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  #1  
Old 04-19-2001, 12:06 AM
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Hey gang,

The other day I was monkeying around with the vacumm lines on my '83 300SD. I was comparing them to my buddy's '83 300D which is having trouble with door locks, brakes, run on, etc.

I would disconnect one of my lines and put my finger over it and see if it sucked and then do the same thing on his car and compare results. Highly scientific stuff here.
Before I did this, I had the "normal" hard shift from 1-2 and 234 was smooth. Now my car shifts hard through all gears. There is vacuum at the modulator valve. That is, if the modulator valve is that thing on the drivers side of the transmission with a vacuum line running to it.
Suggestions?

Second question (big dummy type):
Where is the vacuum pump? I have a 300SD but I only have a 300D Haynes manual. Can't find a picture in there. Is it that thing on the front of the engine with a metal and plastic line that runs back to the brake booster?
I ask this because I could not feel any vacuum on my buddy's car and want to troubleshoot in a linear fashion.

Thanks in advance,

Matt

P.S. I was at least able to fix his tachometer problem thanks to info on this site!

[Edited by mcamara on 04-18-2001 at 11:13 PM]
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  #2  
Old 04-19-2001, 05:17 AM
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Yes you are correct, that is the vacuum pump. Here's some stuff that might help you with the transmission:

http://hsb.baylor.edu/html/easley/autofaqs/atcheck.htm

http://www.mbz.org/articles/trans/diesel.html

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  #3  
Old 04-19-2001, 09:23 AM
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The vacuum source for transmission control comes off the brake booster vacuum supply line (black plastic going from vac pump to brake booster).

The source is one of two tees off the line. These tees are metered holes. They are designed to supply vacuum in small enough portions that if they both were wide open the vacuum strength in the brake vacuum source wouldn't be threatened.

The way it works is the vacuum going from the line to the modulator is teed through the microswitch on the valve cover to a proportioned leak (vac proportioning valve) on the back side of the injection pump.

At idle, full vacuum goes to the modulator (should be around 20in). As the throttle is taken up (before pump movement) the microswitch opens the line to the proportined leak and the vacuum is immediately dropped to around 10-15in. As the throttle continues to rotate to full, the leak should become so sufficient that the vacuum is dissapated to zero.

Since the size of the leak is proportional and FIXED, the volume of vacuum at the source is critical. Too much (a supply hole reamed out) and the vacuum is always high. Too small (oriface is maybe only 0.020in and it cloggs) and the vacuum is too low or non-existant.

Before chasing the vacuum too much, a new vac hose with proper oriface size is a must for those lines with broken tees (don't reglue them).

That is basically what happens. There are multiple adjustments that can change the range of vacuum, the onset of proportioning, the lever of leak, and the result of it all at the modulator. A multitude of contrary commands can confuse some of those hard shifters to play fair, but thats another story.

[Edited by stevebfl on 04-19-2001 at 08:28 AM]
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  #4  
Old 04-19-2001, 06:51 PM
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As far as I know, you cannot compare the vacuum system of a W123 300D and a W126 300SD to solve a door lock problem.
The W126 has an additional electrical vacuum pump for the central doorlock system (under the rear seat) and the W123 has a combined vacuum system with the central doorlock hooked up the engine vacuum system (as described by stevefbl).

greetingz,
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  #5  
Old 04-19-2001, 10:42 PM
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Steve
As always, I really appreciate your taking the time to make a detailed explaination of topics like this. I gain a bit of new knowledge every time I read one of your explainations, and just want you to know how much it is appreciated.
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  #6  
Old 04-20-2001, 12:30 AM
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Thanks for the help! Every time I post on this site a get an education.
stevebfl- I was chasing my tail. What I initially believed to be the vacuum pump was in fact the proportioning valve. I also thank you for your detailed explanation. I echo palangi's sentiment.
Mbenz- Boy, you got that right. They are just different enough to make you wonder if you are working on the same manufacturer's cars. I had heard the pump on mine kick on when I would lock the doors. Buzzing from the rear of the vehicle. I thought that THAT was the only vacuum pump prior to my post.
Speedy300Dturbo- Thanks for the links. I had seen them before but could not remember where. I tend to read things like this and have to come back to you experienced folks for clarification. Luckily, I can.

Anywho, I guess the 300D has NO vacuum. Shall I rebuild the pump?

Thanks,

Matt
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  #7  
Old 04-20-2001, 08:57 AM
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Are your brakes working properly?? If so you have vacuum!!

If not check the check valve at the pump outlet. Remove line and remove the fitting below it. You should not be able to see through it. If you can the pieces of the valve are in the pump and its face must come off. Remove the pieces and replace the valve # 004 997 18 72.
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  #8  
Old 04-20-2001, 10:31 AM
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He said that the brakes are hard to push. This is my friends car. The doors don't lock and the engine runs on after the key is switched off. I decided to check the vacuum when it wouldn't shut off by itself. Asked him if he had trouble with the locks and brakes and he said that he had these problems as well. It was then that I opened the can of worms. His car is the 300D.
Thanks for the tip. I'll check that this weekend.
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  #9  
Old 04-20-2001, 10:34 PM
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Sounds Like a Vacuum Pump Diaphragm Replacement

Greetings,

Just my .03 worth. Been there done that sort of thing. If no vacuum is available for braking or his locking or shut off valve, I'd look to see if the exhaust line from the vacuum pump isn't sucking engine oil to the filter housing. A definite sign that the diaphragm in the pump has a hole in it. Diaphragm can be ordered from parts shop for a minimal amount of green, don't purchase a new pump when the old one can be rebuilt easily.

Charles
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