91 300D Vacuum Leak (?) Rough Shifting/Banging in Transmission - Valve question

[Rhauss]

Hi,

This is my first post, and I appreciate any help and guidance offered!

I have a high mileage 1991 300D. Currently at 340,000! She's as reliable as can be. About a year ago I had a local shop replace the blower fan for the heat, as it had stopped working. When I picked up the 300 D it was shifting hard. As I'd had the transmission replaced in it's recent history - I returned the car to the tranny shop who said " I think you have a vacuum leak ". I have driven the car and almost gotten used to it. Now as I prepare to move this car on to someone else, I'm trying to clear this problem. Today I returned the car to the shop whom installed the blower fan and they said the transmission vacuum line was in tact and had vacuum. They then pointed to a inline valve (located on the right side of the engine behind the engine shut off switch) that is in the transmission vacuum line and said ... "We think this value should only allow air flow in one direction, but yours allows air flow in both directions" and they speculated that could be the problem.

Does anyone know if that valve in question should be a one way, or in fact should allow air flow in both directions?

again, thanks in advance for helping me with my first post!

[JamesDean]

Not sure about the valve flow but could you explain the problems you are perceiving with the transmission?

Does it feel overly hard going from gear to gear? At partial throttle? At wide open throttle? More details will go along way.

[Rhauss]

Hi there,

Thanks for offering help...

As I accelerate..the RPM's increase and then when a well-behaving transmission would shift effortlessly, my 300D has a gear shift you can feel. Then if I take my foot off the accelerator to let the RPM's drop.. I get a powerful "bang" in the rear end of the car.

Does that explanation help?

[JamesDean]

All of the MB's that I've ever driven you can feel the shifts. Especially 1-2 in the 300SDs and my 300D's can be somewhat hard depending on throttle.

Try driving the car with little throttle and letting it shift on its own. Then try it again with say 50% throttle and again with 100%.

I can't remember how it goes exactly but the quantity of vacuum will vary with throttle. Little vacuum results in hard shifts. Lots results in soft shifts.

[Diesel911]

Transmission Vacuum and adjustments
Mercedes-Benz Transmission, Steve Brotherton, ImportCar, February 2002

Of course Mine Transmission is older and not entirly the same as yours. When I accidentlay disconnected the Transmission Modulator Vacuum Line my forward upshifts were nearly the same but it was the Down Shifts that really thuded hard.

You did not mention checking the Transmission Fluid level or condition (color and smell).

I had trouble with the forward upshifts and changing the Auto Trans Filter and Fluid took care of the issue until the B2 Piston Broke abvout 1 year later.

It might be helpful to know what the actual Vacuum is in your main Vacuum Line that the other lines feed off of. This would help determine if the Vacuum Pump and or the Vacuum Pump Check Valve are working OK.

[tangofox007]

Originally Posted by Rhauss

Today I returned the car to the shop whom installed the blower fan and they said the transmission vacuum line was in tact and had vacuum.

That's a lot like a doctor declaring that his patient is in good health because he has a pulse.

A vacuum leak anywhere reduces vacuum levels everywhere.