Measuring boost pressure

[mgt83300D]

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

[R Leo]

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.

[Shellnekov]

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.

[Brian Carlton]

Originally Posted by Shellnekov

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.

[mgt83300D]

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?

[boneheaddoctor]

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.

[Fisherman]

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.

[Brian Carlton]

Originally Posted by Fisherman

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.

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.

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