Well I've had this 89 420sel for about 4 months now. Its had a few issues to say the least lol. I may trail off a little bit, to be honest I guess I could just include a bunch of questions into one thread since I know how it is crowding a forum with one person making different threads for different questions so I'm not going to do that. So this is my first benz, but I absolutely could not pass this one up. To be very honest, I've been a Lincoln enthusiast you can say all the **** you like, but I'm now a benz lover. Anyway, onto my questions. So when I set my cruise control it will get to the speed that I need it to, but I feel it has to really work hard to maintain the speed, like I feel it continuing to rev even on a flat freeway. My accelerator linkage was "modified" after it got stuck when I was on the freeway going about 90 mph after I "resumed cruise control" it went full throttle and it was a real bad situation. So my question is, since I know this is an older year, I was wondering if it was normal for the cruise control to do this. And also if it isn't, is it easy for me to change? There's nothing I hate more than showing up to the service center at Mercedes, they really **** me off, so I like doing the work myself.


Also; I don't know too much about the transmission of this vehicle since that Owners manual and maintenance guide was as useless as ****. When its cold, is when I especially notice it, like I have to release off the gas sometimes to let it shift and sometimes if i'm driving slower it'll shift like a normal vehicle at 25 mph but sometimes it won't shift until 35 and sometimes 45 mph, and I was thinking about putting TransTune in it, but the fluid is full and I don't want to overfill it.. And I know that this vehicle is semi fuel injected as some could say, but it doesn't really have that much pick up and go at all, I've put 3 cans of sea foam over the past few months into the gas tank and oil but I know I can't exactly put any into the booster because its really attached and I was sketched out as to what I should do. I didn't know if it would be safe to put it into the throttle body where the air filter was located (that little hole in the center) or not.

I'm real sorry I'm asking so many questions.. I'm just real new to this type of vehicle and I just want to learn more and do as much as I can to it before winter takes its full grasp. I may also take a trip down to florida at the end of this month and I want to be on point.. I'll also be replacing the exterior temperature indicator because it doesn't even read correctly. And maybe the radio as well, I'll find one from the early 90s because the cassette player keeps spitting out my tapes and I have to push the tape in and hold it in there for it to play.

Again if you've actually read all of this I appreciate it and if you can even give me some advice on even some of my questions, it would be greatly appreciated. I just want all the knowledge I can get about my 420. Thanks a lot guys.

 

Had same problem as you.
First I changed the kickdown switch located under gas pedal then I changed out the stop light switch located on the upper part of the brake pedal.
It cured the cruise control issues and kickdown issue I had. The parts are cheap about $20.00 in total.
As far as your transmission siphon some fluid out of the fill tube and add Lucas Stop Slip. Had same problem as you do. After adding Lucas Stop Slip the transmission operated properly. It is a friction modifier that seals any minor leaks and tightens up the bands by modifing the oil viscosity. Will not hurt transmission.
Worked for me. It costs about $12.00 a bottle.32 ounces.
Hope this helps you out.

 

Thanks so much for your reply brother.. Well I have a pretty interesting update. I had a strange fuel smell that I knew wasn't normal so I checked the distribution head as I had absolutely feared was the culprit, but instead when I examined the EHA valve it was leaking like a siv, I was SO relieved that I didn't have to replace that distribution head. So now that I have that replaced, the lunging has stopped i.e the hesitation and I've gained a lot more of the power that I was lacking. It's also shifting a lot smoother surprisingly and shifts ON time and without such a yank. I'm going to try what you suggested with the pedals. I just found it to be pretty odd when it was doing that and I immediately thought it was the module. The next thing I'm going to do after the pedals is fix an issue with a rusted fuel filter and line. Doing the job shouldn't be so hard, but finding the fuel filter will definitely prove to be interesting. Thanks for your reply man. I appreciate it.