83 300SD Turbo - Vacuum Configuration - PeachParts Mercedes-Benz Forum

wildbore2000 · #1 09-05-2008, 11:31 AM

First off thanks to this community. I bought my 83' 300 SD Turbo a couple months ago and the manuals I have are worthless. This site is a wealth of accumulated knowledge that has helped me out quite a bit.

I cannot however figure out what the proper configuration of my vacuum lines should be. My configuration does not seem to match any of the ones I've seen so far. My specific issue is the connection of the switchover valve. It does not connect to the main feed at the brake booster like the diagrams show. I put the pics at the end.

I also would like to know what a valid test for the vacuum system is. I went out and got a Mitty Vac, but have not found what a valid test for a vacuum valve is. I have a really hard shift from 1-2 especially on the downshift. I put a vacuum on the transmission valve and it losses it really quick, but I do not know if that is typical. I've pluged up the non-functional A/C and door locks so I think I've got the leaks taken care of. If there is not one already, maybe I can start a thread for a walkthru on vacuum testing.

Example:

Step 5 - Transmission Valve: Disconnect feed line to transmission valve. Put 10" vacuum on the line. If it maintains vacuum it is good, if not the valve is bad or the line has leaks.

Do one of these proceedures for each device in the system for a complete system checkout.

_______________
83' 300SD Turbo - 320,000+ miles, odo stopped working a while ago ;-)

SD Blue · #2 09-05-2008, 11:49 AM

You didn't mention what "manual" you have. If it's Haynes, then it's for the W123 series which is very similar but there are a few differences from your W126 series.

First, the W126 has a separate electric vacuum pump in the trunk for the door locks. This will explain the photo with the "Y" connection.

Also, this thread will have plenty of reading for checking out your vacuum and the effect on the transmission:It's CRITICAL... how you set your transmission's vacuum system on your diesel MBZ...

(The photo asking about the missing line is not vacuum. Actually that is for turbo boost. You should have one line that goes from the back side of the intake manifold to that valve in the photo. The other line coming off of the bottom of the valve should be going to the alda, which is part of the injection pump. This provides for added fuel when the turbo spins up and gives pressure boost. A big factor in performance.)

wildbore2000 · #3 09-05-2008, 02:06 PM

The useless manual is the Haynes one. It's usless more because it's so general and lacks any specifics, than because it applies to the W123. I had read this review about it many times here, but bought it anyways thinking it might have some worth. NOPE! When I replaced the ignition switch I spent a couple afternoons dismantling my dash looking for the "locking pin". The last step in the manual just says "push in locking pin and remove switch from column...." OK, what is a locking pin and where do I find it? No pictures, no nothing... This site (well actually a link to diesel giant) had a big long illustrated walkthrough. Found it in about 2 seconds.

I read through that post on vacuum testing a little more thoroughly and still can't find an exact answer for how to check if the transmission valve is bad. If I put a vacuum on it and it leaks does that mean it's bad? I just need to know if I should replace that valve.

While I'm at it, I had read that if you have oil in your lines, the diaphragm on the pump is bad. I disconnected the main line to the check valve and put a vacuum on it that held. I read somewhere that this means the diaphragm is good? Can someone clarify that, I'd hate to do a diaphragm replacement if it's unnecessary.

SD Blue · #4 09-08-2008, 12:01 PM

Quote:

Originally Posted by wildbore2000

The useless manual is the Haynes one. It's usless more because it's so general and lacks any specifics, than because it applies to the W123. I had read this review about it many times here, but bought it anyways thinking it might have some worth.

The Haynes is OK and really, you get what you pay for........a general, basic manual for a cheap price. The frustrating thing is we always want a little more for the money, which is human nature.

This is the diagram I was using when tracing out my lines:
As you can see the W123 has a 5-way feed and the W126 has a 4-way feed. Mine has a "Y" in that place and if you look at the missing line, it is the one going to the switchover valve (81). Those two lines that are "Y"ed off are the ignition switch feed and the A/C (I think).
That is the correct diagram but the thing to keep in mind is there were several running changes in this system, during this time period, and you may/may not have all of the items shown in the diagram. This was due to changes in the EGR system. If your system is like mine, there isn't a switchover valve in the vacuum system.
The door locks come off that other "Y" on the right in the picture that has the golf tees in it. I don't know exactly how the switchover valve works, but I know the turbo is not working. This could be my first step in tackling that problem.

I read through that post on vacuum testing a little more thouroughly and still can't find an exact answer for how to check if the transmission valve is bad. If I put a vacuum on it and it leaks does that mean it's bad? I just need to know if I should replace that valve.
There are a couple of things to check before deeming the modulator valve bad. Check the flex line to the valve and if that little rubber cap, covering the adjustment, is cracked, they will cause vacuum leaks. In fact, what I did was put all new flex fittings and rubber parts in the system and it cleared up many of the intermittent vacuum problems. If the modulator does indeed leak, it is defective but it is a PIA to get it adjusted correctly. It is not plug-n-play. You need not ask how I know.

While I'm at it, I had read that if you have oil in your lines, the diaphram on the pump is bad. I dissconnected the main line to the check valve and put a vacuum on it that held. I read somewhere that this means the diaphram is good? Can someone clarify that, I'd hate to do a diaphram replacement if it's unnesseary.

Your pump will be a later model that doesn't have a diaphragm. It uses a piston and couple of internal check valves to create vacuum. Oil in the vacuum lines could have been from a previously failed fuel shut-off and there is likely an isolator near there to prevent this from happening. IIRC, there was a factory recall to install the isolator/filter.

wildbore2000 · #5 09-08-2008, 11:16 PM

Thanks Sam,

I looked up the rebuild kit for my model and it only has gaskets and plastic parts, no diaphragm like you said. Where does the oil get into the system? My ignition switch was filled with oil and I had to rebuild it to get my ignition working. Should I just clean out the lines with BBs, replace the fittings and call it a day? I don't think I have one of those vent filters so I guess I'll add that as well.

If you have any tips on changing the transmission modulator valve, I'd appreciate it, it looks to be leaking and I guess I'm going to have to replace it.

I was feeling particularly ambitious today because of my frustration with finding a good diagram, so I drew my own. Here is my rendition of what I believe my configuration is by tracing out the lines.

SD Blue · #6 09-09-2008, 12:10 PM

For the oil in the vacuum lines, I believe it is from a failure of a previous vacuum shut-off valve for fuel at the IP. From my understanding, the vent filters (#85) were part of a service bulletin. It may have had some oil in the system and wasn't cleaned when the bulletin was complied with. Also, the ignition switch must be the lowest point, gravity-wise, in the system and cleaning may have caused the oil to migrate there. Mine had the same problem but haven't had any sign of oil since.

For the modulator valve, here is a photo:http://www.allpartsexpress.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleWHD&cookieID=2GL0LXRFX2GL0LZJHV&yearid=1984%40%401984&makeid=MERCEDES+BENZ%40%40MERCEDES+BENZ%40%40X&engineid=1194038%40%40300SD++%2F+L5%2D2998cc+3%2E0L+DIESEL+FI++Turbo%40%40300SD&catid=Transmission%40%40Transmission&mode=PA&subcatid=Modulator+Valve@@Modulator+Valve&clientid=allpartsexpress.com
In the center, you will see a metal adjustment key that is pulled out just enough, and turned, to set it to the next cog. This key is sealed by this cap:http://www.allpartsexpress.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleWHD&cookieID=2GL0LXRFX2GL0LZJHV&yearid=1984%40%401984&makeid=MERCEDES+BENZ%40%40MERCEDES+BENZ%40%40X&engineid=1194038%40%40300SD++%2F+L5%2D2998cc+3%2E0L+DIESEL+FI++Turbo%40%40300SD&catid=Transmission%40%40Transmission&mode=PA&subcatid=Modulator+Valve+Cap@@Modulator+Valve+Cap&clientid=allpartsexpress.com Ensure that it isn't cracked or missing as this is a leak source. If all of that looks to be in good shape and a mightyvac still confirms a leak.......here is the basic procedure: Install a pressure guage at the case pressure port. Disconnect the vacuum and run the engine at 2,000 rpm. The pressure is adjusted up or down, by the small tee handle, to meet the figures on the data file that usually comes with the modulator.

This is the article I used:http://www.mbz.org/articles/transmission/adjust/
(Now you can see why I was saying the modulator replacement is a PIA and try everything else first.)