I apologize in advance if this question has already been answered in another post...I have a 1988 420sel which jumps to around 2000 rpm upon startup, or when shifted into park or neutral while running. I am beating the transmission to death! [] This vehicle has idle control valve part number 408 202/010/002, and I believe that this part could be at fault. Is there any obvious checks to be done before going ahead and replacing the valve? Thanks in advance!

 

TTT [sm=helpout.gif]This has to be my daily driver (as circumstances dictate), and I REALLY cannot afford to destroy the trans. Anyone??

 

No one has ever had and solved this problem? I have replaced the idle control valve, and it did help some (idle speed goes to between 900 and 1700 or so)...I am thinking that maybe I have a vacuum leak as well - anyone have a vacuum diagram or a link to one for this car?

 

Hello, I dont have an answere to the high idle on your 420sel, however I have the same problem with mine, and I have asked the same question on here and nobody seems to know how to fix it or they just dont want to say anything. my 420SELwill idle at about 1,200 rpm , I just bought my 420SEL off of ebay for $600.00 but I have not picked up the car yet, I will have it here by the end of the month so I have not been able to look at it, however I had a 1985 500SEL that would idle at about 2,000 rpm and I know on that model there is a idle control valve and a idle controlrelay andthe relayis located just beside the brake booster to the left. I cant remember what the part number was but when I replaced it the idle went back to normal, I am not sure but I am thinking that the 420SEL would also have this relay and if you unplug the relay if I can remember correctly the car will not run at all or will start but die as soon as you let off the key, I will let you know what I find out with mine when I get it, that is the first thing I am going to fix come hell or high water. in the meantime, since you have the 420SEL there with you, you might try to see if there is a relay on the firewall just beside the brake booster to the left. thereare 3 relays in a row in the same area, I cant remember but it was black and maybe 3 inches long or so.one relay has a red top cover and that is not it, I think it is the relay in the middle of the 3 butthis is just a guess so keep that in mind as I dont know much about these mereceds yet, but like you I am learning one repair at a time.oh and check for vac leaks while you are under there too. OH, here are a few pics of the 1986 420SEL I just bought on ebay.

 

I had a 1990 300SE that I was using for parts and I ended up using my idle control relay and it works wonderful. it is on the left side of the brake booster with a red fuse in it covered by a glassy type top.