83 300SD - Unidentified Failing Object... - PeachParts Mercedes-Benz Forum

pantherpaw · #1 11-01-2007, 03:55 PM

Anyone know what that small device is on the fire wall (drivers side) just beside where the main vacuum line goes through the firewall to the brake booster? It has two small hose attachments on the bottom (maybe vacuum) and a 3 or 4 wire harness plug (also on the bottom)...both hoses have been cut, but the wire harness looks intact...I would like to know what it is and how it works so I can decide whether or not to fix it...its cylindrical in shape with a smaller cylindrical "cap" on top...

D.Blake · #2 11-01-2007, 04:56 PM

Me thinks you are describing the switch-over valve which dumps or shuts off intake manifold pressure to the IP. This is designed to reduce fuel delivery in an overboost condition. These also have been known to gunk-up which will limit power by not allowing manifold pressure to the IP. If you follow the pressure line back to the manfold, you'll find the fitting where it attaches. This also can get gunked up and limit your boost effectiveness. Hope I guessed right and this helps.

pantherpaw · #3 11-01-2007, 05:14 PM

I think i can see on the diagram the correct way to hook it up to the vacuum system and the intake manifold...if i do this, how can i determine if the device is working properly? Also, where is the pressure converter located...#84 on the vacuum line function diagram...I cant find it...

thanks...

D.Blake · #4 11-01-2007, 05:27 PM

I don't think it hooks to the vacuum system, one rubber hose goes to the intake manifold, the other to the ALDA on the IP. These things are normally open so you should be able to blow low pressure air throught it when you are not in an overboost condition (which would be most of the time). You can put 12VDC across the wire terminals to see if it closes. I am not sure how you could test the pressure detection switch easily. If you simply bypass it, test drive it and then hook it up and test drive, there should be no difference in performance. Of course during low RPM, high power situations, you may trigger the circuit. A boost guage would be helpful here, but difficult to set up and I am not sure worth the effort. If someones knows more, please feel free to chime in here. I am doing this from rusty memory neurons!

pantherpaw · #5 11-01-2007, 09:30 PM

i drive the car very slowly, and i'm not hard on her at all...so i'm not exactly sure how an overboost situation could occur (or even what causes an overboost). As much as I would like to return the car to mint condition, I'm not sure this device is life critical for the car...I mean, enough to spend a lot of money on it right now...

my goal is to learn as much as i can about the entire car as fast as possible, because i am so excited about getting her back to perfect...its the most awesome car I've ever driven...

Every question i ask on here brings another...ha, ha...for example I'm not sure what an ALDA is...pretty dumb huh! Actually, I'm an engineer, but structural, so, its been awhile since physics lab...lol

any and all help, tips, advice is much appreciated...

crashone · #6 11-02-2007, 12:25 AM

Keep asking the questions and you will get there, lots of people here to help you on your way.

toomany MBZ · #7 11-02-2007, 07:54 AM

Yes, and yes, keep asking! I also think it's the switchover valve/overboost valve. Many have been disconnected. My CD, for instance, an MB mechanic mentioned, it's not a problem to take out of the system, very little danger. However, the one on the SD is operational.

vstech · #8 11-02-2007, 09:25 AM

no highway speed driving? OVR should not be bypassed, but like said, if you don't drive very hard, it's probably not an issue. shouldn't cost anything to reconnect. did they CUT the hoses? or did they just bypass them? I would pull the banjo bolt from the manifold, spray som carb cleaner in it to clear it of gunk, reconnect, then do the same on the ovr and hook the hoses back where they go.

peterhardie · #9 11-02-2007, 10:24 AM

My guess is you can see nearby where the two hoses which used to be connected to that switch have been re-connected together. Otherwise I think your ALDA won't supply fuel at all.

I have had mine bypassed on all my diesels to no harmful effect. I am not sure what happens at "overboost" except that excess fuel is dumped into the cylinders, causing excess smoke. A few others have also bypassed that switch. Two lines of thinking on this, one is that Mercedes engineers are clever fellows, the other is that any way to get a little extra zip out of 20+ year old cars is worth doing.

I reconnected mine recently, drove it for a few weeks, then reconnected it. Lots of highway miles, lots of hard acceleration. Hard to notice much difference, more smoke under hard acceleration, more boost sooner (under 2K rpms).

pantherpaw · #10 11-02-2007, 12:20 PM

vstech,

"vwbuge" (member) mentioned a "banjo" bolt...where is it on the intake manifold? front? back? top? bottom? I would like to inpect it and clean it...

Yes the hoses have been cut; however the hoses are completely missing. I would like to replace them, but i don't know what an ALDA is (or Where), and I don't know what a "banjo bolt" is either. I've got some hose here, just want to make sure I install it to the correct devices...

pantherpaw · #11 11-02-2007, 12:31 PM

hey crashone;

i am also trying to find my vacuum pump for the central locking system; i have diagnosed all the door vac solenoids...and the driver door lock switch is delivering 12V to both the blue and yellow wires...(both should go to vacuum pump in rear of car)...one is lock and the other unlock (reverse pump direction)...everything is working except the mailn central lock vacuum pump...need to find it, inspect it, and replace it (if necessary)...

pantherpaw · #12 11-02-2007, 12:48 PM

well don't i feel like a dumbass now....i just discovered the diy section of the web page...and holy crap...IT DOES LOOK LIKE A BANJO...

...I even play the banjo (bluegrass buddies)...so, I really should have figured that one out...ha...

vstech · #13 11-02-2007, 01:35 PM

if you look at the top of your injector pump, there is a metal cylinder with a clear plastic hose attached to it. follow that hose, and you will find a spot where they linked it to another clear hose, that is where the OVP switch needs to hook back up. continue following the clear hose around the block to the intake manifold, you will see the bolt that goes through the "banjo" fitting. disconnect the bolt, clean out the hole so it flows freely, and put it all back together.