My 83 300sd seems to have lost some ability to maintain speed on the highway. Used to be able to maintain 70+ uphill on the highway. Now it loses 10~20 mph depending on the hill, and surges under hard acceleration. Problem first appeared after replacing a coolant tee for my block heater, I broke the vacuum line to the banjo bolt. (thanks to this forum and diesel giant for info on finding that) So, removed the banjo bolt, cleaned it, repaired the vacuum line. Only slightly better, but the surging went away.
Then, I changed both fuel filters. Better still, but not how I remembered it.
Next was a comprehensive throttle linkage adjustment. Huge improvement, but still seems like it is lacking something. Today, I noticed that there seems to be no black smoke when under full acceleration, sitting or at speed. I recall seeing smoke before, (loaded and at speed, not at idle) and am under the impression that a diesel should smoke when being worked hard.
Could the ALDA get clogged, or go bad suddenly? I would rather save ALDA adjustment for last case scenario.
What kind of pressure readings should come out of the banjo bolt? or the overboost protection solenoid? When is that thing supposed to be open and closed?
The car is getting a valve adjustment next, I've had it for about 6000 miles, and I have no idea when it was done last. Also, because it's next on the "fixing other issues, not the main issue" list.
Car has 283000 miles.

Ideally, you want no smoke if everything is in tune, since that means everything is all "cleaned" up.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

Mine runs great. Plenty of power. I adjusted the turbo to 11 pounds boost. Ripped the cap off the ALDA and screwed it out 1 and 3/4 turns. Runs great now. 100% NO SMOKE....ever. I believe that being tuned properly combined with Ultra Low Sulfer diesel= no smoke.

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Anything is possible if you don't know what you are talking about........

1985 Mercedes 300D
2007 Mercedes C-350
1980 Mazda Rx-7 (SA22C)
2005 Jaguar XJR
2007 Jeep Wrangler

I'm over the desire to see the smoke, guess I must have been mistaken about diesels creating smoke under load. Only other diesel I've ever owned was a GM 6.2L, which is does a disservice to boat anchors everywhere.
The improvement came after bypassing the overboost protection solenoid.
Car that would barely maintain 65 uphill was now at 85 and climbing. Same highway hill, over and over - my own little testing ground.
After bypassing that valve and getting the performance I was looking for, I tried cleaning it out. Flow was definitely restricted before being blown out with carb cleaner. Post cleaning, but with power disconnected from the solenoid, pretty close to same performance. Connect power, and right back to stage 1, no highway performance.
Next step, check the pressure sensor and associated wiring.
Big thanks for all the info on this forum, really is the best thing I've ever encountered for any vehicle.

P.S. With the overboost solenoid disconnected, I am also getting VERY smooth shifts. Shifting was acceptable before, but has gotten markedly better.
Never encountered vehicles before that were so interdependent.
"Foot bone is connected to the shin bone is connected to the knee bone" and if your knee is out of spec, your foot wont work.

Get yourself a boost gauge and some nylon tubing. T into the line from the overboost protection solenoid to the ALDA. Run the gauge through the driver's window so you can see it while you drive.

Take it for a test drive and bring it up to 3500 rpm in third gear with your foot on the floor.

Read the boost gauge and report back with your findings.

Brian, got any recommendations for a boost gauge?
I don't have one, and can't seem to find any friends with one either.
If I am taking a reading off the vacuum line, could I use my Mityvac?

That means the system is doing its job and protecting your engine from damage.

Get that boost gauge and fix your wastegate asap. Driving with the solenoid disconnected is only doing harm.

A gauge similar to this will be perfect:

http://cgi.ebay.com/ebaymotors/Parts-Accessories_Car-Truck-Parts-Accessories__Boost-Gauge-SRT-4-OEM_W0QQitemZ300293878213QQddnZPartsQ20Q26Q20AccessoriesQQadiZ2865QQddiZ2811QQadnZCarQ20Q26Q20TruckQ 20PartsQ20Q26Q20AccessoriesQQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item3002938782 13&_trksid=p4506.c0.m245&_trkparms=72%3A543|65%3A12|39%3A1|240%3A1318

You can't use the typical Mityvac as it won't measure pressure. However, some of the premium versions will measure pressure..........the gauge will tell all.

Thanks again Brian, I'll get one this week.
Forced Induction, isn't it possible that the solenoid is bad?
I would like to think that these cars can maintain 70+ uphill on the highway.
Not talking about going up Pike's Peak road, just regular East Coast hills.
I am all for having the protective measures that the car's designers felt it needed. Obviously lots of other people on this forum feel that the overboost protection is a waste of time, judging by their posts, but seems like when the system is working correctly, it shouldn't be noticeable.
This car is my first with a turbo, and only my second diesel, so I am still getting to know how they work.
My understanding is that the line coming out of the manifold banjo bolt goes to the overboost valve, which then leads to the alda. The wires attached to the valve should run from the fuse panel to the valve to the sensor. (which I haven't found yet) If bypassing the valve, or unplugging the wires results in better performance, then there is some problem in the system. Next I should be determining what the actual pressure is coming off the manifold. (should be ~11psi ? ) If the actual pressure is within spec, then it narrows down to sensor or valve or wiring.
Does the sensor send power to the valve to close it when overboost occurs, or does it break the circuit and rob the valve of power during overboost? Can the sensor be cleaned?
Also, how will the engine get damaged? Isn't the danger of overboost from the turbo a lean condition? Wouldn't getting more fuel by way of the ALDA be a good thing? Seems like I am missing a connection here. This overboost protection, is it somehow connected to the wastegate?
Thanks for putting up with all the questions, but this forum is the best thing this side of diesel mechanic school.

It all depends on the grade of the hill. There is a hill in CT...........just west of the West Rock tunnel on the Merritt Parkway where the SD can maintain 68 mph. It's only got a 2.88 diff, however, so some HP is lost. With a 3.07 it can probably hold 71 mph.

This is a perfectly running 617 that will do 12.8 for a 0-60 time.

They only have 123 hp...........nothing is going to change that fact.

Get the FSM. There is a wealth of information on these systems even if it can be tough to read sometimes.

There are two safety systems. The first is the automatic wastegate. This tends to fail safe in that when it goes out you have no boost. *Edit* - This morning thinking about it I realized this comment is incorrect. If you lose pressure on the regulator hose then it will not fail safe. *End Edit* As the spring weakens with time this is an item prone to failure. This is the primary safety system as it stops the intake from over pressurizing in the first place.

ALDA by itself is not an overboost protection. It is a fuel economy/emissions system that limits the fule supplied in lower RPM situations. As low RPMs result in low boost pressure whenever the ALDA "reads" low boost it limits the fuel.

The the overboost ties into the ALDA. If the sensor on the intake manifold triggers it closes the circuit (If I read it correctly) which causes the overboost protection valve to vent the line from the intake manifold to the ALDA. The ALDA now "sees" low pressure and limits the fuel which ultimately will slow the turbocharger. The only part that usually fails on the overboost is the switchover valve and this is usually fixed just by cleaning it. A 10 minute job at worst.

The systems look like a kludge at first but when you study the manual and see how they work together it is actually a quite elegant and simple system. Especially when you consider that these things will work for decades needing only minor annual tuneups.

FWIW I am going through the same issue. It can feel like tuning and checking all of these systems is getting nowhere as they don't solve the problem. But in reality much of this work should be part of the annual tuneup anyway and when I do find the source of the major problem in the meantime I've solved several minor issues that were either masked by the major problem or would have become a problem soon anyway. In the long run it is better that you do check all of these items and get them in the original working order.

Regarding the sensor itself the manual suffers from a bit of "Japanglish" in the translation from German. If I understand it correctly the circuit is normally open and when the intake manifold overpressurizes it closes the circuit. If this is the case this is a fail-deadly design and not good engineering IMO. It should fail safe like the automatic wastegate. If you were to cut the air line for the wastegate you would lose boost. Cutting the wire for the sensor would allow an overboost condition.

Diesels are always "lean" in most conditions. This is not an issue as with gasoline engines. The issure of too much fuel is that my understanding is that it is not good for the engine in the long term.

Before this problem arose, I could cruise 70~75 on the Merritt with understandable loss of speed on the hills (meaning 5 or 10 mph) depending on how you hit them. I typically get on the Merritt by Sikorsky, where the parkway and 95 are closest, so I haven't gone through West Rock Tunnel with this car.
What is the desired amount of pressure coming out of the manifold?
Presumably that boost is adjusted with the wastegate?
But, like Gene said, tracking down these kind of issues is a great way to get the maintenance list taken care of.

The hill I'm referring to is right at Sikorsky..........headed west from the tunnel exit.

No, you cannot maintain 75 mph on that hill............I'll bet $20. on it. If you can go into the hill at 71 mph and maintain 71 mph with no loss of speed..........the vehicle is performing per specification and no additional time need be spent to search for a problem that doesn't exist.

The desired boost pressure is 12 psi.

The boost is adjusted with the wastegate spring, but it's a real PITA and most folks opt for a boost controller.

Thanks for the info. I will be traveling to NY in about a week, so I will pay attention to how the car does on that particular stretch. I am assuming we're talking about the hill heading west past the Housatonic?
I will be making some 0-60 runs as well. The boost gauge is on it's way, so I will have some actual numbers to report soon.
So far I have only driven the car about 20 miles with the valve bypassed, and have no plans to take it any distance until I get it sorted out.