from the separate shut-off valve (21) several times and put them back on again. The valve should click audibly.

Otherwise, connect a test lamp between the plug and ground. If the lamp lights up, replace the valve. If the lamp does not light up, check fuse #14 or the electrical wiring according to the wiring diagram.

Start the engine and let it idle. Remove the plugs on both valves at the same time, the engine should stop immediately without running any further. Start the engine again and remove the plug from the left valve, the engine speed should drop significantly or the engine will start to jerk. Then put the plug on the left valve and remove it from the right valve, the speed should also drop. Otherwise, replace the corresponding valve.

Note: On carburetors without a central adjustment screw for CO between the idle code shut-off valves, the valves can be replaced with the carburetor installed.