HELLO TO ALL, FOR THOSE WHO HAVE ASKED ABOUT THE HIGH IDLE PROBLEMS WITH THE W126 420/500/560SEL 1986 TO 1991 MODELS I MAY HAVE THE ANSWERE FOR YOU. I NOTICED THAT THERE ARE A FEW OUT THERE THAT HAVE THE SAME PROBLEM WITH HIGH IDLE AS I DO AND NOBODY HERE COULD TELL ME OR ANYONE ELSE HOW TO FIX IT, WELL, I AM HERE TO TELL YOU HOW TO DO THAT, AS YOU KNOW THERE IS A IDLE CONTROL VALVE ON THE ENGINE, THAT VALVE IS CONTROLLED BY A RELAY ON THE FIREWALL JUST TO THE LEFT OF THE BRAKE BOOSTER, YOU WILL NOTICE 3 RELAYS IN A ROW THERE, THE CENTER BLACK RELAY OR THE RELAY TO THE LEFT OF IT IS THE IDLE CONTROL RELAY .THE RELAY TO THE FAR RIGHT HAS A REDDISH TOP ON IT AND THAT IS NOT IT,THE IDLE CONTROL RELAYWILL NEED TO BE REPLACED, ALSO CHECK FOR VAC LEAKS WHILE YOU ARE UNDER THE HOOD. ANYHOW, THAT RELAY SOLVED MY HIGH IDLE PROBLEM ON MY 1985 500SEL, HOWEVER I AM NOT SURE IF THATWOULD BE THE SAME FOR THE 420/560SEL MODELS BUT IT IS WORTH LOOKING INTO, I JUST BOUGHT A 1986 420SEL OFF OF EBAY THAT HAS A HIGH IDLE ISSUE, HOWEVER I DONT HAVE THE CAR HERE TO LOOK AT RIGHT NOW BUT WHEN I DO GET IT HOME I WILL CHANGE THE RELAY AND LET YOU GUYS KNOW IF THAT WORKED, UNTIL THEN GOOD LUCK.

 

I have the same sympton, & when this condition occurs, the ABS light is on ... when the idle is "normal", the ABS light is off! Obviously, I have checked the brake pads, wires @ the wheel, etc., & everything is as it should be ... I suspect there may be a computer error? btw; the car JUST passed Colorado Smog Test with flying Colors ...