Understanding and adjust the VCV and direct linkage - PeachParts Mercedes-Benz Forum

kuene · #1 09-08-2016, 02:55 PM

OK - so I'm methodically going through many steps to verify my vacuum system in the 82 300D Turbo. Finding and stomping out leaks, replacing rubber, etc (throughout the vehicle).

I'm now at my VCV and looking to test out some adjustments - I have some shift flaring that I'd like to tackle. The flaring reduces as the vehicle warms up/drives. I've recently changed tranny fluid and filter. Now, whether or not I can impact my shift flare with the VCV, I just want to understand how it should be working and how adjustments in either direction change things.

I inspected the unit on, and payed close attention to the linkage and how the throttle position moved the lever. Then I removed it so I could better inspect the control itself - I realize that I will be making and evaluating adjustments while this is back on the engine.

Here is what it looks like at rest:

When I move the throttle to full open by pushing the linkages with my hand - it appears that the VCV lever doesn't quite go all way to full stop - looks like so:

I can still move it further open(shut?) by hand, all the way to the stop - but the throttle linkage as controlled by accelerator pedal wont move it this far:

So my question is, before I start tinkering with the adjustment and vac readings, do I need to adjust the linkage rod so that this DOES go ALL the way to full stop when I have the accelerator mashed to floor?

tyl604 · #2 09-08-2016, 04:11 PM

Several years I replaced my tranny and went through this to get the clunk out of the shifts. Here is what I remember:

The vac modulator on the tranny adjusts how hard or soft it shifts. Turn it CW for sharper shifts - like one or two clicks each time - not much. This might help your upshift flare. Vac is the key.

The vcv regulates how much of your 21 inHg vac (from main vac pump) gets delivered to the tranny. When fully open there is supposed to be a 0.5mm gap at the stop. If not you remove the plastic dome from the vcv and adjust the screw just a little - like an eighth to quarter turn. You should get about 15 inHg between the vcv and the tranny if it is adjusted right. I had 9 inHg and successfully adjusted the vcv to get 15 inHg delivered to the tranny.

When you hit the throttle the vac is supposed to go away so that you get smooth upshifts - regulated by the vac modulator and however many clicks you adjusted it.

You might take a look at my old thread "Tranny goes clunk." Some parts of it are appropriate. If you are having flaring at upshift, I would guess that you have too much vacuum and not enough has bled off when it upshifts. I am guessing that your downshifts are smooth because it likes vac on downshift.

This might help too: directions from Steve Brotherton from another of my old threads:

Yes, needle nose pliers did it. Just snag the edge of the dome with the pliers and gently rock it back and forth pulling outward. It will finally release and is just a friction fit. The dome has about a 3/4 inch shroud which fits inside the vcv. The shroud has two square cutouts which must be fitted properly to reinstall. Once the dome comes out, just turn the nut. I asked Steve Brotherton if the nut was just a stop for the copper block which was snugged to it - thinking the copper block needed to be moved then the nut snugged to it again. He said it is only necessary to turn the nut. Maybe the copper block adjusts downward to the nut under presssure; not sure. \\

So turn the nut, check the new vac reading, and push the plastic dome back in after you get the proper inches of vac. 15" in my case.

Would have been much easier if someone had posted this earlier. But this should be helpful to folks in the future.

The tranny shifts quite well now except for a small flare.

By the way, this is my understanding and folks like Kerry could probably give you a better answer.

sixto · #3 09-08-2016, 09:52 PM

That's a good description but the modulator sets transmission operating or governor pressure. It should be set during a road test then left be. It's a band aid to adjust the modulator to compensate for shift flaring or harshness, but we do what we must if a rebuild isn't in the cards. IIRC the VCV spec is 15" Hg at idle to 0 by 100- or X% throttle.

Sixto
83 300SD

Junkman · #4 09-08-2016, 10:30 PM

I have at least 1 trans manual if needed for slightly newer trans. The modulators are colored plastic. The modulator pressure is similar for a given color of modulator regardless of what trans it is in. That will get you close enough for guvt work.

kuene · #5 09-09-2016, 11:03 AM

thanks all. will keep working.

fahrvergnugen · #6 09-09-2016, 01:44 PM

Kent has a good write up on how to tune your tranny. I'm going through it now and find it very thorough.

kuene · #7 09-09-2016, 03:02 PM

I have the Kent manual. Sometimes her over explains things and sometimes he skims over important things. It's hit or miss - but its definitely one of my sources.

I also found this guide which I find fairly helpful... so far. Any peachparters have opinions on it?

http://jenniskens.livedsl.nl/Technical/Tips/Files/Mercedes%20722.4%20adjustment%20guide.pdf

tyl604 · #8 09-09-2016, 05:24 PM

K - that is a pretty good write up. It explains a lot that I learned by trial and error and with the help of Kerry when I got a new tranny installed in 2011. I finally got it pretty much sorted out with his help - and JT.

Junkman · #9 09-09-2016, 05:30 PM

For the part about having a 10mm gauge, just take a nickle and eyeball the center and drill a 1/8 hole in the middle. Glue 2 nickles together if you want to get fancy and have a thicker gauge.

Measure the nickle if you want to check it before drilling. I forgot the numbers but the nickle diameter/2 - 1/16" ~ 10mm close enough to work and you don't even need to run to the parts store.

Or ask Professor Duck Duck Go (if you don't want your IP logged by Google. http://www.answers.com/Q/What_is_the_diameter_of_a_US_nickel

kuene · #10 09-12-2016, 03:10 PM

update on my 82 300D.

as mentioned in original post, in the 7 months I've had the car, trans has been flaring for a while, then didn't for a bit, (clunked on final downshift) but that was resolved when my mechanic noticed the line to trans was cracked - replaced that and fixed the downshift clunk, but flare returned.

Things I did:

1. Isolated or fixed any vacuum leaks.

2. increased the length of the little rod to VCV lever from 120mm to 122mm as per spec. Seemed to now make the VCV arm go to within .5mm of stop when accelerator fully mashed.

3. tested VCV in place, goes to zero when fully open, goes to around 6 when up against the 10mm gauge. seems to be working correctly and so I did not adjust the VCV itself.

4. I noticed when adjusting my throttle linkage that the lever controlling the bowden cable was not engaging at all, had jumped over the little tab/stop. I resolved that of course - wondering how long that has been like that - could have been as short as 2 weeks since I had some unrelated work done, or it could have been like that for a couple months. would that create flaring - no bowden cable?

5. When I mityvac test the line to modulator, I get a leak down. for example, if I pump to 15, it leaks back down to 0 in about 15 seconds. Is this a bad leak or acceptable leak?

Results and residual/remaining issues:

1. the flaring appears to be 100% gone, seems to shift pretty well and about when I would expect. However, the shift out of P into D or R now feels pretty abrupt, kinda makes the car lurch a tad. Is this a big problem I should worry about? and is this a well-covered topic?

2. I was checking the trans fluid level yesterday and I noticed it had a stronger smell than I recall - perhaps this is the burnt smell people talk about. Had not noticed this the last time i checked. Within the last few weeks I had the tranny fluid/filter changed. DexIII and an MB filter. He charged me for 8 quarts which is ridiculous, I know it can't fit much more than 7 on a good day. But anyway, I'm suspecting there is too much in there. However I can't seem to get a clean reading on the stick, it's all over the map when I pull it out. It definitely is above the max mark though. So two questions: could overfilled tranny fluid contribute to a burnt smell/darkening of fluid, and does anybody have any tips on getting an accurate reading on the dip stick? How much should I take out each time in order to not remove too much?

tyl604 · #11 09-12-2016, 09:41 PM

Did you remove the shift linkage underneath, perhaps to insert new bushings? If so, maybe you got the front linkage a bit long (or short).

I am addressing the lurch when you shift to drive or park.

kuene · #12 09-12-2016, 11:05 PM

no, I didn't touch any shift linkage. to clarify, flaring is gone now, vcv seems fine. - but i'm wondering about the possibility of too much trans fluid...maybe its a bit burnt smelling..

also, little lurchy when i shift out of park into drive.