I am having a problem with my (relatively new and leak-free) modulator - I turn it the T handle but no matter what it doesn't affect shift quality. If I disconnect vacuum from the trans I get really hard, even shifts. With the vacuum system connected ( that has been totally gone over - thanks Brian C.!) If I connect the modulator directly to the trans, and pump to 25mmHg, I get great even shifts at min throttle.


So something is messed up internally that interprets the vacuum signal or responds to modulator adjustments. I am looking for a mechanical diagram to piece this together. does the modulator go into valve body? How do they interact?

I am ready to rebuild/replace the valve body if needed.

Thanks in advance - this goes way above and beyond what the pros in my area are willing to investigate (and I dont blame them),

dd

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

bump

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

>> So something is messed up internally that interprets the vacuum signal or responds to modulator adjustments.

Sorry, I don't follow your logic.

If I have read your post correctly, the tests you have done by removing all vacuum, and imposing maximum vacuum have worked, showing that the modulator is working, and affecting the shifts in the correct way. If anything, I would say your test results point to something easy and cheap, outside the gearbox, rather than something expensive and difficult within it!

Ditto. Something is messed up in your vacuum system.

Thanks for the replies fellas...let me explain a little more what I think is wrong

-vac system has been gone over - mod holds vacuum, vac pump puts out 23mmHg, VCV bleeds vac as it should, EGR lines to 3/2 removed, new shutoff valve, ACC vac bypassed for now, central lock system tight etc.
- i have previously set the vacuum system to factory spec according to the FSM. thats the procedure with setting it to 4 mmHg with the 1 cm disc.
-I have also set the modulator hydraulic pressure to factory specs using the factory method (i wrote the wiki article for it - http://www.peachparts.com/Wikka/Trans722VacMod)

This made my shifts too hard still.

-so I increased the vacuum at idle from about 8 mmHg to 15 mmHg. Better, but still shifts hard at min throttle, and actually slips a bit at max throttle on the 1-2 shift. I think this is because the VCV is not bleeding off enough vacuum at top throttle - if I disconnect the vacuum the thing shifts really hard
-so, as the Steve Brotherton article points out, you have to make adjustments based on feel on an old trans like this, so thats what I did.
- I have never noticed any changes when adjusting the modulator, so I adjusted it, trying to soften the shifts. No change, even if I turn it a turn or two to soften shifts (CCW out). This is strange, since I can see the modulator adjusting the hydraulic pressure on the gauge when I have it attached.
- as I stated in post #1, I have indeed verified that the vacuum is changing the shift qualities. But, I need a lot of vacuum (25 mmHg using the mity vac) to achieve the desired results at min throttle.

So I am thinking something is messed up since the modulator is not making any sort of shift quality change. I know mercedes issued a TSB about the opposite issue on the 722.315 - the modulator pin was too long resulting in not being able to firm up the shifts.

I was thinking to install a superior shift kit (aka rebuild valve body) anyways to take a more hands-on approach to the 1-2 (2-3 is kind of harsh too, not mushy like most people report). I wanted to take a look at what the valve body internals might have to do with the modulator issue, before I ripped into it.

Thanks for reading...this has gone beyond just wanting to repair my problem, but I also want to really learn how an automatic works. I wish I had time to fully rebuild one.

Does anyone have the original and elusive 722.3xx transmission manual?

dd

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Have you confirmed that the VCV on the injection pump is actually bleeding off vacuum? I accidentally tightened the nut a bit too snugly once, and found that it would not bleed off vacuum as the rack was opened up. Perhaps the slipping shift at full throttle is due to this condition. Have you t'd a vacuum gauge into the line to see how the vacuum going to the modulator changes with acceleration/deceleration?

I do find it odd to hear you mention that adjusting the modulator does not appear to be making a difference in shift firmness - I have always noticed a change with only one or two changes in T handle position.

all of the pictures I have found online have an incorrect picture of the valve body. best bet is to be very careful with it and only change the k1 k2 springs and dont split the valve body. you can change these 2 without taking the valve body out of the trans which is safer too. there is only one change that requires you to split the valve body for the retaining clip that is buried inside. if you must you can change all of them but that one without taking the body apart. I ruined mine so I now have a manual trans.

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2001 Excursion Powerstroke (to go dependably)
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Thank you for the replies.

i have T-ed into the line and everything looks OK, I think. Vacuum bleeds down with the throttle. I have not tested at all throttle and acceleration conditions - I am going to do that in the next couple days now that spring is here (finally).

The modulator not affecting shift quality is what strikes me as something more going on...

i have been gathering info on the secrets of the valve body. I have a part from the dealer that is the pin behind the modulator. Its what the plastic pin behind the modulator pushes on. If I do crack the valve body open, this is what I am hoping is the culprit of my flukey modulator- or - something else pops out at me.

I am weighing the merit of that against getting a new valve body from Sun Valley mercedes and going from there.

Ill post some more info soon. A lot of other issues on the car have been worked out and Im ready to attack this one full force...

Thanks again for the thoughts - any more appreciated greatly.

dd

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car