722.315: Vacuum... again - PeachParts Mercedes-Benz Forum

CDTurbo001 · #1 01-15-2018, 09:43 PM

Just spent 2 hours going over, and going back over, the vacuum lines and appliances on my turbo coupe. I checked the line pressure the other night (Bowden cable had slack, modulator unhooked - ran it at idle and at 50 km/h, same pressure) and found it at 5 bar, which I adjusted back very close to the spec'd 2.9. It shifted smoother but first gear still wouldn't tolerate much power off idle and the other shifts (except 3-4) were spongy and would borderline slip under heavy throttle - okay, maybe I just need to bump pressure back up a little. But this evening while letting the engine oil drain I went back over the VCV, line routing between all the tees, and the thing connected to the throttle linkages on top of the valve cover (what the heck is that thing?). Upon firing it back up and testing vacuum on the VCV output to the modulator, I had about 15.5 hg at idle, but as soon as the throttle linkage moved it dropped to zero because of the thing on the valve cover dumping all the vacuum. After disconnecting the valve cover thingy and plugging all pertinent fittings I had gradually decreasing vacuum (the way it's supposed to be... right?) from the VCV as the throttle opened so I hooked it up and went for a drive. Slipping like crazy in every gear except 4th. I parked it and called it a day.

The modulator is supposed to have a variable vacuum signal which, in turn, ups line pressure to prevent slippage, right? So what's the explanation for the transmission behaving so awful when it does have that varying vacuum signal? Is the thing on the valve cover supposed to drop all vacuum as soon as the engine comes off idle?

Diseasel300 · #2 01-15-2018, 09:53 PM

The earlier cars have some switches under a cover on the valve cover that control the EGR servo. If the servo is disconnected or if the diaphragm is torn, your vacuum will bleed off to nothing when those valves open.

The VCV should have high vacuum (12-15") at idle, and bleed off to 0 as you progressively press the accelerator. Since you have that, your vacuum system is doing what its supposed to. The control pressure drops with higher vacuum to provide softer shifts. If your transmission is worn out or the valve body is damaged, someone may have cranked up the modulator pressure to make it shift acceptably.

I'd go back and verify the operating pressure you messed with and see if it's still set correctly. You have checked the fluid level?

CDTurbo001 · #3 01-15-2018, 10:08 PM

Ah - I wondered if that gadget wasn't purely for the EGR. I removed the line to the EGR valve a couple months ago so I suppose that can stay cut out, as well?

While the shifts are a bit soft with the pressure at 2.9-3.0 bar, it's still basically just as drivable as it was with pressure at over 5 - the tipping point is having the modulator pressure at 14, gradually decreasing, vs. having it drop completely out (aka, the signal to the modulator is the same as if the engine were at WOT) like it apparently was with the EGR servo hooked up. That seems like it would take an awful lot of wear to make it behave well with high line pressure and no vacuum, and basically undrivable in every gear with correct fluid and modulator vacuum pressure.

I changed the fluid and filter a few weeks ago and checked it a couple days ago - didn't lose more than a couple drops doing the pressure check. Running, in park, right?

Wait... I just thought it through and this could make a difference. I noted that the modulator on the car appears to be an updated type? with a little handle on a plastic cap rather than rubber. It's green. I've also checked it in the last couple months and it doesn't leak. If the modulator is the wrong one for this transmission and it's calibrated for less vacuum or more line pressure, couldn't that explain this behavior?

dude99 · #4 01-15-2018, 10:31 PM

You can completely remove the valves on top of your valve cover on an 83, no issue, just block the source line to them

Diseasel300 · #5 01-15-2018, 10:37 PM

The VCV is designed to bleed off vacuum to the modulator at full rack travel for firm shifts under high power to prevent slippage. If your VCV is bleeding to 0 well before that, you'll have rough/abrupt shifting behavior if the modulator pressure and control pressure are correct. With the vacuum line pulled off the modulator, it should just about feel like a teenager learning to drive stick. With gentle throttle input (Grandpa driving), that 1-2 shift should just about feel like you got rear-ended with no modulator vacuum.

As mentioned in Post 4, simply plug the vacuum source to the EGR valves and move on with life. The EGR is a common delete due to the soot fouling it does in the intake manifold. If you leave the vacuum source connected and have the EGR deleted, you'll have a vacuum leak when the switches activate.

CDTurbo001 · #6 01-15-2018, 10:43 PM

Ditto on the EGR gadgetry - it'll clean up the valve cover, anyway.

I didn't disconnect the VCV to test its reaction but it did appear to take WOT to get it down to zero - wasn't going to hold it there with the engine running.

The other day when adjusting pressure, out of curiosity I hooked the vacuum line back up to the modulator to see how pressure changed, and at idle it dropped to basically zero. Is this correct or should there be at least some pressure at the test port, with everything hooked up?

Diseasel300 · #7 01-15-2018, 10:47 PM

I'd expect pressure even at idle with vacuum applied. Are you sure you're connected to the correct test port?

CDTurbo001 · #8 01-15-2018, 10:50 PM

On the driver's side just behind the modulator - 12mm hex head plug?

Errr... reading over this http://manualzz.com/doc/6307340/722.3-and-722.4-transmission-adjustment-guide write-up apparently VCV output is only supposed to be 3.3-4.7 inches for my tranny. That could be the problem! The only externally visible restrictor orifice in the line from the main vacuum supply (to the booster) is a white one which isn't even shown in his list. I'll have to check input vacuum to the VCV and, if it's up past 15 like I suspect it is, find some other orifices and get it down.

Diseasel300 · #9 01-15-2018, 11:05 PM

Start out by making sure your vacuum system matches what is in the FSM for your year model. It's not uncommon to have the orifices missing if someone's been in there before and just thought they were a connector. Some "orifices" look like a check valve, my car has a green one just before the VCV on it (different system tho).

Mxfrank · #10 01-16-2018, 07:48 AM

May help.

CDTurbo001 · #11 01-19-2018, 12:11 PM

Okay, follow-up.

The supply line to the VCV had one white orifice in it - I don't have gauge pins to measure it and don't find a white one mentioned on the 'net but it appears smaller than 1MM. I'm in the process of making a few to see if I can get input vacuum to the VCV down to <10 inches, but in the mean time I adjusted the VCV output to approx. 3.8 inches, bumped the modulator screw back up 1/2 turn (probably at 3.2-3.4 bar now?), and it was acting much better. A member of a Facebook W123 group said the transmission actually rests in second and drops to first upon takeoff, and this function can be exaggerated by wear or a too-loose Bowden cable, so knowing that line pressure and modulator vacuum were real close I began tightening the cable back up.

Per the suggestion of at least one board member (can't remember who) I was attempting to set the Bowden cable such that WOT upshifts were at around 4,300 - but even with 1/8" of play in the cable at idle it would still pass 4,500 and keep on going, such that each time I tested I felt compelled to back out of it a little to let it shift. With the new info about the 2>1 shift upon takeoff, and an apparent consensus that the 3-4 shift, under near-zero load, should happen around 30 MPH when mine was happening at 23-24, I adjusted the cable in several turns initially - no change to the 3-4 shift yet but first was acting better. Another turn and a half got it up around 25-26, and yet another put it up about 31 with shifts being a little hard and letting the engine spool a little hard before shifting. I backed it down 1/2 turn before going into the coffee shop to post this and I expect it to be good enough to leave alone for a while. I don't know what my WOT shift speeds will be, but I'll live with having to judge that with my foot and have drivability 99% of the time - maybe the governor needs adjusted to get both traits? If so I'll delve into that at some point.