Over boost protection valve 1985 300SD - PeachParts Mercedes-Benz Forum

koya1893 · #1 07-05-2010, 05:37 PM

Like any engine vacuum is critical for proper operation. I check on acouple of vacuum lines and found acouple of old and crack vacuum line being used as coupling. Replaced those and I found this valve in the photo I've attached. Can someone assist me identifying what attaches to it, and maybe a vacuum diagram. I also found this filter. Can this filter replaced with a inline fuel filter, they look similar? Thank you.

whunter · #2 07-05-2010, 05:42 PM

Quote:

Originally Posted by koya1893

Like any engine vacuum is critical for proper operation. I check on a couple of vacuum lines and found a couple of old and crack vacuum line being used as coupling. Replaced those and I found this valve in the photo I've attached. Can someone assist me identifying what attached to it, and maybe a vacuum diagram. I also found this filter. Can this filter replaced with a in-line fuel filter, they look similar? Thank you.

That is the over boost protection valve = intake manifold boost pressure to ALDA.

RAYMOND485 · #3 07-05-2010, 07:08 PM

1984 300D TURBO 153K
NO FILTER NEEDED IN LINE FOR PRESSURE TURBO BOOST, REMOVE REAR 12 MM PLUG AND CLEAN BANJO BOLT NEAR TRANS DIP STICK,RIGHT REAR ENGINE.

micalk · #4 07-05-2010, 07:59 PM

As the venerable Mr Hunter said, that is the overboost protection valve. It has an electrical connection to +12V in the fuse box, and is grounded through a switch on the intake manifold. It also has a boost (air) pressure line from the intake manifold (banjo bolt) to the bottom of the valve, and from the front of the valve to the ALDA. If yours is bypassed, the boost pressure line is probably routed directly to the ALDA.

Sometimes they get clogged so instead of cleaning it, a PO may have simply bypassed it.

koya1893 · #5 07-05-2010, 09:52 PM

First let me apologized for not identifying the year of the car. It's a 1985 300SD. Now my stupidity to the components identification. What's a ALDA? Where is it located? The valve does not have anything hooked up to it. Noted the small vacuum tube with a clamp, I assumed something was attached to that.

micalk · #6 07-05-2010, 10:24 PM

Your OBP valve is bypassed, as evidenced by the picture you posted. But the electrical connector is in place. On top of the intake manifold near the rear of engine you'll find a pressure switch with a single terminal on it. That's the manifold overboost switch and it should have a wire going to the electrical connector on the valve. On the very rear of the manifold is a fitting with a tube attached. This is the banjo bolt. Follow the tube; it should be routed to the ALDA, a square thingy on top of the injection pump. Read my previous post for how the tubing is _supposed_ to be routed.

There's spirited debate as to whether the OBP valve is needed and those who tune their engines for max power would tend to bypass it. But for the most part, if you are going for original then you'll probably hook it up. If you attach it you'll want to make sure the banjo bolt is removed, cleaned and re-installed. Same for the OBP valve. Until you're ready to dive into the circuit, though, feel free to leave it alone.

toddyvol · #7 07-05-2010, 10:30 PM

The filter...is it possible that it could be hooked to the shut-off vacuum?

Isn't that pretty common?

micalk · #8 07-06-2010, 12:31 AM

OMG! I didn't even realize what I was looking at.

That's a fuel filter. It could be hooked anywhere. There's a small filter that allows vented air from the inside of the vehicle to enter the vacuum system. This could be used in place of that, but its an absurd replacement.

There's a sticky on the top of the forum discussions that contains vacuum diagrams and links to diagrams. Find the one for your car and start going through it identifying each item and how it's hooked up. Come here for answers. But someone has seriously messed with the vacuum.

How's the transmission shifting for you?

toomany MBZ · #9 07-06-2010, 09:25 AM

Quote:

Originally Posted by koya1893

First let me apologized for not identifying the year of the car. It's a 1985 300SD. Now my stupidity to the components identification. What's a ALDA? Where is it located? The valve does not have anything hooked up to it. Noted the small vacuum tube with a clamp, I assumed something was attached to that.

The ALDA is a self adjusting altitude (atmospheric pressure) device. It will restrict fuel to the engine when at higher altitudes. There is "less" air, therefore you don't need/can't use the same amount of fuel as you can at sea level.
It has a ballon inside that will expand with lower pressure. It is connected to the intake manifold via the over boost protection valve (switchover valve). If the banjo bolt at the manifold gets clogged up, it will not receive any signal, thereby severely limit fuel available to the engine. The same with the line going to the switchover valve, whose job it is to vent pressure were the engine to produce more than the engine can take. I've been assured by an MB mechanic, it won't produce too much boost and the engines are built to take it anyway.
As mentioned, there is a debate on the switchover valve. My CD's has been bypassed before I bought it. My SD still has it in the loop. The line in my hand is a vent line to the cabin, is vents vacuum the transmission can't use.

tangofox007 · #10 07-06-2010, 01:04 PM

Quote:

Originally Posted by micalk

There's spirited debate as to whether the OBP valve is needed...

That's a lot like debating the merits of having a spare tire. The car will operate perfectly without one. And, as long as it's not needed, you don't need it.

whunter · #11 07-06-2010, 02:29 PM

Quote:

Originally Posted by koya1893

Like any engine vacuum is critical for proper operation. I check on a couple of vacuum lines and found a couple of old and crack vacuum line being used as coupling. Replaced those and I found this valve in the photo I've attached. Can someone assist me identifying what attached to it, and maybe a vacuum diagram. I also found this filter. Can this filter replaced with a in-line fuel filter, they look similar? Thank you.

Answer: Picture #1.

"Hens tooth" 1985 300D vacuum filter dark black
http://www.peachparts.com/shopforum/showthread.php?t=259240

Answer: Picture #2 and #3.

Diesel Injection:
http://www.peachparts.com/shopforum/showthread.php?t=142405

ALDA issue threads
http://www.peachparts.com/shopforum/showthread.php?t=239597

koya1893 · #12 07-06-2010, 04:57 PM

How's the transmission shifting for you?[/QUOTE]

After reading all of these great feedback from everyone and photos Toomany attached my ALDA has been bypassed the vacuum line that should be attached to the valve is hooked up directly to the ALDA.

micalk,
The trans shifts fine, it shifts around 1800-2100 rpm and very smooth. Since the ALDA is supposed to limit fuel during high altitude operation. Is there a way for it to operate in that manner all the time. With the hydrogen system onboard the engine does not need all the scheduled fuel as the hydrogen is augmenting the combustion process.

Thank you for all the great info I will check all the link posted.

toomany MBZ · #13 07-06-2010, 05:50 PM

There is an adjustment screw, visible in my second pic. Note there is no black cap, but the metal cup is still there. Remove at least the black plastic cap, loosen the locknut and turn the screw clockwise, this pushes the restricting pin down, reducing fuel. Not sure what pressure you want, stock values are in the attachments.
The third pic is from the FSM, it shows how it works.

micalk · #14 07-06-2010, 09:39 PM

Did I speak too soon? The fuel filter is a mod to the vacuum system to identify vacuum pump issues before they become problematic, is that correct? My apologies for any confusion.

The fact that your transmission is operating correctly is a good sign that your vacuum system is setup properly.

Just a mild correction. The line going to the ALDA is boost pressure, not vacuum. If you want to operate the engine without the ALDA simply disconnect the tube. My understanding is that this will cause it to act more like a normally aspirated engine as far as the amount of fuel delivered is concerned. The tube is not very flexible, so don't try to clamp it closed.

whunter · #15 07-06-2010, 10:32 PM

Quote:

Originally Posted by micalk

Did I speak too soon? The fuel filter is a mod to the vacuum system to identify vacuum pump issues before they become problematic, is that correct? My apologies for any confusion.

The fact that your transmission is operating correctly is a good sign that your vacuum system is setup properly.

Just a mild correction. The line going to the ALDA is boost pressure, not vacuum. If you want to operate the engine without the ALDA simply disconnect the tube. My understanding is that this will cause it to act more like a normally aspirated engine as far as the amount of fuel delivered is concerned. The tube is not very flexible, so don't try to clamp it closed.

On the 1985 300SD that filter is a FACTORY modification to PROTECT the vacuum system from vacuum pump issues, and diagnostic tool.