do i understand this system ? pressure switch boost air pipe trigger switch
 

1980 mercedes 300SD with a OM617.950 turbo

Can someone tell me if I understand the system correctly…..

Ok the pressure switch boost air pipe trigger switch in picture number 20.

When boost hits 15 PSI the switch grounds the switch over valve picture # 21

This in turn cuts the pressure to the ALDA picture # 14 and stops sending extra fuel to the pump.
If this is all correct, I have a bad pressure switch boost air pipe trigger switch in picture number 20 because when engine is shut off it is still grounded at the post….

http://i63.photobucket.com/albums/h1...oostsensor.jpg

You need to know if that switch no.20 is a normal open or normal closed one.

Any numbers manufacturer imprints on it? like n/c or n/o, brand or part numbers?
Interesting that Mercedes uses a second safety device. Usually the boost pessure control valve is the only safety device.

The switch is normally open, closed by 16psi manifold pressure.
The solenoid is normally open through from manifold to ALDA, supplied a constant +12v power with switched ground. Closes manifold side and vents ALDA side when switched.

This system is the only safety device Mercedes uses. If the wastegate hose fails, its the only thing that will save the engine from overboost.
Easy to tell when its active, the engine will buck at full throttle as boost pressure goes up and down.

Then it is super simple to tune.
Disconnect it, adjust the ALDA for more fuel and install a manual boost controller like the Forge MUNOS, Just make suke you can observe EGT's and Boost pressure.
Ah, and an oil cooler is a good idea...

The ALDA does not control maximum fuel. Turning it up will not increase maximum power, only low boost smoke.

Mercedes already uses a very large oil cooler.

I know about the cooler, next to the radiator- it is not sufficient...

Actually its more than ample for anything the engine can do. These engines were used in 8,000lb box vans, RVs and Unimogs without oil cooling issues.
The oil system uses a thermostat to make it run 230f, thats why the cooler always feels hot.

I don't know what you do with your diesels- but we do have a little bit more hp in europe.
It is no very seldom to find an OM603 or OM606 with 300 up to 400hp. And they do get hot.
We do not use the engines from a Mog of the 60's. They are lame. At least you put a HX40 on it and install bigger injectors.

Your argument is 100% invalid here in that case.
His 617 (not related to 603 or 606 in any way) is capable of less than 150hp and his transmission can barely handle stock power as-is (thats why his car has a torque limiting system to prevent boost from giving extra fuel under 2000rpm, "b" in the picture).
His oil cooler is 100% sufficient.

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Why are you so obsessed with getting this guy to install a bigger oil cooler on his 110hp engine when the stock one has been more than ample for the last 32 years?

Simple, you have a speed limit of 70mph max. Have you ever been longer than a couple of seconds at full speed? We have the Autobahn. Going 140mph is no problem- because you can, but your thermal system can't keep up if you have modified the engine. Going 120mph for half an hour- no problem.

Yes you are right, you might never have experienced heat related problems in the past. Because you can't go like that.