here's a simple question...
I'd like to learn how to check the boost pressures on my 83 300D and 85 300SD. I've searched the forum and found a reference to placing a T in the switchover valve line but i'm not sure which one that is. I'd also like to determine which brand of turbo my cars have. Are there any identifying marks? Will a normal pressure gauge work as long as it can measure up to 20 psig? I've read that i should be developing no more than 13 psig under full acceleration.
Thanks to you all for all the valuable information I've learned. Working on these incredible machines is fun.
Mike

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1983 M-B 300D, 148K, daughter's ride
1983 M-B 300TD, 120K, my ride
1985 M-B 300SD, 102K, wife's ride
1969 HD FLH, 45K, restored/hobby
1996 Chev Suburban, family taxi/tow vehicle
1990 John Deere Model 240
1942 John Deere Model LA
1985 16' 4Winns ski boat

Not sure on the SD but the switchover (or overboost protection) valve is mounted to the firewall, behind the brake booster on the W123s. Plumb a plastic tee in there and snake a line into the cabin and hook it to a 0-30 psi air gauge. Here's how I made one (look at the 10th or 11th post).

FWIW, my SDL has a similar-looking valve that is the cold shift valve so, don't hook to the wrong thing on yer SD.

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Never a dull moment at Berry Hill Farm.

I put a T in the overboost line from intake manifold to overboost sensor. Standard gauge,mine was 0-50lbs. Had I long piece of tubing to the gauge and ran into car. Giver and read pressure, mine was 11 max. You will see a tubing connected by banjo fitting on back of intake. That is the line I used.

That's fine, and the reading of 11 psi is perfect.

Now, just to be sure that the overboost sensor is not restricting the boost, reconnect the T to the line from the overboost sensor to the ALDA.

The ALDA is on the top of the injection pump. It's a round cylindrical device and has a short piece of hose attached to it which quickly changes over to the hard plastic tubing.

You should see the same 11 psi during this test.

ok, i know the line you're talking about... it's the banjo fitting on the back end of the intake manifold I removed and cleaned the sludge out of before i adjusted the ALDA. What about identifying my turbo? Any suggestions? Can you get at the wastegate adjustment easily on both brands?

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1983 M-B 300D, 148K, daughter's ride
1983 M-B 300TD, 120K, my ride
1985 M-B 300SD, 102K, wife's ride
1969 HD FLH, 45K, restored/hobby
1996 Chev Suburban, family taxi/tow vehicle
1990 John Deere Model 240
1942 John Deere Model LA
1985 16' 4Winns ski boat

The KKK is easiest the wastegate sticks sideways...the Garrett is a pain it faces the front..try a search on wastegate adjustment...some very good threads have been written on this procedure.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Is this the wrong place? I figured to be on the safe side measure the actual turbo output as it goes into the manifold.
The wastegate location should give you the same reading unless your line to the overboost switch is plugged in which case you would not get an accurate reading anyway.

FWIW, I actually just put a T in the line from the turbo to wastegate and then rigged a piece of 3/8" hose that runs up to the drivers side corner of the windshield, wrapped my gauge in cloth (to protect it from bounces) and layed it right in that corner of the hood so that I can easily see it while driving.

It's a temp install anyway. Just drove 1100 miles like that and it worked fine.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

When you are testing turbo pressure.
Second gear pres break pedal and push accelerator max.
You can see your max pressure.
Max pressure under max load.
Looks your switch is no good - over boost control.
Second you are doing somthing wrong.
meeter conect to the valve over boost is more easy.
Regards

A little unconventional, but, I don't see any reason why it would not give you the manifold boost pressure. It's just a whole lot easier to work on the lines that come and go from the overboost protection valve. Put a T in one of those lines and run the gauge right through the driver's window.

I put a brass 1/8" npt 90 degree nipple in the 10mmX1.0 threaded hole in the intake manifold (the one with a 5mm allen plug). I then ran a vacuum line into my instrument cluster and installed an inexpensive glycerine-filled 0-30psi boost gauge where the non-working clock used to be under the tachometer.

Works perfectly.

Jim

Somebody used a vacuum (economy) gauge from a M-B Gasser adjusted to indicate boot pressure, which would fit neatly into the stock instrument cluster.
Here is the link, but few details are given (3rd post down).
Boost gauge

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1991 M-B 560SEL Arctic White/Grey 99,000 Miles
1987 M-B 300SDL Ivory/Palomino 229,000 Miles (sold but never forgotten)
2006 Volvo XC70 Blue/Beige
1999 Porsche Boxster Arena Red/Savanna Beige
1986 Porsche 928S Goldweiss/Brown

There is no need to put a tee in the alda pressure line, or even find a fitting to fit into the blocked off hole in the intake manifold.

Simply hook the pressure line of your boost guage to the bottom of the overboost protection valve, there is really another place to put a line in there, but it is not used in the stock configuration. This overboost protection valve is found mounted to the false firewall on the drivers side near the IP.

It is a lot simpler doing it this way..

Dervman,

I have done this. Look here if any interest (~ middle of 2nd page): Turbo boost guage advice needed

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Now you got me thinking...I am going to look at that tonight.....never even noticed that before.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

Yeah, it is also helpful when you break one of the valves... if you break the top nipple off, you can simply use the side one and the bottom connector.. a lot easier than buying a new one.. this is what I did, because I broke my top one.