I do have a boost gauge that I ordered a few weeks back. I will give that a shot.
I have driven another 1985 300D although it was ten years ago. I borrowed it for a week and drove it all around the countryside. Indeed this is why I jumped on this one. It was a pleasure. I simply do not recall it behaving like I ever had to put the pedal to the floor in any scenario. This is a dangerous car to bring on the highway simply because it can't get up the on ramp and onto lane reasonably.
How do I know if it's starting in first gear?

Are you getting three shifts from 1-2, 2-3, 3-4?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

IIRC your car starts in second gear by design.

You can force a first gear start.

Downshift to second while stopped, then upshift to drive.
You'll start in first with a quicker launch, and reach the 2000 (or so) rpm needed to activate boost much quicker with less initial smoke.

If you don't downshift to second beforehand, and start normally by mashing the accelerator pedal to activate the kickdown switch, you'll start in second but force a quick downshift to first, and create a bit more smoke.

__________________
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

So, speaking in terms of what's actually indicated on the "shifter"... P,R,N,D,S,L

Presumably having been driving in D, then come to a stop at a stop sign/light, while stopped I shift to L (which is the mode for "Upshift to Second Gear Only" in the User Manual), begin accelerating and it will be in 1st gear????

...Then it will automatically shift to second, and after that i can choose to shift up to D in order to hit 3rd and 4th gear?

It would be amazing if this was the problem the whole time. User error. But hey at least my turbo system maybe will be better at higher speeds. Even just having cleaned out the clogged alda system makes me feel like SOMETHING has to be better.

An 85 300d should not be starting in 2nd gear. Early 300d's were designed to start in 2nd gear. 85's weren't. It should start in first gear.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Not to beat a dead dog but here was my experience with the switchover valve - 1981 300SD - when I replaced it. At first I hooked up the vac lines to the wrong nipples and I blocked off the third nipple not knowing it was supposed to be vented to atmosphere.

Took it for a drive and could not get over 30mph. I have had this car for 35 years so I know how it is supposed to pick up. Very disappointing.

Took another look at my old pics of the old switchover valve and realized I had it hooked up wrong. Switched the vac lines to the proper nipples and unblocked the third nipple.

Took it for a test drive and immediately all the power and pick up were back. That was maybe ten years ago and I still have the same amount of pick up.

I have no idea how the 123 would run without a switchover valve but my 126 ran like a dog with it installed wrong. If your car was designed to run with a switchover valve, I would add it back. Perhaps someone can tell you exactly which vac lines to hook up on the 123.

From my experience this might have been your only problem from the beginning. Hate to say that after all the work you did to replace the ALDA. In fact I hope your old ALDA was bad but at least you are good for another 30 years.

A boost control valve doesn't have any vacuum lines does it?

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Kerry - actually I have just assumed the lines on the switchover valve were vac so I do not know. They are the same size and material/plastic as all my vac lines so I just assumed. Since one nipple is vented, maybe it is not vac. I figured there was some sort of a one way flap inside to hold the vac but have never looked inside.

On mine the top plastic line goes to the nipple at the rear of the turbo almost touching the firewall. The bottom plastic line has a banjo bolt on the other end which is bolted to the square metal gizmo with round topper which sits just in front of the VCV. Believe this gizmo is a part of the injection pump.

They are boost lines which sense the amount of boost.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I took it for another spin and tested out the methods of "forcing first gear", starting from L and working my way up. There are two results:

One, it does work better when starting from stop. I would even go so far as to say it is performing something like 30 - 40% of what i'd expect a 300D to do from start. This helps. Before i'd say it was like 10%.

Two, i am not at all convinced that this is starting me in first gear, but rather just second gear. When in L, which is supposed to give me a maximum gear of 2nd, i do believe it is still starting me in 2nd. When i get up to like 4500 RPM's, which is beyond my comfort with this car in it's state, it is really cranking but never does it shift above what it's currently in. And given that the pickup from zero still only feels like 30-40%, it makes me think it's starting in second.

But, if L is an improvement from D, then this is a little weird. If my thoery above is true, and my note of marked improvement at pickup is true, then it would mean that D is in fact starting me in third. If my theory is half wrong, and i am crazy and there is no pickup improvement, then it would mean D is starting me in 2nd.

I haven't ventured out onto any quicker roads yet as i'm still being very cautious. But the fact that L gets me from zero up to 4500 rpms and i'm cruising along neighborhood streets and it NEVER shifts up to something unless i jump it up myself, it says to me that L is starting in 2nd. OR that the whole shifting ensemble is totally messed up somehow.

What speed at 4500 rpm's? Are you saying it never shifts at all out of the initial gear even if you back off the pedal?

4th gear should be around 3000rpm's in the 60mph range.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The 'forcing 1st gear thing' isn't applicable to an 85. It should start normally in 1st gear. If it isn't, something is wrong.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Not an expert here. From a stop how many times does it shift?
Also, from one of your pictures there is a clear "thingy" looks like a filter, that needs to be vented into the cabin. I also have an '85 and my firewall looks like yours no check valve. After I rebuilt my turbo I had 10 pounds of boost.

Ok i took it out again. Starting to get excited because the car is fun to drive. The starting in L thing makes a big difference in that i can actually feel comfortable getting safely around town, albeit with a bit of a cloud of smoke from stop still. Not as bad as it used to be though.
Here are some rough results. Unfortunately my speedometer movement is stubborn and stays where it last was until you press the trip reset button. And it keeps working if you hold that in rather firmly. So i have to take "speed samples" haha. I've known this was an issue and will be opening up the dash to perhaps address that trip reset switch with some DeOxit maybe. Somehow it's definitely related. I don't think i'll be passing inspection without the speedometer working.

I went up to around 55 or so, and still only ended up using three gears. So, two shifts.
Gear A, from the "L" position from stop: 30mph, 3800-4000rpm
Gear B, after switching to "S" position and noticing the gear shift: 30mph, 1800-2000rpm / 40mph, 2200-2500rpm
Gear C, after shifting to "D" position and noticing the shift: 50mph, 2000-2500rpm

I didn't go above 55. Speed limit on the "major" roads around here is 45. Didn't want to take it on the highway.

Haven't done the boost pressure gauge test yet. Looking forward to that!

So based on those readings, what gear am i actually in when i start, and what gear is gear C? It feels good to drive up there. I feel like it kinda has to be 4th gear, but what do i know. I certainly could have picked up comfortably it seemed. The engine was purring nicely and didn't struggle up there. Maybe this is the result of all the work i've done. I felt like i could have managed fine on the highway once i was up at that speed range.

I did notice a faint whistle occasionally when letting off the gas at moderate to high speeds. But i only noticed this once or twice. Must be the turbo. Sounded kinda sweet, but hopefully that's not some new issue i've introduced.

Not sure what clear thing you're referring to. I took a look at pics but don't see it. Can you point to it somehow?