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Q: Check engine light on all the time on 1990 Mercedes-Benz 300TE

I have replaced the transmission with a new one. Replaced the O2 sensor, the cam sensor, the BDU, and had a full service on the car. All to no avail...The car is fine to drive around town, smooth responsive. On the freeway at higher speeds its great! great until you leave the freeway and then it stalls, leaving me with no power in a busy and dangerous situation. It has been checked, endlessly and repaired, according to the shop! A day later the engine light is back on and it stalls again. So, normal town speeds good, higher speeds good, until you slow down. Shop says its a engine management problem, but still they have not cured it. Any ideas?
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I have a '92 300 TE,w/194K miles. Just had minor repairs, plugs, fuel/air reset. Runs great, 200 miles later, the BAD BOY CEL came on. My tech said don't worry to much! Still bothered me,turns out the computers sensed a low battery! Sure as Hell, the Interstate bat had 4 dead cells. Installed new bat, CEL is out. Guess I got lucky.
I would think that your SHOP has the proper diagnostic computer to plug into the test module on the engine fire wall, driver side. With out the test computer, your bill go ballistic!

Great ride!
alternator or voltage regulator might be bad. Dealers make lots of money not getting to the problem for a lengthy amount of cash flows through.
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