vacuum mystery, shifting problems... - PeachParts Mercedes-Benz Forum

mgcanuck · #1 02-24-2008, 12:26 AM

I have a 79 300td. I had a used tranny installed, which flared and shifted poorly, so the shop said i should rebuild it. Did, and problems weren't solved! initially, they blocked off the check valve to the door locks, and the green ac line. The car shifted very softly and early. However, since i got it back, it is late shifting at the top of the RPM range, kickdown won't engage, and it won't shut off.

The shop found the 't' fitting too large, so the filled it and drilled it out to 20 tho/inch...The gauge pulls 12.5 in vacuum at the 't' fitting, and at the line to transmission from the 3-2 valve atop the valve cover.

I have replaced all the rubber vacuum lines/connectors--both in the engine and at the key shut-off terminals. I have replaced the two brown vacuum lines.

The shut-off switch functions, but only periodically and takes a while to engage (hitting the thottle). Sometimes it only seems to get 4 hg

At some points i get NO vacuum reading at the line to the trans modulator. Then it will kick in, and the vacuum control (proportioning) valve does it's job--leaking down vacuum with throttle opening/rpms.

SO...sometimes it shifts like butter, sometimes hard shifting. I've also tried blocking the line to the shut-off valve, to no effect.

Is my vacuum pump leaking?

**Just read the long sticky thread on transmissions: it would seem possible/likely that the vacuum line going from the 3-2 valve after the VCV might be cracked/leaking! This would explain the intermittent shifting. I don't have a vacuum pump gauge, just a dial gauge. So i will go get one tomorrow to see if the vacuum line to the modulator on the tranny is holding vacuum!

popscat · #2 02-24-2008, 02:41 AM

i have a good trans from a 79 300d come off a turbo 617-950

rhodes2010 · #3 02-24-2008, 03:06 AM

The 3-2 valve on the valve cover leaked on my 1980 and was
the cause of all my shifting problems.

It is easy to test with a vacuum pump (mitey-vac type).

I can't be sure from what you wrote that the valve was tested.

The small arm on the valve can be replaced separately from the whole valve.

On these particular years, that valve has to be operational.

Was the shop that did this work a Mercedes oriented shop ?
Vacuum is crucial to the operation of a Diesel Mercedes.

Not everybody gets that.

In addition , I had to carefully arrange the vacuum connections to
feed trans, heat/ac system, door locks, and shutoff.

For me on my old car arranging them in the wrong way on the tees
had adverse effects.

With a vacuum pump , do some isolated testing of each part and see
what you get.

mgcanuck · #4 02-24-2008, 11:05 AM

The 3-2 valve was the first thing we replaced, thank you. It was not a mercedes shop, it was a generic transmission shop (my car had a limited warranty when it failed, so i had to take it there, and they didn't cover it anyway).

I have replaced the 'y' and 'x' connectors for the lines, as well as plugged them with golf tees (a/c, locks) to isolate as well. The only thing i haven't tested is whether the transmission line holds vacuum. I don't have a mighty-vac, though i could just put a T connector between the 3-2 valve and the line to the modulator to see if it holds. Also, it's possible the T at the booster line is leaking, which is why i have intermittent vacuum (rather than the vacuum pump).

kerry · #5 02-24-2008, 11:13 AM

The shift points are controlled by the bowden cable (or rod depending on the year) and not by vacuum. So, the symptoms of high rpm shifting can probably be resolved by adjusting the bowden cable one way or the other. It may even be stuck since your symptoms have appeared on member's cars with stuck cables.
I'd get a Mity Vac. They're not very expensive and tracing down the vacuum problems is going to be MUCH easier with it.
I'd T into the main vacuum line, run the gauge or Mity Vac into the car or out onto the cowl where I can see the readings and drive around for a while. Your problem could be as simple as a sticking check valve in the vacuum pump.

mgcanuck · #6 02-24-2008, 11:20 AM

Thanks, Kerry. So why does it go from high rpm shifts/rough, to intermittent soft, easy, consecutive shifts (when i plug/unplug lines, for example).

Which control rod are you referring to? The one on top of the valve cover? Is it the one that has some slack in it, adjustable with a nut?

kerry · #7 02-24-2008, 12:01 PM

High rpm shifts are always hard if the system is working because more vacuum has been bled out from the modulator vacuum line via the VCV, resulting in harder shifts.
Yes, bowden cable/rod goes from top of valve cover down to right side of transmission and is adjusted with a nut. My 79SD has a rod and not a cable. Length of the rod determines shift points. It sounds like yours is functioning intermittently. The other possibility is that your kickdown switch may be functioning intermittently because when it sticks open, it pushes the shift points up to very high rpm's. You can disconnect that switch and see if it makes a difference.

All this presupposes that a Japanese 123 is similar to a US 123. May not be a safe assumption.

Brian Carlton · #8 02-24-2008, 12:20 PM

Quote:

Originally Posted by kerry

The shift points are controlled by the bowden cable (or rod depending on the year) and not by vacuum. .

Kerry, prior to a certain year, the shift points were controlled by vacuum because there was no Bowden cable.

Do you know the year of the changeover on the W123?

mgcanuck · #9 02-24-2008, 12:20 PM

Kerry...Just followed your instructions at put my gauge in-line with the t fitting at the booster line. Low vacuum. As soon as i plugged the a/c line, it went back up. The car shifts perfect and shuts off (albeit slowly, though that might be vacuum needing to come up a bit since i unplugged bits).

I had the same 'fix' a few days ago, then when i hit the kickdown switch, it stuck on high rpm shifts. I think the switch may be faulty or sticking, as you suggested. Where does that switch connect and how do i replace it?

Thanks!

kerry · #10 02-24-2008, 12:50 PM

Quote:

Originally Posted by Brian Carlton

Kerry, prior to a certain year, the shift points were controlled by vacuum because there was no Bowden cable.

Do you know the year of the changeover on the W123?

You're right that's way too broad a statement and I'm making assumptions about the 79Td. My 77 300d has no vacuum control valve and the 'bowden cable' is actually a series of complicated rods operating bell cranks going down thru the intake manifold area then turning 90 degrees to head back to the transmission.
My 79SD has both a vacuum control valve and a solid rod going from the top of the valve cover back down the right side of the engine. But it is a turbo, whereas the 79TD isn't.

mgcanuck · #11 02-24-2008, 12:56 PM

Mine's a 123.190

Ok , so i think mystery solved. This might explain why i have constant heat even when the temp is turned to off (vacuum diaphragm/control behind center pod is shot or the line disconnected or rubber connector worn?). Also, why it worked before until i hit the kickdown switch. But then if the switch was still connected, and i plugged the a/c line, wouldn't the kickdown switch still be stuck open?

mgcanuck · #12 02-25-2008, 04:36 PM

After reaching operating temp, the car shifted perfectly today. Then after sitting 3 hours it went back to late shifts and slow shutdown. I'm thinking my vacuum pump must be leaking or somewhere else?

kerry · #13 02-25-2008, 07:13 PM

I think you've got vacuum problems in the climate control system. Turn the heat off completely (push far right button) and see if it improves. Alternatively, plug off the green vacuum line exiting the engine compartment. I think this because when the engine is cold, the system shuts off the climate control system until the engine reaches operating temperature. Something in that shut off mode is leaking.
All this assumes you have an automatic climate control system like US market cars.

mgcanuck · #14 02-25-2008, 10:13 PM

The cc line is already plugged! That's how I got things shifting. Also, the car is a manual climate system..I've never been able to switch heat from the top vents to the floor only...

It's doesn't appear to be a leak in the shut-down valve--no oil there, it works when i pull vacuum on it, and when i plugged it, the car wouldn't shift either. However, when i did tee into it, it dropped vacuum after shutoff..

My last thought is maybe it's the actual line to the tranny modulator...when i get this symptom (needs to heat up) the shut down is also extended...mystery. Maybe just a slowww leak in the shutdown valve?

mgcanuck · #15 02-27-2008, 01:20 PM

I'm still stuck. Tested good vacuum at switchover valve, to the tranny modulator. Put the vacuum gauge on the SOV and pumped up 15hg vacuum and it held. Still, when the car is shut off for over 1/2 hour, it won't shift correctly until i rev it high and shift into third and/or it's up at full operating temperature. Then the shutoff valve slowly works. I've replaced both shut-off lines and the rubber connectors to the key switch. Maybe the key switch is bad?? Also, maybe a slow leak at the break booster line?