6.9 control pressure rod position

[daantjie]

Hi guys
Calling 6.9 drivers with transmissions shifting right with no slip or flare and with nice firm positive shifts across the band.
Please snap a pic of the control pressure rod as it sits in the spring clip at the throttle linkage.
I know each trans has a unique setting beween modulator and control pressure  so this will not be scientific of course.
Just interested to see where the "happy medium" is.
Reason I am asking is that the service manual shows that you have to push the rod all the way towards to firewall and then clip it in this position, but this will result in very low control pressure, thus I do not know if this "blanket" adjustment is correct for the W3B 050 trans
Cheers

[rumb]

Here is mine. also check bushing at transmission, if missing will give you 1/2" of slop.

I think the position mostly controls at what rpm the trans shifts when you are accelerating, light load vs flooring it.

[daantjie]

Thanks Robert!  Almost exactly matches my setup (I guess I need to post a pic of mine now eh )  will snap one tonight and post.  I have a new bushing pressed in so I have no slop.
Interesting again that yours also is quite forward in the clip or nowhere near the "pressed full back to the firewall" as the manual states.  Appears that the trans manual covers many different transmissions, at least the one I have covers all the 3 speed ones and I guess they use a blanket setting in the manual, which is kind of odd
By the way looking spiffy in that engine compartment also with all that plating and new vac lines! I will be "slightly" embarrassed to show mine in comparison
Another point of interest (for me at least) is the setup of the throttle linkages.  The manual states that the "roller in gate" must rest free of tension "against stop".  I am a bit perplexed by this, but it appears as if you have the right setup as per the manual in the the plastic roller is "nestled" in the gate at the end of it's travel so to speak?

[daantjie]

Here is pic of my control pressure rod in the clip:


Then also the position of the "roller in gate".  It is hard to tell from the service manual but I think it should be resting in the position marked with red square if properly adjusted:

[daantjie]

Any other 6.9 divers care to weigh in?

[TJ 450]

This is tough to get right and it relies on the bushing at the trans end being intact. You also need to make sure you can achieve WOT before adjusting the control pressure rod, which I have found usually needs to be pulled forward, not backward.

Tim

[daantjie]

This is tough to get right and it relies on the bushing at the trans end being intact. You also need to make sure you can achieve WOT before adjusting the control pressure rod, which I have found usually needs to be pulled forward, not backward.

Tim

Absolutely Tim. This is very frustrating as per usual the service manual tries to cover a blanket adjustment but clearly there is a lot more finesse involved.
Does not matter what I try I cannot get the roller to rest in the curve of the gate so I am thinking the pic in the manual showing this is for reference only and not an absolute position.

[TJ 450]

Yes, I think where that roller rests is good. If it were right up in that corner you'd have a fair bit of extra play in the pedal before anything happens.

Tim

[raueda1]

Yes, I think where that roller rests is good. If it were right up in that corner you'd have a fair bit of extra play in the pedal before anything happens.

Tim

I've recently been fiddling with this stuff.  My roller was originally "floating" a bit like Daniel's.  After a lot of adjusting, readjusting and adjusting more it ended up the same as Rumb's.  Now there's no play in the pedal at all (and I had a lot!).  To do this I disconnected the pedal connector with the spring and fiddled with the linkage on the engine till there was no slack anyplace and throttle was opening and closing fully.  I then reconnected the pedal and messed with the bolt until there was no play AND WOT was really WOT.  It took several iterations.

Now I'm trying to find the sweet spot for the shift control pressure rod. Mine was originally quite far back.  Under light load the car shifted very early, or so it seemed to me.  The car would shift at about 1800 rpm, realy just loafing along.  Under WOT the 2nd -> 3rd shift was also premature (~25 mph too soon).  Pulling it forward, about like Rumb's, raised the shift points across the board.  Under light load it now shifts about 2500 rpm.  At WOT it shifts later but still not corresponding to the speedo dots under WOT.

Now the question is, what's optimum?  Setting so shifts at WOT correspond to the speedo dot and letting the rest fall where it may??  Or does it depend on how/where you drive the car?  Around town it feels like something between very-far-back (where it was) and farther forward (where it is now) would be best.  However, that would also reduce WOT shift speed which is already too low to begin with.  Farther forward should better calibrate the WOT shift speed but at the expense of reving higher when city driving.  Does anybody know what the 2 -> 3rd shift speed ought to be under light throttle?

Wouldn't it be great to have a brand new car to drive as a benchmark?  I'm going to pay closer attention to shift points and write them down so it isn't as subjective.  Thanks,

[raueda1]

Very nice progress to report and share.  I played with the shift control rod through all ( or most) of its range.   See picture. Settings A, B and C correspond to the rear of the clip, colored blue, overlapping with the red marks on the rod. Results:

A (rod very far back in clip towards firewall):  Definitely the wrong place to be.  Premature upshift under light throttle.  1st->2nd and 2nd->3rd only about 1300 rpm on level ground.  Downshifting under partial throttle doesn't realy happen, need to floor it.  Highway downshift under WOT depends on speed.  Doesn't seem to downshift at all over about 80mph.  Below that, say 65mph, it downshifts but upshifts from 2nd->3rd at about 75-80 mph.  All this makes car feel sluggish and is clearly out of whack.

C (opposite of A, rod very far forward in clip):  Also very wrong.  Even more wrong in fact.  Shifting is very delayed.  1st->2nd and 2nd->3rd occur at about 3000 rpm.  On a grade it hangs in gear unless you take your foot of the gas.  Not really driveable at this setting so I didn't bother going on the highway.

B (approx between A and C):  Getting closer, best so far.  1st->2nd and 2nd->3rd occur at about 1600 rpm.  The highway WOT downshifts as it should, though the 2nd->3rd  upshift occurs at about 82 mph - still a bit too soon but headed in the right direction.  Shifts are positive and firm without being harsh.

Setting as pictured, rod slightly farther forward than B:  By far the best, seems close to optimum, probably as good as I'll get without messing with something else like the vac modulator.  The rod is just 2-3 mm forward from B but the change is substantial.  1st->2nd and 2nd->3rd occur at about 2000-2100 rpm under light throttle.  Shifts are firm but smooth, very satisfying I must say!  Partial throttle downshifting at about 40 mph (such as on a secondary road) is excellent and it will downshift directly from 3rd to 1st if not going too fast.  It's easy to make it shift the way you want based on throttle position, not at all the case with settings A and C.  Finally, the WOT shift point from 2nd->3rd occurs at about 90 mph.  That's still a hair low but I can live with it.

I have to say, the driveability and throttle response at the 'as pictured' setting really transforms the car.  The differences from the next best setting is not small.  B was OK, but just 2-3 mm to the final setting made a great improvement.  It therefore seems pretty clear to me that this isn't a one-size-fits-all tweak.  I'm going out on a limb maybe, but it seems like everybody should go through a similar process instead using generalities like 'all the way forward' or 'all the way back' etc.  There really was one setting to rule them all, just had to find it!  Hope this is of value to somebody besides myself.  Happy shift optimization to all and cheers,

[daantjie]

Very nice write up there, Dave !

It always amazes how this rudimentary tech of a simple rod, and small adjustments thereof have a huge impact on shift timing and quality.  Not sure how NA cars of the same era got adjusted, but I am guessing Benz was very far ahead of the game when it came to fine tuning their transmissions.

What you have shown here is that it takes a fair bit of finesse and trial and error to get the sweet spot, and indeed I think for each box there is a different sweet spot, and as always it is marriage between the control pressure rod and the vacuum modulator.

I think many a good Benz transmission was sent to an early grave, purely due to folks here in NA not knowing how to set the 2 crucial components properly.  Slushy shifts are for sure not what you want on these older Benz boxes, they always should give a firm, positive upshift with no hesitation, and under WOT the shifts should be really firm and crisp
Cheers