1983 300SD TD Sudden Downshift

I have not seen this problem before. It may be something internal that will require opening up the transmission. Although unlikely, it might be a short in the kickdown switch located under the throttle pedal. This could activate the kickdown solenoid on the transmission by accident. Try disconnecting the wire to the kickdown on the transmission and test drive again. If the problem persists I recommend you contact a transmission shop very familiar with Mercedes diesel transmissions.

Kent, I hope it is this solenoid. Do I disconnect the wire from underneath the car. I can see the step down switch under the gas pedal.

I will say that the kickdown switch clicks, but when I am going 45 and step down on it nothing happens.

If you can direct me to how to get to the wire to disconnect I would appreciate it.

The easiest approach would be to unplug the wire at the rear of the transmission. It is a black wire plugged straight in to a pin connector at the kick down solenoid. Pull straight out on the plug. Tie the wire up out of the way and test drive to see what happens.

I unplugged the switch with no change. It still slipped. Then I checked the fluid again at operating temperature and it seemed there was too much fluid. So I drained some off and test drove the car. It was shifting properly. So I took a 40 mile drive and tested it late at night. All went well. The next morning I took it out on the freeway at speeds up to 80mph and it ran flawlessly for 70 miles. So.......I thought it was fixed. Then I took a trip to an auto parts store across the city of Phoenix to get what I needed to do a Diesel Purge. It was hot and it will only get hotter here in Arizona. So there were lot's of stops and starts with the A/C on. The engine temperature gets over 100 C when it get this hot out here. It was about the time the temp had gone a bit over 100 C when it started to downshift again. Would the auto tranmission fluid temperature be causing this. Would a transmission cooler help with this? Any advice or insight would be appreciated. Thanks Rick

As it turns out, when transmission fluid heats up, it expands as it thins out. My transmission did ok the first two summers in Arizona, but as age and wear took their toll, those seals that hold pressure for shifting became worn. While cold the car would still shift ok, but with the fluid hot it began to slide by and compromise the seals designed to hold it. This is what was causing the car to downshift unexpectedly. So this will require a rebuild to go in and replace those seals, clutch packes, etc. So she'll be as good as new in a few days. I hope this information can help someone else who may have the same problem. Rick

1985 300TD original owner.

Recenttly started clunking on me when it downshifts into 1st as I come to a stop. After the car warms up the clunk disappears. Trans fluid looks good. Any ideas?

The condition you describe is usually caused by a vacuum leak somewhere in the system that is preventing enough vacuum from getting to the modulator. (ie leaks at brake booster, climate control system, etc.)It could also be caused by low vacuum output from the pump.

I do not suspect it is the transmission modulator or it would do it all the time.. If you are not familiar with checking for leaks please see my Diesel Vacuum Troubleshooting Manual.

that's very good to know!! i have a 4spd 240 but i am in the market for a 300. these cars never cease to amaze me. just the particulars of that very tight german enginering. good luck with the rebuild.

Aaron P.

Looking for tales of lost treasure and, good places to metal detect.