Hello All,

Please see the attached pic to help explain my descriptions below.

We all know there has been much discussion of vacuum issues and t-shooting for our deisels. I have read lots of people say their 3/2 valves "do not hold vacuum" or "hold vacuum". In the process of eliminating my own vacuum leaks, I have found that what appears to be the "inputs" to both of mine do hold vacuum when the valve is closed (the lever is not pushed down). My question is, what is the behavior supposed to be when the lever is depressed?

When I plug both of what I would consider to be the "outputs" of the valve, it will still deplete the vacuum I apply to it with the Mityvac, and relatively quickly at that (about 1 second). If I do NOT plug one of the outputs, the vacuum is lost immediately.

My inkling is that with both outputs plugged it should still hold vacuum, simply redirecting it out the other two nozzles. However, to my knowledge, nobody has yet described the expected operation of a working valve in its open and closed states. In fact, I can imagine some folks have applied vacuum to the wrong nozzle and seen a valve not hold vacuum when perhaps it would have on the input.

Elaboration on this point will hopefully help me and others in the future know when we need to drop the $60 to replace a pair of these or not, so your collective wisdom is greatly appreciated as always.

Kindest regards,

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Garrison R.
'83 300D

Which vehicle are you referring to?

If it's the 300D, the 3/2 valves only serve to operate the EGR system and all lines to and from the plug can be eliminated.

I'm sorry for not specifying... This is a newly-acquired '83 300D Turbo, and I have inherited vacuum problems in a variety of places, which I am slowly pinning down each one thanks to the wealth of info here.
One of my three lines goes back to the lower nipple on the VCV atop my IP. One is a direct input via a couple of 3-way splits from the brake booster line, and the third heads off to the EGR. So not everything here is isolated to EGR function, though I do have my EGR line plugged to help me diagnose the rest of the system.

I was actually not intending to get into my own specific problems here since it appears there's plenty of existing threads that descibe my problems and diagrams to help me... But in the process of t-shooting my two 3/2 valves, it occurred to me that I'm not sure how they're *supposed* to work. I can get them to hold vacuum on the one port but, as described, they quickly leak their vacuum when the button is depressed. Knowing that vacuum venting is a part of this whole process, I wasn't sure if these things vent vacuum when depressed, or are supposed to transfer vacuum to the other two nipples in a leak-free manner.

Thanks for your quick reply.

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Garrison R.
'83 300D

Trust Brian, he's the expert when it comes to anything and everything vacuum-related.

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1983 Mercedes-Benz 240D
Automatic, A/C, Power Sunroof, Power Right Side Mirror
231K Miles FOR SALE MAKE OFFER

With the 300D, it makes the most sense to eliminate all the lines to the valve cover because the 3/2 valves only control EGR.

We can do that for you, if you wish.

The function of the valve is as follows:

When the valve is "closed": The consumer (EGR) is connected to the vent line.

When the valve is "open": The consumer is connected to the vacuum supply line.

Brian, I have a difficult time accepting that the operation is as simple as you describe. A purely binary operation (vacuum present or vacuum absent) would appear not to require the usage of two different valves which are both influenced by throttle position. These valves almost certainly have a specific function as it pertains to the shifting behavior of the transmission- which I don't currently understand.

Also - on the 1985 model, there is an electronic replacement for these valves - a device which is also influenced by throttle position.

I would be very interested to hear an answer from a Mercedes engineer who is familiar with the function of the device in question.

A good question. And, I haven't delved into the details of each individual valve. But, it's a certainty that they do not have anything to do with the transmission and are only present to control the EGR on the '81-'85 vehicles.

The routing of the vacuum lines to the black box and onto the EGR valve confirms this.

The SD has nothing connected to the black box.........and it shifts flawlessly.

Perhaps each of the two valves serves as a switch - allowing the EGR to operate between a certain range of throttle positions. As the throttle is actuated, perhaps the EGR may function above a certain RPM above idle, but then is turned off again to allow maximum engine power once the throttle has opened past a certain point (indicating full throttle acceleration). Just speculation on my part.

I think you're correct with that theory.

I still don't figure on why they couldn't use a single valve for that purpose.

Here is how it works:
At "closed throttle," Valve A is closed, preventing vacuum from actuating the EGR valve. At slight throttle and higher, Valve A routes vacuum to the EVR valve so that it opens. At near full throttle, Valve B vents the EGR vacuum to the atmosphere so that the EGR valve closes.

So, the two valves combine to operate the EGR valve only when the throttle is in a "middle" position. And they prevent EGR operation at closed throttle and at WOT.

That is a common misconception which, undoubtedly, has its roots in the fact that replacing the 3/2 valves often improves transmission performance. The reason for improvement, however, is the fact that a vacuum leak was eliminated, not that the 3/2 valves have a direct role in the operation of the transmission.

My thanks to both Brian and TangoFox for their input.

I have a 79 300td, no egr. The 3/2 valve is there to allow the car to remain in 2nd gear if if the throttle is backed off, from what i understand. I put a brand new one on and i still have no/intermittent vacuum. I isolated the (used) vcv i put on, and it seems it has a major vacuum leak, so i assume that is the source of my leak.

That said, when the throttle is closed, should the cam that contacts the 3/2 valve's allow it's piston to be fully extended (so that the 3/2 valve is totally open?). Experimenting with the adjustable throttle control rod, there seems to be a finely tuned position where the vacuum operates correctly--which seems to be when the valve is partially closed. However, this might all just be a function of intermittent supply from the vcv.

Thanks!

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Mike
Vancouver, BC
'79 300td 60k miles!
import from Japan

I recently replaced broken levers on both 3-way valves, and the transmission shifted horribly, especially when downshifting.

Hooked up a Mityvac to the vacuum supply line that feeds the 3-way valves from the brake booster line - and had the same symptoms described above.

I have concluded that either my 3-way valves are leaking, or there is an EGR valve related leak.

Having read Brians posts on this topic, I decided to disconnect the supply and vent lines to the 3-way valves and have temporarily blocked off the 'T' connections with a couple of golf tees. Car shifts perfectly again!

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Neil.


1991 190E 2.6 170k
1995 E320CA 57k
1999 Chevy Tahoe 98k
2000 R1150GS 45k
1983 300CD 178k - sold, but not forgotten

hi, thanks for your comments. I did some testing. First of all, the 3/2 valve itself is brand new.

Second, i put a new vacuum line to the tranny.

Third, i unbolted the vcv, and played with the spring. it was passing no vacuum at all. I banged on the sucker with a wrench, which seemed to free up the spring inside and boom, vacuum again. I've got a replacement vcv on order. I guess the springs wear out on these and they don't return correctly to supply full vacuum. No point bypassing the 3/2 valve if the vcv itself is flawed.

As you can see mine is a very simple setup.

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Mike
Vancouver, BC
'79 300td 60k miles!
import from Japan

The '79 predates the setup on the turbos and the 3/2 valves may be used for the transmission on that vehicle. I'm not all that clear on the valves on the n/a engines, since I never owned one...........but, you can't just eliminate them as is common on the turbos.