I'm having issues with my '75 240D not wanting to up-shift into 3rd. Even after a valve body swap. I feel like there may be a modulating pressure issue. Kickdown solenoid has been ruled out as the culprit. I have the "Technical Data Book" for 1975 and wanted to see if I can try myself at diagnosing what is going on. Has anybody tried this on their own before? Are the test ports all the same threading? I'm assuming I need a gauge that can handle 250psi as well.
Any insight is appreciated.

The following got me thinking since my speedo needle bounces quite a bit at low speeds.

I think what Frank was trying to say is "Check the nut on the output flange". If it's loose, your speedo and governor drive will be slipping.

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Does the trans start in 1st, shift to 2nd but not 3rd ( or higher ) ? Or does it start in 2nd and not shift into 3rd ( or higher )?

I made an adapter for a 722.5 ( the 5 speed version of the .3 / .4 ) from a 6 mm thread bolt drilled through the center and a -4 JIC hydraulic adapter or male plug welded to the head.

When selecting a gauge, ideally you want your measurement range to be 25 to 75 % of the gauges full scale reading. You can use the low end of the scale with some loss of accuracy but don't go over the full scale reading unless the gauge is retarded.

Dry gauges are just fine for what you are doing.

Starts in first, shifts to second but won’t go to third mostly. If it does shift to third then it shifts to 4th fine. Downshifts seem to be just fine.

When the trans does not shift into 3rd, does the engine RPM flare or just stay at what you would expect for being stuck in 2nd gear?

During the expected 2 - 3 shift, if you let of the throttle does the trans complete the 2 - 3 shift?

Do you have an apply chart for this trans? ( This shows what clutches/ bands apply to get a certain gear )

Engine RPM stays in line with what I'm doing with the pedal. The feel is like a manual in second gear and your not shifting up purposely.

If I let off the throttle there is a 50/50 chance of getting the upshift to happen. I usually let off, count to two, and then give it a hair pedal and sometimes it will catch. 3-4 after that works just fine. It also is starting in 1st and I'm pretty sure that's not normal.

I found the modulator adjustment and adjusted clockwise 2 full turns with no change in behavior.

I do not have a chart.

Some trans start in 1st but you would have to talk to others to see what specific ones. Generally 1st is only used when the throttle is pushed all the way to the floor / kick down.

We need to find an apply chart. If the 2 - 3 shift only picks up a clutch / band, rather than something releasing and another element applying, you might have a fault with the element that is picking up.

If anything, raising the modulator pressure (clockwise) will further delay the upshift.

Backed the modulator adjustment way out and the 1st to second is really soft. Hangs out in second still though. Looking at the technical data manual page 218. Shifting members are as follows:
1 B2,F
2 B1,B2
3 K1,B2
4 K1,K2
R B3,F

Maybe brake band 1 is getting stuck since all the other shifts seem to work ok?

BP:

If B1 were not releasing, and K1 were simultaneously applied, the trans would try to lock, and the car would decelerate.

What is the source of the modulator vacuum? If there is a vacuum control valve (VCV) is it providing vacuum to the modulator at small throttle pressure?

Also check the adjustment of the control pressure linkage; if it is too short the shift points will be delayed, and if way too short there will be a 1st gear start!

This is a 722.108 so no vacuum modulator.
The control linksge has been shortened and lengthened several times to no avail. Curious thing is when I can get it to go into third, if I hit the kick down it goes to second and when I let off as a normal person would as I get to speed it will upshift back to third easily and then into fourth. And downshifting always seems to go through all 4 gears when coming to a stop.
The car doesn’t feel as though it’s locking up so it’s something with the transition from the B1-K1 I would think. If this were a working pressure or any other system pressure issue, wouldn’t it have an effect on the other gear changes as well?
Maybe both the original valve body and the one I swapped in have a similar issue?

Are there any check balls in the trans case? This really feels like a check ball has beat the seat out / plastic check ball is worn out.

Was the valve body you installed a known good unit? Any metal in the fluid of the replacement valve body?

What is the history of the current trans and was the fault sudden / slowly got worse.

Where would the check balls be located?
The unit came from a Mercedes tech friend of mine that gave me a parts transmission for this purpose. He stated that it would probably flare but should work. It sat for 15 years though so who knows. I didn't see any swarf when I pulled it out of the parts tranny. The original valve body wouldn't shift out of second at all when I got the car so I figured that this replacement VB was an improvement. When I first put it in I was able to drive the car around down and only 25% of the time did I have shifting issues. Now over the past couple weeks it has gotten progressively worse to the point that its very hard to go to third at all. Seems like it gets worse when its warmed up too. Maybe its just worn out too and I should just bite the bullet and get a new VB?

The KD behavior may offer a clue as to a possible fault. Upshifts depend on rising governor pressure; the KD solenoid valve dumps governor pressure. If the KD valve is leaking internally, there may not be enough available governor pressure at the usual road speed for a 2-3 shift. At the road speed for a 3-4 shift the governor pressure will be correspondingly higher, and the shift will be normal.
Cycling the KD valve may temporarily decrease the leakage. Cleaning the valve and ensuring that it seals, and that the solenoid moves freely may be your next step.

Maybe I’ll pull the KD valve off of the parts trans and see if there is any difference. That sounds plausable though. An absurdly low governing pressure might explain the start in first as well.
Question: what is the effect on pressures when I would abruptly let off the pedal and then give it a hair after 2 seconds and it would shift to 3rd. Is there a quick spike in governing or modulating pressure that would cause it to bump into third?