I think I have two possible problems.I have a 90' 420sel that runs rough when the cruise control is set and also the trans won't up-shift when accelerated hard (fluid level is correct,however its never been changed with 98k miles),and when it DOES shift to the next gear it jerks your teeth out! If I speed up SLOWLY the trans will shift correctly. This just started all-of-sudden like. About a week before that event, I noticed that the engine wouldn't smooth out when the temp reached around 80* cel like it used to when it was warming up (I assume the cold start valve is cutting off at that point). So I thought the thermostat was stuck open because the engine temp woundn't go above about 70* cel, so I replaced it. No help. I still can't seem to get the engine temp up. Could the rough running and trans problem be interlaced with the temp problem? Is there a vacuum relationship between the cruise control unit and the transmisson shifting problem? Does the trans have a vacuum modulator? Vacuum leak maybe? Any help wound be GREATLY appreciated!!

First off I would suggest a trans fluid and filter change and see where you go from there. I know that when my trans is cold that it takes longer to shift up and is slightly firmer than normal. Vacuum leaks will give you rough running and shifting problems so trace out all the vacuum leads and see what you find. My cruise control works but the car is slightly jerky when the car is attempting to maintain speed, apparently a new amplifier will sort this out. Good luck.

Matt.

I think Matt is on to something. I think you have a vacuum leak, but more significantly, I think you have a vacuum leak for the transmission vacuum modulator, this would explain the harsh shifts. Check the vacuum line from the intake to the vacuum modulator.
I don't think the temp problem is related to the shift problem. Very strange to not have the engine warm up enough, possibly the gauge is wrong?
Gilly

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Gilly is on the money with the guage, however maybe one of the temp sensors is faulty which is telling the guage that the car isn't warm. To test this you should find a large hill and chuck the air con on and run it hard up the hill. See what the guage does. If it's warm outside you could hit 105 mebe 110 so compare what you get. If the guage doesn't move much then I would say that a sensor or something is faulty as your car must heat up with a run like that. If the cause of your trans problems is the modulator do a search here and you will find the instructions on how to adjust the modulator.

Matt.

Matt:
What I mean is that a vacuum leak at the modulator line can be causing the driveability problem, as well as the hard shift (low modulator vacuum=hard shift, high vacuum=soft shift, so what I would recommend is seeing if there is vacuum to the modulator at all before attempting to adjust the modulator. Adjusting the modulator will be useless if there is no vacuum to it. Possibly just the vacuum line for the modulator has come unplugged from the manifold.
By stating that maybe the gauge is wrong, I didn't mean to give the impression that the gauge itself is defective, simply that the gauge is reading lower than the actual engine temp. I agree the most likely cause for an inaccurate gauge would be the sensor, but it is pretty easy to test the sensor. The best way to test it is to remove it and allow it to cool (or warm) to a known temp, usually just "room temp", then test the ohm value of the sensor and compare it to the resistance chart for the sensor. If this passes, then a resistance value can be fed into the gauge electrical connector and the accuracy of the gauge compared to the known value fed into the harness.
Gilly

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Matt and parrothead:
The more I think about the shifting complaint, the more I think it could just be the throttle valve cable for the transmission adjusted too tight. Worth checking if there is vacuum to the modulator, another simple check. Unless Parrothead is the original owner of the car, I kinda doubt the trans fluid has never been changed (possible I guess, but doubtful). It should be done every 30,000 miles though, so if you know it's way over that,i'd get it changed.
Gilly

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Thank you very much for the info guy's, I purchased this car from the orig. owners with approx 50k miles on the OD. The car was serviced at the local Benz dealer and never altered in any way.I haven't adjusted the shift cable, so I leaning towards a possible vacuum leak. As I stated before, this problem appeared all-of-sudden. Which leads me to believe a parts failure of some sort. I DO need to change the transmission fluid, even though it's very clean with 98k miles on it now. As far as the cooling problem, it reached the high 70's here in N.C. yesterday and the temp. went to around 110*......I quess thats O.K. I would like to ask one more question, my expansion valve sprung a leak and needs replacing. Can it be accessed under the steering column? Is it necessary to pull the whole dash assy. out? Thank again guy's!!

Very common problem on 420SEls is for the vacuum hose to come off (the hose to modulator). It can be seen with the aircleaner off, down the back side of the motor. It attaches to the same area the large brake booster vacuum hose originates. It is very hard to see down there but the vacuum line to the tranny can usually be identified and then just be sure its plugged into something. I usually stir it with a 12in screw drive and see if its attached. BTW this is special hose to resist the heat from the exhaust crossover which is right there also. Don't use normal vacuum hose or you will be back soon.

The expansion valve is replaced without removing the evaporator case. It is not fun on the cars after airbags as the reinforcment is right in the way and the fitting are often real tight (if not corroded). We had one the other day that we had to replace the evaporator as the fitting was so corroded to the valve.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Thanks Steve for that info. I'm going to purchase a manual off the internet on DVD-rom for this car. I'm going to need it! Can the expansion valve be reached on the drivers side? Or do you normallly have to pull the passenger side airbag? I'm an commercial airline mechanic by trade, so I have a LITTLE bit of mechanical experience! LOL But with that said, I believe I would rather change the engine on a Boeing 737 instead of jacking around with an airbag! I have visions of it going off in my face!! Once again, thank you VERY much for taking the time to answer my questions! It's GREATLY appreciated!