Quick input sincerely appreciated here...I have spent literally hours with the Search function and reading threads en masse: 88 420 SEL w/138K mi. I'm 3rd (new) owner. Starts fine, idles rough 'till warm. In Neutral, idle pushes 1100. In Drive, idle @ ~550. I've replaced rotor & cap, have wires on the dining room table tonight and will gap new plugs tomorrow. Any further suggested action if this doesn't help? Sorry to bring up the "mundane tune up question" but no Haynes guide available on this model, and my $25 aftermarket copy of a supposed "shop manuel" wasn't worth it PS Happy New Year

The Overvoltage Protection Relay can affect idle if it is bad. It is the little silver box with the red top and clear plastic cover about 8 inches left of your fuse box up against the fire wall. Check the 10 amp fuse in it as they often go first. For our cars the overvoltage protetion relay has been upgrades since our cars were new. I also has a rough idle that nothing cured (including almost all new ignition parts from coil to wires, a new fuel distributor and injectors - (all of these helped somehwhat, but OVP relay fixed it all up good. It cost me $120 Canadian with tax, I know they sell it in the FastLane here and you can replace it yourself in about 10 seconds, it just pulls out and plugs in. I used some dielectric grease to prevent corossion on the contacts. This might work, but your best bet is probably to take it to a mechanic and had a thorough diagnosis done.

PS. You can get pretty good shop manuals on CD-ROM from MBUSA

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired

Jason,

Thank you for your concise answer...I'll check out the module you described if plugs & wires don't cure the problem.