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Old 09-14-2008, 03:36 PM
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Join Date: Jul 2008
Posts: 25
which one is bad

Hi, after tracking down my vac leak (1991 300D) found the line that runs to a blue covered part on the drivers side of the engine then goes to another blue covered item on the driver fender (with mb vac on it)
Both will let air pass both ways, wish I knew the proper names to help with this though they are the only blue plastic cover parts that can be easily seen
what I found out is my climate controls will work when this one vac line is pluged off but wont when it's hooked up
any sugesstions?
and what will happen if I run the car with it pluged off for winter?

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Old 09-14-2008, 09:19 PM
compress ignite's Avatar
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Join Date: Feb 2004
Location: 32(degrees) North by 81(degrees) West
Posts: 5,554
Vacuum Diagrams

#s 65 (On the side of your IP) and 123 (On left front "Wheelhouse") are the
two BLUE devices you mention.
65 changes Vacuum porting on the attached lines based on IP control movement.
123 is the "Vacuum Amplifier."


LINE c (which provides Vacuum power to the HVAC system)connects through 61b to the "Four Way" on Top of the Vacuum Pump.BUT the line right next to
it in the "Four Way" feeds vacuum through 61a to 65 (which is how your HVAC
Vacuum feed is being "Bled" off).I'm going out on a limb and suggest that you do not have "OverBoost" protection either due to the same leak.
AND if you run the engine with THAT Vacuum line plugged...
You do not want to KNOW!
(See the device #103 it's the ALDA and it will CUT the fuel supply in an
"OverBoost" scenario)
(Device R5/1 is monitoring the Boost Pressure for the EDS [computer]...
when the Boost goes Too High the EDS tells Y30 to limit fueling through
control of 103...and that control is exerted through Vacuum.)

So, finding and fixing the Vacuum leak that's robbing your HVAC could save
your engine TOO!
Attached Thumbnails
which one is bad-screenhunter_01-sep.-14-20.13.gif   which one is bad-screenhunter_02-sep.-14-20.14.gif  
'84 300SD sold

Last edited by compress ignite; 09-14-2008 at 09:41 PM.
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Old 09-14-2008, 09:58 PM
2.5Turbo's Avatar
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Join Date: Feb 2006
Location: Tacoma, WA
Posts: 900
There are two vacuum related blue covered thingies in the engine compartment of a 300D 2.5T.

One is the vacuum modulator for the transmission (second device down):

The others are the vacuum transducers for the EGR, intake flap, and turbo wastegate (new model looks like this; old one looks similar). However, they're all on the passenger side:

If it's the former, cutting vacuum to it will result in hard shifts. A line comes off a valve on the IP which changes the amount of vacuum applied to the modulator depending on how much the rack in the IP has been opened. This mimics the vacuum behavior of a gasser with a throttle so that both types of engine can use the same transmission system. Check all the lines between the transmission, the modulator, the valve on the IP, and the vacuum supply. Make sure there are no leaks. Almost the entire climate control system works off vacuum, so if there's a leak somewhere causing the vacuum to bleed off, it won't work well.


1990 300D 2.5 Turbo -155k
2000 E430 - 103k
2014 Jeep Grand Cherokee Ecodiesel 4x4 - 11k
2014 VW Passat TDI SE - 7k Bro's Diesel
2006 E320 CDI - 128k Pop's Benz

Pre-glow - A moment of silence in honor of Rudolph Diesel
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