Hey guys,

I just bought a 1987 300SDL and it is in absolute perfect condition except for one little problem... It doesn't like to upshift and when it does it doesn't hold the gear. I really don't think the transmission is slipping. Once it is in gear it is fine but under light throttle it drops into neutral. Does this sound like a vacuum issue? I know these cars are famous for faulty vacuum pumps. I check mine and I see no screws on the front cover so that tells me it is the original (pre-update). If it is the transmission I am just going to rebuild it myself. I can get an entire kit for ~$250 plus whatever it will cost for the brake bands. It would be nice if I could drop a 4 or 5 speed manual in it, but I know that is near impossible. Does anyone have any input on what the issue may be? Like I said once it is in gear it doesn't slip but it struggles to upshift between all gears... Especially 3rd and 4th. This car is a dime and I would really like to make it mint.

Thanks,
Landon

__________________

• 1993 Mercedes-Benz 400E *146,000 Miles*
• 1985 Mercedes-Benz 300SD Turbo Diesel Biodiesel *SOLD @ 346,000 Miles*
• 1984 Mercedes-Benz 300D Turbo Diesel Biodiesel *SOLD @ 284,000 Miles*

I should also add that when I shut the car off and pull the key out it continues to run. I know these use a vacuum shutoff solenoid so right off the bat there is a clue. Or is it just a coincidence and the solenoid is bad too?

__________________

• 1993 Mercedes-Benz 400E *146,000 Miles*
• 1985 Mercedes-Benz 300SD Turbo Diesel Biodiesel *SOLD @ 346,000 Miles*
• 1984 Mercedes-Benz 300D Turbo Diesel Biodiesel *SOLD @ 284,000 Miles*

Transmission vacuum controls shift quality, not shift timing. The transmission will shift very harshly with no vacuum signal, but it will shift when it's supposed to. Shift timing is more the province of the pressure control cable. There's a cable that runs from the passenger side of the transmission to the throttle linkage by the oil filter. The cable should be just slack with the throttle linkage in the idle position. More tension holds gears longer, less tension gets you to top gear quickly. What you describe sounds like an internal problem. I hear of talk about a B2 piston which can be replaced without tearing into the tranny. Maybe it's something 'easy' like that.

Apply vacuum directly to the shutoff actuator directly atop the IP and see if that kills the engine. If it does, your problem is between the vacuum source and the key switch. If the shutoff actuator doesn't hold a vacuum, replace it.

Sixto
87 300D

That is really good advice. I didn't even think about the bowden cable.

And if the shutoff is bad and leaking that could be causing the whole problem. Now I just have to find time off to work on it!

__________________

• 1993 Mercedes-Benz 400E *146,000 Miles*
• 1985 Mercedes-Benz 300SD Turbo Diesel Biodiesel *SOLD @ 346,000 Miles*
• 1984 Mercedes-Benz 300D Turbo Diesel Biodiesel *SOLD @ 284,000 Miles*

I just ordered a new vacuum pump since I figure it needs to be replaced regardless of the transmission issues. I will update after that is done.

__________________

• 1993 Mercedes-Benz 400E *146,000 Miles*
• 1985 Mercedes-Benz 300SD Turbo Diesel Biodiesel *SOLD @ 346,000 Miles*
• 1984 Mercedes-Benz 300D Turbo Diesel Biodiesel *SOLD @ 284,000 Miles*

Slight correction to Sixto. Your upshifts will be smooth without vacuum; the system is supposed to bleed down the vacuum at WOT. However the downshift will clunk without vacuum. You can test this with a Mityvac. Unhook the vac line to the tranny and hook it to the Mityvac. Drive around the block holding the Mityvac. The upshifts will be fine. When you get close to a stop sign, pump up the Mityvac and you will see the clunk in the downshift go away. At least that's the way my 81SD works and I assume yours is the same.

I don't follow. If I remove the vacuum connection to the modulator, low rpm upshifts knock my fillings out. Though I agree there is a lurch with downshifts that are not perceptible with proper vacuum. I also agree that upshifts are smoother at higher rpms than at lower rpms with no vacuum.

Sixto
87 300D

Sixto - that's really strange. The VAV is usually set around 12-15 inches of vac. When you open the throttle it bleeds off the vac line going to the vac modulator, so when you shift up you have zero vacuum and it is smooth. However you absolutely need vac for downshifting. If you hook the Mityvac to the VAV you will see it bleed off the vac really fast when you hit the throttle.

Not sure why yours does not react the same. Went through all this with the excellent help of Kerry a couple of months ago. He really knows his stuff. Of course I am only speaking of an 81 SD and those others that are designed the same way but I guess the SDL is the same. I think the 300D is designed the same way but not positive.

When you upshift at WOT you should have zero vacuum. With light throttle input there will be some vacuum continuing past the VCV to the modulator. The VCV isn't an on-off switch. It has a response curve based on throttle position.

For kicks, disconnect the line from your VCV to the modulator and try a few low throttle input shifts.

Sixto
87 300D

My issue is upshifts, not downshifts. It only upshifts with a certain amount of throttle... Usually WOT. Sometimes not at all.

__________________

• 1993 Mercedes-Benz 400E *146,000 Miles*
• 1985 Mercedes-Benz 300SD Turbo Diesel Biodiesel *SOLD @ 346,000 Miles*
• 1984 Mercedes-Benz 300D Turbo Diesel Biodiesel *SOLD @ 284,000 Miles*

Disconnect the Bowden cable and see if it upshifts very quickly. If so, maybe your Bowden cable is sticking.

Sixto
87 300D

Sixto - agreed; that's the way it works.