PeachParts Mercedes-Benz Forum - View Single Post

Jeremy5848 · #11 05-23-2009, 12:51 AM

Quote:

Originally Posted by TN TRACTOR

Bought an '85 300DT. Drove it 65 miles home with a notice of a little lack of power going up hill on interstate. I changed fuel filters, oil filter, and air filter and drove it and it shifts hard and will not pull a hill, big loss of power. I'm going to change transmission fluid tommorrow.
1.What kind of transmission fluid does it take?
2.Any ideas what else might cause lack of power?
3.Low compression?

Any Ideas?

Uses standard Dexron/Mercon transmission fluid (GM kind, not Ford). Mercedes has their own fluid, preferred by purists but not necessary. Remember to drain the torque converter as well as the transmission, will take about 6 quarts. Turn the engine by hand (clockwise from front) until the drain plug in the torque converter can be seen, then remove to drain. Pull the pan, there's a filter to replace, most foreign car parts places will be able to get it or a Mercedes dealer for sure (for more money, of course). Remember to put plugs back in and don't strip the aluminum threads.

Hard shifting probably due to incorrect vacuum level going to transmission, especially if the blue flying saucer has been bypassed. There's a vacuum control valve (VCV) on the fuel injection pump (IP) that changes vacuum level in response to the footfeed at the IP. The blue flying saucer combines vacuum with boost from the intake manifold (IM) to produce the vacuum signal that the transmission needs to shift smoothly. There's also a Bowden cable from the throttle assembly on top of the valve cover (VC) that tells the transmission when to shift.

Poor power can be caused by a turbocharger not working properly but is most often caused by clogging of the line that leads from the IM through the over-boost protection solenoid to the ALDA, which is a disk-shaped thing on the IP. ALDA looks at the boost (pressure from turbo) in the IM; if allowed by ALDA, the IP gives more fuel when there is boost (more air to burn the more fuel). If the boost line is plugged (with oil/soot from EGR) or disconnected or if the overboost solenoid isn't working, ALDA won't know to have the IP give more fuel, hence you end up with about 85 HP max instead of 123 HP.

The boost signal also goes to the blue flying saucer and will affect shifting times (but not as much as it affects power). Without boost, the transmission will shift at lower RPMs and you will feel as if the car is lugging (which it is).

There are lots of ALDA-related threads on this forum. If you will use the search function, you'll find a whole weekend's worth of reading.

Don't worry about low compression until you have made sure the ALDA and the vacuum system are all working correctly. Think about getting a service manual. Haynes is OK although often panned here. Factory manuals hard to find and expensive; factory CD-ROM version may still be available ($20--cheap), check Mercedes web site, look for "classic" section.

Jeremy