Transmission Modulator -- Which Way For Harder/Later Shifts? - PeachParts Mercedes-Benz Forum

jkoebel · #1 12-23-2007, 02:43 AM

I've captured the last gremlin that was my vacuum system, plugging a leaking port that had been left open for some reason and capping off my door locks. Now everything is as it should be.

With the vacuum leak, my transmission shifted very hard and very late -- 1-2 at ~15, 2-3 at ~35, 3-4 at ~60 mph. Now, it shifts so smoothly I'm actually worried about my clutch packs, and so early that I can't get up to highway speed even on a 1/2 mile long on-ramp+merge lane. 1-2 almost instantly (I feel it shift, it's not starting in 2nd, it's just shifting there at about 3 mph), 2-3 at about 20, 3-4 at 35. It's very nice, it feels almost like when I had my hybrid with the CVT, but it's robbing me of what little power my engine had to offer.

I'm going to crawl down and turn my modulator, I just want to know: which way is "harder"? A thread I found with search said "screwing the adjustment knob in raises pressure, screwing it out lowers pressure" but I'm not sure which one of those I want.

I was thinking of giving it 1/4 turn in the respective direction, taking it for a test drive, and repeating until satisfied. How's that sound? Also -- this might be an odd question -- is there anything I could attach to a vacuum line and let it leak "a little bit", like a valve or something? There's a T fitting on the vacuum line before the transmission (for some reason, that's what was leaking in fact) and I was figuring that, since plugging that one changed the shift characteristics of my car, I could put a knob to turn up there and tweak the vacuum pressure going to the tranny from the engine bay, or in the car itself, rather than under the car.

This should be a simple answer, but I couldn't find it.

300SDog · #2 12-23-2007, 07:22 AM

Since leaky air line caused hard shifting before then *decreasing* the pressure or turning the T adjust counter-clockwise should stiffen it up.

Bill Ladd · #3 12-23-2007, 09:09 AM

Quote:

Originally Posted by jkoebel

...and so early that I can't get up to highway speed even on a 1/2 mile long on-ramp+merge lane. 1-2 almost instantly (I feel it shift, it's not starting in 2nd, it's just shifting there at about 3 mph), 2-3 at about 20, 3-4 at 35. It's very nice, it feels almost like when I had my hybrid with the CVT, but it's robbing me of what little power my engine had to offer.

Bowden cable controls shift points. I learned here to start by taking all the slack out at idle and adjust from there.

It's attached to the throttle linkage mounted atop your cam cover.

Good luck and post back with your results!

Magoo · #4 12-23-2007, 09:10 AM

You should make sure everything else, i.e. vacuum system, bowden cable, is set to spec before messing with the modulator.

JimmyL · #5 12-23-2007, 09:51 AM

Do later 240 automatics have a Bowden cable?

Quote:

Originally Posted by jkoebel

I'm going to crawl down and turn my modulator, I just want to know: which way is "harder"? A thread I found with search said "screwing the adjustment knob in raises pressure, screwing it out lowers pressure" but I'm not sure which one of those I want.

That adjustment procedure you noted above sounds like the vac. modulator valve assy that is on the Injection Pump. Down on the modulator on the tranny you remove a cover and rotate a pin.
Others more familiar with your particular setup will chime in.....

Douge · #6 12-23-2007, 10:05 AM

Not to hijack the thread but I need to check the adjustment on my transmission as well. The upshifts are a little stiff though not bad, pretty livable although not as smooth as I think they could be based on comments I have read here.

My main concern is that when I come to a stop, I get a hard downshift. I don't know if its 3-2 or 2-1 but it is hard, almost a full slam.

Will the Bowden cable and modulator adjustment address this issue?

jkoebel · #7 12-23-2007, 10:37 AM

Quote:

Originally Posted by JimmyL

Do later 240 automatics have a Bowden cable?

I haven't seen one but that's not to say I haven't noticed. Will check it out.

Brian Carlton · #8 12-23-2007, 10:50 AM

Quote:

Originally Posted by jkoebel

I haven't seen one but that's not to say I haven't noticed. Will check it out.

From your description of the symptoms and the response, I'd guess that the 240 does not have a Bowden cable and that vacuum regulates both the timing of the shift and the speed of the shift.

I'm quite sure it's possible to make an underhood adjustment to the vacuum level to achieve the desired results and I would not disturb the modulator until all other potential avenues are exhausted. You never quite know what the modulator will do for you.

If we can dig up a proper vacuum diagram, I'm sure there is a method for adjustment.

jkoebel · #9 12-23-2007, 11:24 AM

http://www.peterschmid.com/vacuum/1977_1985/616_617_91/616_617_91.jpg

Rick H · #10 12-23-2007, 11:46 AM

There is no Bowden cable on a 83. I think they stop using them in 80. I had the same problem with my 81 240. I adjusted the linkage on the motor side of the injection pump it`s either a 8 or 10 mm nut. It`s concected to the vacumm regulator. Just move it a little at a time tell you get it right. It`s kind of tight getting to it. It helps if you have a ratcheting box end. If you don`t have any maybe you could ask for some for Christmas. Any way hope this helps. Rick

jkoebel · #11 12-23-2007, 11:56 AM

On that diagram link I posted there, there's a 3 way connector going between the transmission, another 3-way, and the valve cover vacuum ports.

My car had no line going to the valve cover -- just one to the tranny and one to the 3-way. The spot that was supposed to go to the valve cover was open (leak) and it would shift hard and late. Plugging with a golf tee, it shifts very soft and very early. Connecting it up properly resulted in a car that shifts medium hardness and still very late.

But, I figure, if I'm to be adjusting something -- that's the line I can do it on. Now to find some sort of adjustable vacuum valve that goes between "full leak" and "closed" and anything in between.

Rick, do you have a picture of what you're talking about? I sort of think I see it but am not sure.

jkoebel · #12 12-23-2007, 12:14 PM

Can anyone post a good PICTURE or clearly-drawn DIAGRAM of the 3/2 switch vacuum assembly on the 240D w/ EGR and Auto? (1983)

I think mine is wrong (it looks like its missing some pieces) and I can't make it out to read it even on this good page:

http://home.comcast.net/~phantoms/vacuum/fuel_egr_240d_auto.jpg

its just not clear enough, how those things are supposed to go. I think it also explains why leaving that vacuum line unplugged and plugging it into the 3/2 works the same, because there's no plug and no line on the back of the port it plugs into on my car.

Rick H · #13 12-23-2007, 12:21 PM

Don`t have a pic but if you go straight down from the last glow plug you will see the linkage with the nut right next to the head. If you loosen that nut it will slide up or down in a slot and you can adjust it right there. It`s hard to see the slot.

jkoebel · #14 12-23-2007, 12:33 PM

The 5-way connector by the 3/2, it looks like, 1 2 3 go to the 3/2 (1 on the left, 2 lower right, 3 upper right) and 4 5 loop back and join each other? on mine 4 and 5 are just empty.

jkoebel · #15 12-23-2007, 12:49 PM

Here's what mine looks like right now. It's a diagram.

Is that reasonably correct?