I've been messing with the vacuum system on my '85 wagon, so I can say that on these at least, no vacuum makes it shift hard while accelerating (upshift), vacuum makes it shift soft. I know it has the blue amplifier, however I've driven the other Western Slope member's wagon (83 I believe) and his also shifts hard with no vacuum on acceleration.

They also shift hard on deceleration (downshift) with no vacuum. I thought I ran over a big rock or something once, but no, a line had just come unplugged.

-Rog

What we have here is "Failure to communicate," to coin an old phrase.

The concept of 'vacuum as a supply,' was my biggest hurdle to overcoming my misunderstanding about MB transmissions of this era.

Let us consider your statement '...the upshift is designed to happen with no vac and it bleeds off immediately when you go WOT...' I agree.

The VAC (vacuum control valve) mechanically bleeds vacuum in relationship to the accelerator linkages' position. The more pedal the less vacuum. That is probably confusing until one breaches the understanding that vacuum is a commodity which is generated for smooth low RPM shifts.

Thus, when the pedal is slightly depressed the less vacuum is drained from the transmission. It's affect on transmission vacuum is realized by the modulator valve on the tranny immediately since the VAC is a part of the supply (suck) of the transmission vacuum line. In fact, on my 84 SD it is a branch off the transmission vacuum line.

We have the exact same position on the affect vacuum has to the shift harshness but we are failing to use the same terms and advance the concept to the soft shifts needed when acceleration is light.

Several respected member here recommend a vac supply of 12 mg at the VAC. If less was needed for transmission vacuum why require a such a supply?

If vac bleeds off as the accelerator is pushed (which is the recommended test for the VAC) why should we think a soft acceleration would not allow for more vacuum to the transmission?

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84 300SD
85 380SE
83 528e
95 318ic

Clemson - I think we are in agreement too. Here is my hands-on experience: On my 81SD I had an indy replace the transmission. I tested it by unhooking the plastic line from the VAV to the tranny and I plugged that line into the Mityvac so I could pump some vac to the tranny. Upon driving around I found that the upshifts were good and smooth - no vac. However the downshifts were quite clunky.

So I drove around more and tested the downshifts by pumping up some vac before coming to a stop. Applying vac got rid of the clunk and I got a smooth downshift.

I was never exactly able to figure out the vac modulator or its purpose other than to read that adjusting clockwise gave you a firmer shift and counterclockwise gave a softer shift (could have that backwards - do not remember). In fact I adjusted the vac modulator several times and do not recall feeling any vast change. And I think you are saying that the vac modulator clicker will adjust how much or how little vac it bleeds off when you downshift.

Clemson - your post sounds right and I think I just plumbed and compared the two extreme opposites - vac and zero vac.

When you let off the accelerator the vac to the tranny increased.

I never considered that. I struggled to make my car up-shift without slamming gears and jerking the automobile but thinking back I remember it clunking when the transmission downshifted too. My vacuum was always low so I bought and installed a new pump.

Yes, the instructions I've read on adjusting the vac pressure for each shift point give lower vac pressures for higher engine RPM at each shift point. Those vac level are achieved by adjusting the VAC.

__________________
84 300SD
85 380SE
83 528e
95 318ic

[QUOTE=Clemson88;3604443]When you let off the accelerator the vac to the tranny increased.

The transmission was disconnected from the VAV and was instead hooked into the Mityvac. So there could not have been any effect on the tranny when I let off the accelerator as there was no physical connection.

There was no vac applied to the transmission on upshifts because the plastic line to the vac modulator was open. I closed it and pumped it up when I got ready to downshift and it downshifted without a clunk.

However to get back to the original poster's thread, I think he needs to make sure the pump provides about 12 inches or so of vac and that the vac modulator on the tranny is connected by the plastic line and does not leak.

If he does not have 12 inches, he removes the dome from the VAV and adjusts the screw ever so slightly. And he may also need to check the gap for the VAV. Something like setting points.

Just so we're all clear on how the transmission works, it was designed for a gas engine, then with some complicated plumbing adapted to work on the Diesels.

On a gas engine, when you press the gas pedal, manifold vacuum drops. The further the throttle is open, the lower the vacuum. Low vacuum means high load, and requires a firmer shift. High vacuum means low load and requires a softer shift to prevent the abrupt jerking associated with a firm shift at low RPM.

The diesels don't have a manifold vacuum (especially if they're turbocharged) and have a relatively fixed vacuum source via the vacuum pump. The older models have the VCV, the newer ones have the blue "UFO" vacuum amplifier that modulates vacuum to the transmission to mimic the manifold vacuum changes of a gas engine.

On any engine (gas or diesel) with the Mercedes transmission, without a vacuum source you will get firm, jerky shifts and possible weird/early shift points. If you don't have the VCV or equivalent in place and functioning properly so that you just have full vacuum applied all the time, you'll have mushy/slushy shifts and flares between gears.

What a great explanation. What I have missed all along is the action between zero vac and full vac; ie the moderation of vac and its effect on shifting. That is something I did not experience when I was troubleshooting my new tranny.

Great explanation. And we were all somewhat right and somewhat wrong. Or as they say - a little knowledge is dangerous.

Where have you been all this time?

So have we helped the original poster any?

I learned a lot about the vacuum systems on the vintage Mercs working on the SL over the last 18 years or so. It used to have TONS of vacuum leaks (and ran like a crippled turtle as a result), and one of the most ALARMING things it did was shifting into 1st at a stop light! Felt like someone ran into you! *WHUMP* "WTF was that?!?!" Tranny fought you to downshift, upshifted WAY too fast, and at low RPM's was truly violent until I got the vacuum problems fixed. The SL is a Euro and vacuum diagrams are really hard to find for it, so I learned a lot in the last several years hunting down and troubleshooting problems in that car.

Just about everything in these cars is mechanical. When you think about how it's all made and the thought that went into designing it, it's really a work of art. Newer Mercs do very little for me (90s and later), but mid-late 80s and earlier I simply admire for their engineering.

I've read PeachParts for years, but only recently started posting when I got a 300SDL. I'm pretty well versed in the M117 and the R107, but learning a lot about the W126 and OM603 from this site and getting greasy hands.

SD you right there .With no profile to go on its guess work to help any one .Said 1984 right away thought W123.

Thankf for all the replies, this is a great forum and a very active one !

I havent gotten the vaccum to the transmission since i was more worried about starting with ETHER, i ordered new glow plugs and installed them , Starts first try cold no problem !!!!! This car is amazing !! for the shape its in the motor runs great, i will be powder coating the rims soon , and then start messing with the vac lines again