Vacuum modulator confusion
 

Bit of a long story but I have isolated a drop in vacuum between the modulator and top pipe, this is after Y junction to diaphragm on side of pump.

Going to have another go at remove/replace, one of the allan bolts is rounded off due to previous attempts despite lowering box.

Couple of questions;

Kent talks about (at 2.48) the cap retaining vacuum on both of these modulators, is this correct as there is no seal inside the cap which is fairly loose anyway.

If the cap does need to be air tight i can search around for another O ring and make good.

Thanks

https://www.youtube.com/watch?v=gM4KZvwsCW0

This video shows the cap/modulator in more detail, I would be surprised if the vacuum relied on the cap rather than say a small O ring around spindle.

https://www.youtube.com/watch?v=GIXR7gNXOws

https://i.postimg.cc/MTgwz4PK/s-l500.jpg

If yours is like my '81 300SD, remember that the transmission needs vac only for downshifts. The vehicle is designed to have about 12inHg of vac at the VCV, but as you apply the throttle, the vac almost immediately drops to zero. Yes, it is theoretically a gradual release but if you hit the accelerator it goes to zero in about one second.


In fact you can drive with the line unhooked to the trans modulator and you will get smooth upshifts and clunky downshifts.

What exactly is your problem? What are the symptoms and why are you thinking vac modulator?

Thanks, its been an ongoing fault which I visit every now an then hoping to find a solution.

1st and 2nd shifts are lumpy, valves (B1,2 K1) have been changed along with 3 ATf flushes , attaching aMytivac to the T piece (pump top) shows a rapid fall in vacuum to the modulator.

The modulator should hold vacuum just like any other vacuum pod. Supposedly the cap does make a difference with vacuum holding, but both my SDL and SD will hold vacuum without it.

If the bowden cable is adjusted properly, the throttle linkages are adjusted properly, and the engine is making the correct power, even with no vacuum you should have a reasonable (but firm) shift. If you're having sloppy shifting or hunting, the modulator likely isn't your culprit, something else is out of adjustment or worn out.

Just went out to try unplugging the modulator altogether, upshifts are a bit firmer as if there is no cushion, downshifts appear the same.

As another sort of test, plugged the black vacuum line (which goes to the box via Y shaped pump fitting) directly into pump, this made a big difference, all shifts pretty much normal.

Wondering if its just lacking vacuum due a crack or something, plugging into pump direct help boost vacuum?

Diesel, not convinced about the cap as it needs to be removed for adjustment, if it was part of vacuum circuit all would be lost making adjustment impossible.

He is right. If your modulator will not hold a vac, it is bad. What color vac modulator do you have? It must hold a vac to work properly.

On the original 1981 vac modulators, the cap was just a black rubber cap. You removed it, turned the T pin, and replaced the rubber cap. The rubber cap was to hold vac.

If your Mityvac will not pump up and hold a vac by placing it on the trans modulator nipple, the trans modulator is bad. Or maybe it was installed cockeyed.

Yup, its deffo bad, there is some resistance there but gauge drops from half to zero in maybe 3-5 seconds.

Dealer looked up correct unit which is brown (but looks orange), three day delivery door to door from Germany at around $100.

Weather is starting to turn here, will get the old one off next week or two and report back -

Last attempt removed trans cross member which wasn't enough, this time will take off engine mounts letting the engine rest on wood - hopefully this is enough to get that last bolt.

Bowden cable has been tweaked to hold onto gears longer, will need to re-adjust - have the procedure somewhere but need pressure gauge fro proper job.

You might consider giving our sponsor Pelican a call. Maybe they have one. Also think Bergstrom has them.

It would be great to support Pelican but unfortunately other side of pond in UK, just came across this excellent Corteco manual for 722.3, 4

https://www.scribd.com/doc/24267811/Mercedes-722-3-722-4

Going to have a goo at pulling this out tomorrow, not much room between cross member and sump for lowering - thinking of undoing and removing one engine mount then twisting engine and box enough to access stuck bolt, is there anything that might snag ?

Is the gear selector linkage cable or rods, that might cause a problem if it is, cheers

Okay, four hours later and one vac modulator removed - the above approach worked well loping the engine to one side, i did have to undo rear trans mounts also.

Bit of good news also, the modulator looks fine saving $100 - the loss of vacuum stems from having the T bar rotated left, i assume at this position the modulator is discharging vacuum at that setting, turned to the right several turns reduces the drop to almost nothing -

..here's the key, it uses the hole which the T piece sits i as a vent, covering this stops all discharge at any setting, this makes sense as there is no other visible vent.

Subscribed .

In an attempt ot get my shifts perfect a few years ago I dove deep into the modulator along with other components. After testing the trans pressure port with a gauge (there is a good guide on here somewhere) I was somehow convinced I should change it. I did have a small vac leak - though it was also obvious the original cap was cracked and brittle. I should have started with the cap but....

Gaining enough access in situ and swapping out my modulator in my 300D was physically the hardest thing I've ever done on my car. disconnected drive shaft at flex plate to be able to shift the trans to side more. In the end, the new (correct color, genuine mercedes part) one would not seat correctly and not leak no matter what. It was pure hell for like 2 days, destroying one new modulator in the process. I just reverted to old original modulator and then put on a new rubber cap. Doesn't leak vac, good enough for me. I will never try that again!

Update & conclusion
 

Fitted parts in reverse order in the dark, all good - used 10mm captivated bolts instead of Allan bolts, tbh probably made it harder due to recess and steep angle.

For anyone reading wanting to test the modulator or diaphragm integrity, this can be easily undertaken with the part insitu.

Mark the current position of T bar then rotate fully clockwise, at this point the valve will be closed leading to 100% seal, the reverse is true anti-clockwise.

As I understand it, the operation of the modulator affects both the hardness and response of shift points, this is achieved by regulating the vacuum against the spring loaded diaphragm - the vacuum varies according to RPM of engine, the modulators response to this regulated by the valve which the T bar sits in, clockwise closes the valve making the response to vacuum load far more responsive, anti-clockwise no vacuum load and possible no modulator intervention.

The cap has no bearing on the modulator operation other than to provide fine adjustment and secure T bar in place, if this were fully sealed it would close off the valve altogether.

Couple of points of note, whilst you can check the vacuum in situ you cannot see the piston operation, in my case it was almost certainly stuck or at best slow to operate – this could be down to the setting which essentially left it in one place rather than moving back and forth.

End result, different car – silky smooth changes in unison with the engine, before it changed up just one after the other without reference to much else.