FredR

Bill,

As long as there is nothing untoward with the vacuum being pulled you will have eliminated one possibility. Although it has been discussed not sure if you tested the vacuum at the tranny end of the system- quite possible the vac line could be blocked unlikely as it may seem.

Apologies for asking but have you checked the current set point of the modulator?- if that is set at one end of the adjustment spectrum it will change fast but harsh as I found out once when the vac line took a walk

Bill Ball

Dave and Fred: Yes, baring some other discovery, I will get the car up in the air so I can check things from the tranny end. Right now the lift is occupied by the BMW doing rear subframe bushings. Thanks!

Bill Ball

OK, I got it up in the air. Initially, it looked like the vac hose was loose on the modulator...a bit flared at the end of the hose and not fully seated. I checked that the modulator held vacuum and that when the tube was disconnected I could pull free air through either end of the tube. So, that verified everything was intact and communicating. I cut off a smidge the hose to get a fresh end, seated it well and went for a test drive. No change. The shifts are too harsh. Harsh enough that I think they will do damage over time. Looks like a modulator adjustment is called for, but I sure don't recall any significant harshness until now. So, I'll make an adjustment to the modulator and report back again. It would be good to see the vacuum readings during the test drive, as also suggested. I need to get a length of vac line.

I suppose this could all be coincidental and that some pressure regulator is stuck in the trans. Argh! if so.

Bill Ball

OK..continuing the saga of the harsh upshifts....

I adjusted the modulator 1-1/2 turns counterclockwise. No change. So, that argues either it or a shift valve is stuck. This is a NOS tranny that sat in a warehouse for 30 years. It worked perfectly right out of the box, but it certainly could have stiff seals and stuck valves. Next up.....Trans-X!

Tony

Is the transmission warmed up at all.? Perhaps just some miles and normal heat buildup will put it right if it is stuck?

worf928

Ok. So, before I remove big bits from a 928 I like to be really, really sure I need to. With regard to being sure and asking the "are the lights on Sir?" questions and at the risk of attempting to teach you to suck eggs:
- you have verified that manifold vacuum (17+) is present at idle from the 5-way via a gauge on a dampener (or the regulator?)
- With the SC install, I can't help with specifics ('cause it's not like mine) but you need to be sure you're getting vacuum from a source before the supercharger. Right?

Since you know that the vac line to the transaxle isn't blocked, if you are sure that there's appropriate vacuum on the transaxle-end of the line then... pull away.

Bill Ball

Hi Dave: I can say yes to all the above. Everything is confirmed. After a bit of driving tonight, most of the shifts except 1-2 were good. Often downright perfect. The modulator and/or the Trans-X did something. And the more or less full throttle shift are still perfect. I wasn't sure how much to turn the modulator. Some of the later 722 trannys have a half turn adjustment specifically for people complaining of harsh shifts. A half turn did nothing, so I went 1-1/2. At least that appears to have had no detrimental effects. I'm happy enough with it now to drive it extensively even if 1-2 stays a bit harsher than I remember.

FredR

Bill,

Not sure if it makes any sense to you but my tranny modulator was set about 25% from the fully harsh position or 75% from the fully soft position- how those percentages translate in terms of turns I have no clue but it sounds as though you are somewhere around where my tranny is or was when last checked. Even mine is getting a bit harsh so I also have to go through this when I do my annual maintenance next month.

Any tips/pics about adjusting such appreciated

Bill Ball

Hi Fred: I can't give any well-informed tips. The WSM and all the Mercedes guides I have found just discuss adjustment to target pressures. I'm not going to go so far as to hook up pressure gauges. It doesn't mention anything other than 1 turn changes the pressure by 0.4 bar. The pressure spec is extremely tight at 4.4 +/- 0.05 bar. I saw nothing about turns stop to stop. All the full power shifts are fine, with still maybe a little too firm 1-2, while partial throttle is fine except 1-2, and it's not as bad as it was.

Without removing any exhaust shielding I was able to remove the cap on the modulator and pick out the little t-bar adjuster. It is not attached, so it flew out onto the floor. It's shiny metal, but it still took me a minute to find it. Reinserted it part way, turned it counterclockwise. I found reference to a variation on the tranny that has an external modulator adjustment for harsh shift complaints, and it appeared to go counterclockwise about 1/2 turn to deal with harshness.

On further review, the Bowden cable, also called the pressure control cable, may actually be the issue. If so, I feel real stupid for taking everybody's time here. I always associated it with shift points more than harshness, but it is the first thing mentioned in the WSM under troubleshooting partial throttle shifts jolting. Mine may have changed slightly due to changes in the throttle geometry with the SC installed, although I didn't think the resting position of the throttle arms changed at all. BUT my Bowden now has about a mm or 2 of slack when I pull the ball off the socket. I don't think it had any slack before, and I have been aware that no tension is how it should be adjusted. I just didn't think that small amount of slack was significant The only adjustment point I know of, at the throttle quadrant end, cannot be tightened further. Is there another adjustment point? I feel like an idiot (again) if that is the root cause here. People discuss adjusting this cable one way or the other to change shift points, so I assumed if your shift points were OK, leave it alone. Hmmmm... What do you think? Is a tiny amount of slack in the Bowden an issue?

dr bob

Late to the discussion --

From a Monday-morning armchair expert who has never supercharged anything... Add salt as needed.


The vacuum you want to the modulator decreases as the throttle is opened and load increases. It's easier for me to look at it as manifold absolute pressure, if that makes any sense. In a supercharged car, the manifold absolute pressure ramps from Small at idle, goes Up from there as the SC helps flow with throttle and RPM, drops to pretty much Zero on trailing throttle and RPM still up. The modulator (and the FPR...) need to see actual intake pressure downstream of both the throttle and the supercharger. Pretty much what the intake valves see. I don't know where the throttle body fits in your Keel installation, but if it's upstream of the kompressor you'll want to move the loa- sensing vacuum connections (FPR and AT modulator) so they see actual engine manifold pressure. The dampers should be plumbed to a port that never exceeds atmospheric pressure; Those connections are only needed to support a leaking diaphragm. If you've been sharktuning with the FPR reading somewhere other than actual engine intake pressure, you'll want to take a new look at the higher-load cells, as the fuel pressure profile will now more closely follow load.

The vacuum system that used to include the little Venturi was there to maintain vacuum when the engine is under high load, so the vacuum-actuated HVAC, flappy, and cruise control functions can be maintained while full-throttle cruising. The system is plumbed to the brake booster, but that booster can reservoir has a check valve so there is initial vacuum maintained. This gets more important in supercharged and turbocharged cars. There's a far-out chance (same as 'always') that you would want to have full vacuum brake boost available while the engine is at full load, and it was an easy connection to make. Point being that the "load sensing" vacuum system is different from the brake booster and accessory system, and also different from the ported vacuum system that provides distributor vacuum advance based on charge velocity through the throttle body.


If this helps: FPR and transmission modulator should be plumbed to the same port where your boost gauge is connected.

FredR

Bill,

First of all the whole point of these forums is to work on the premise that many hands make light work- it is somewhat foolish to have issues and not discuss them.

My views on the Bowden cable [ I refer to it as the kick down cable assuming we are talking about the same thing] may not please everyone but as I am concerned the stock method for setting the Bowden cable is somewhat deficient. As I recall the stock setting is such that the cable should be set with the cable one turn slack after the cup aligns with the socket- I set mine about 3 to 4 turns tighter than the WSM procedure. I did this initially to facilitate first gear starts as I found the second gear starts somewhat awful. Nowadays I also run with the kick down bypassed as well. My adjustment criteria is set by what happens when rolling on at about 70mph. Gentle throttle pressure should allow to car to roll on without shifting but anything more than a gentle touch and it changes down instantly. As I am aware this does not change the shift points per se but it makes car feel much more alive.

Needless to say it is one thing ensuring first gear starts with ease every time but that does not mean one has to take off like a bat out of hell every time. As I am concerned if the box cannot handle first gear starts it should not be on the car- common sense being the order of the day. This mod coupled with the PSD system makes driving the thing effortless- just wish it had the 6 speed box in my Cayenne Turbo S [and its power delivery for that matter].

worf928

What I know about automatic viscous-coupled transmissions in general and the 928’s automatic transaxle specifically might fill a thimble; 90+% of my clients are 5-speed owners.

With that caveat, I have had issues with the kickdown cable. But those issues translated into shifts at the wrong RPM not shift firmness IIRC. I can imagine that there might be a relationship between the two such that a messed up kickdown cable might cause your issue. But, 2mm is not, I think, sufficient mis-adjusment.

Your post helped jog my memory. And also - see above - that I needed to rethink for the kickdown cable.

The “fuel stuff” needs to see the external pressure at the injector. Thus they need to be plumbed after the supercharger as you wrote and not as I posted above.

The lines from the stock 5-way and the 4-way at the booster need to be plumbed differently on a positive displacement SC (like the Keel kit.) Everything but the ”fuel stuff” should get vacuum from between the throttle plate(s) and the supercharger inlet such that they see more-or-less the same vacuum behavior as they would in an N.A. engine (e.g. 17-18 vacuum at idle to ~0 at WOT.)

Lizard928

I browsed through but not extremely thoroughly.

What is the level of vacuum that the modulator sees with the car at full operating temp idling?
I would recommend that you monitor the vacuum level with the car driving (at the transmission with a T fitting).

I am working on rebuilding a Keel kit that was on an 87 and drove great and then didn't. The intake leaked more boost than it held and through everything out of whack.
The top plate is held to the surround with mostly just sealant. The bolts holding the IC to the top pate were all leaking as were IC pipe connections. The lower plate had some weld ground down and it was completely split left to right and would have leaked really badly.
I've welded the top plate to the surround as well as welded in new injector pieces and am in the process of redoing the IC mounting to ensure everything is as rigid as is needed.
I'm also making it so that the IC can be pulled off without pulling the entire intake off so assembly is MUCH easier.

My point with those comments is that it is likely that you have many vacuum and boost leaks which will vary drastically as you drive.
It is also possible that the seals and valve body in your old transmission had a slightly higher leak past rate which can also affect how the transmission shifts etc.

Bill Ball

dr bob, Dave, Colin - you've added a lot for me to understand and respond to...and of course, I now have more questions. It's going to take me a bit to organize a reply that doesn't confuse things further. In the meantime, the shifting issue is now minor to nothing and I'm doing more Sharktuning. THANKS!. Oh, and Tony, I didn't mean to overlook your comment. I won't.

andy-gts

so trans x helped a lot it sounds. .....cool