Ok so my 81 300sd shifts way too soon.. by the time i get ti 25 miles an hour its already in overdrive. I adjusted the tranny shift cable - its all the way out.. but it still shifts way soon unless i have my foot in it hard
I have the cable adjusted so its at the end of its travel at wide open throttle

Any thoughts?

thanks!

-Dan

 

Have you tried driving another 300sd from the same year to see if it's normal response for that year/model? If not, I'd suggest doing that first before assuming there is a problem.

If this is your first diesel, you might be babying it too much and if it has a turbo, you can actually damage the turbo by trying to drive for fuel economy.

 

LOL well im pretty sure im not babying it...... BUT when im into the pedal it does seem to shift with more seperation between 3rd and 4th... Thats a good idea to try drving another one. I'll give it a shot! Thanks!


Another question - my tach isnt working as of yet.. so whats a good cruising speed on the highway? is 80 too fast? I drive it about 250 miles the other day and if i went even like 85 for very long it seemed to heat up more then just keeping it bvetween 75 and 80

thanks!

-Dan

 

Hey did you ever figure this problem out? I have an 80 300SD that does the same thing. Shifts from 1st to 2nd and then like 3 seconds later shifts to third. I don't think I have an overdrive. There's a long pause between gears as well. I'm assuming I need to adjust the bands in the transmission. Has anyone done this? The shifting point though has me baffled
I repaired most of my dash issues but my tach isn't working either. I ran from OH to NC and it seemed to be screaming at 80MPH. It didn't heat up and used 26 gallons on a 500+ mile trip which I don't consider to be too bad.

Mcgyver01

 

As for the shifting issue. I'm not 100% sure, and you should check a manual, or another member may be able to clairify. I think the cable from the throttle linkage to the trans works backwards. In otherwords, turn the adjustment in (make the inner cable long) to increase the shifting duration. One problem I have come across a few times is the throttle kickdown switch going bad. I is located under the gas peddle.

Cruise speed of 80 MPH is okay. Actually, that is where the 300SD is most efficient. I have run from NY to Miami a couple times, and 80 gets the best gas mileage. You should be in the neighborhood of 3000 RPM's @ 80 MPH. I have had mine up to 100 a few times for short spurts, but would not recommend over 85 for an extended period. Watch your temp guage, that will indicate if the engine is working too hard.

 

Log onto www.dieselgiant.com and read about vacuum issues and how its regulated on a diesel. Great site.

 

Have you tried adjusting the Automatic Transmission Modulator Valve?

On the right (passenger) side of the transmission toward the rear you should see a round device with a vacuum line attached to the top cap. It almost looks like a very large oil pressure sending unit that is screwed into the transmission. Use a needle nose pliers to grip the squeeze clamp and disconnect the vacuum cap, usually made of white plastic. Inside the modulator valve you'll see a metal key.

Make a note of the current position of the key and then pull it out slightly. In order to make the transmission wait longer between shifts you need to turn the key, in one position increments, to the right until the desired shifting is achieved. If you are brave you can try advancing two positions at a time, but make sure you road test after every adjustment.

If this doesn't help, or you are not noticing definite changes in the way the transmission shifts as you make valve adjustments check for a vacuum leak between the valve and the engine, or you can try replacing the AT mod valve altogether. Cost is about $43 from www.thepartsbin.com.

 

Its been my experience that the modulator doesn't go out of adjustment but the rod linkage can, as well as vacuum leaks developing causing shifting problems. My point is the modulator should be the last thing adjusted. Verify vacuum control circuit is working as designed first, then throttle rods adjusted to spec then lastly adjust the modulator to fine tune to your preference.

 

When the mod goes bad or begins to go bad sometimes an adjustment can temporarily improve shifting. Based on my experience with M-B diesels it sounded like the modulator was the culprit. I don't think I implied that the mod goes out of adjustment. And It's cheap enough to replace, so if one plans to keep the car for a while then the expense would be justified. Just trying to help.

 

Point taken. My concern is maybe from lack of knowledge the poster does not logically do the diagnostics in order. Vacuum leaks and proper vacuum signals need to be verified first. Thank you for your imput.