85' 300D Late Transmission Shift

[AndrewTaller]

Hello everyone,



Recently I took my 1985 Mercedes 300D on a 40-mile drive on the freeway. I've owned the car for about a year and a half now, and everything was working great. The transmission shifted as it should, engine ran very well, etc. I got home, parked it and then started it up again 3 hours later because I had something else to do. When I got on the road, the transmission would not shift into 2nd and kept revving. Confused and annoyed, I pulled back into my driveway to investigate. That was three days ago.



Since then, I tested many things to see if I could find an issue, but still nothing. I'll go ahead and list them below for reference. The car has 223k miles on it with a 722.315 transmisson.



1. Checked fluid level; a little high, so I used a vacuum hand pump to get the excess removed. No change.



1. Checked Vacuum; 25 inches of Hg at idle from direct pump line. 18 from VCV when at idle and bleeds off to ~13 when accelerator is pressed. (Checks out, no issue found.)



2. Checked Vacuum modulator; holds vacuum as it should. (Yes, I used a different mityvac from the one used to drain the atf)



3. Unplugged kickdown switch and made sure it wasn't stuck.



4. Dropped transmission pan to look for debris. Pan was clean as can be, but I changed the filter and fluid anyway. Added trans-x both before and after I dropped the pan.



5. Checked the tightness and adjustment of the bowden cable. Cable has tension when pulled (while disconnected from the throttle, proving it hasn't fallen apart.) Cable was re-adjusted too. This caused no change.



I was also able to get all my gears back if I revved the engine high enough, but the transmission only shifts at 3.4 k rpms now, while it was shifting at around 2.0 k rpms before.



Please help... I'm completely stumped. Feel free to ask questions, as I will be happy to answer them.

[billmerserve]

Did you find a fix?

[Diesel911]

Originally Posted by AndrewTaller

Hello everyone,



Recently I took my 1985 Mercedes 300D on a 40-mile drive on the freeway. I've owned the car for about a year and a half now, and everything was working great. The transmission shifted as it should, engine ran very well, etc. I got home, parked it and then started it up again 3 hours later because I had something else to do. When I got on the road, the transmission would not shift into 2nd and kept revving. Confused and annoyed, I pulled back into my driveway to investigate. That was three days ago.



Since then, I tested many things to see if I could find an issue, but still nothing. I'll go ahead and list them below for reference. The car has 223k miles on it with a 722.315 transmisson.



1. Checked fluid level; a little high, so I used a vacuum hand pump to get the excess removed. No change.



1. Checked Vacuum; 25 inches of Hg at idle from direct pump line. 18 from VCV when at idle and bleeds off to ~13 when accelerator is pressed. (Checks out, no issue found.)



2. Checked Vacuum modulator; holds vacuum as it should. (Yes, I used a different mityvac from the one used to drain the atf)



3. Unplugged kickdown switch and made sure it wasn't stuck.



4. Dropped transmission pan to look for debris. Pan was clean as can be, but I changed the filter and fluid anyway. Added trans-x both before and after I dropped the pan.



5. Checked the tightness and adjustment of the bowden cable. Cable has tension when pulled (while disconnected from the throttle, proving it hasn't fallen apart.) Cable was re-adjusted too. This caused no change.



I was also able to get all my gears back if I revved the engine high enough, but the transmission only shifts at 3.4 k rpms now, while it was shifting at around 2.0 k rpms before.



Please help... I'm completely stumped. Feel free to ask questions, as I will be happy to answer them.

This is a partial description how the trans works: "


1st Gear

In 1st gear, brake band B-2 is applied and one-way converter

clutch is locked. In selector lever position "2", clutch K-2 is also

engaged. Both planetary gear sets are involved in gear reduction.

2nd Gear

In 2nd gear, brake band B-1 and brake band B-2 are applied.

Both planetary gear sets are involved in gear reduction.

3rd Gear..."

Notice that you need the B2 and B1 to be working to go to 2nd.

The classic B2 piston issue is you loose forward gears completely but when it has not yet failed completely it causes issues but still keeps working for awhile. There is a lip seal in there and there is an aluminum bushing (my 84 had the aluminum busing)and as time passes when you pull out your B2 piston you can see were that bushing has been grabbing on it. The replacement is a Nylon. Plastic one.

This is what is in the trouble shooting guide gut it does not exactly fit your issue.
"Transmission Slips When Starting Off In 1st Or 2nd (Reverse

Is Okay)

Band B-2 shift valve sticking (that would be the Piston). Band B-2 piston worn or

damaged. Band B-2 adjusted incorrectly or worn or damaged. Adjust

brake band B-2 by installing a longer thrust pin (if necessary). If

transmission operates properly with selector lever in "2", but not in

"3" or "D" position, one-way clutch may be slipping.

Transmission Slips In 2nd Gear Or Shifts From 1st To 3rd Gear

Check control valve B-1 for ease of operation. Replace valve

body (if necessary). Remove and install brake band piston B-1, check

sealing ring and replace (if necessary). Replace brake band B-1 and

thrust body for B-1. Command valve binding.

Transmission Slips During 2-3 Upshift Or Slips Initially,

Then Grabs Hold

Check modulating pressure and adjust (if necessary). Check

for temperature throttle installation (if equipped). Valve body worn

or damaged. Replace valve body (if necessary). Replace inner plates of

clutch K-1 or recondition clutch (if necessary). Check front cover

Teflon ring."




WILL NOT UPSHIFT


Incorrect governor pressure. Defective governor assembly.

Check for stuck kickdown solenoid or for constant voltage to solenoid

caused by a defective fuel pump relay or sticking kickdown switch.

Valve body dirty or valves sticking. Repair or replace valve body.



"UPSHIFTS AT HIGHER SPEEDS THAN SPECIFIED
Check pressure control cable engagement (bodwen cable), condition and adjustment. Check for stuck kickdown solenoid or for constant voltage

to solenoid caused by a defective fuel pump relay (gasoline engines) or sticking kickdown

switch. Check governor pressure. If regulator pressure is too low,

replace centrifugal governor. Ensure control pressure regulating valve."

If you have the vacuum amplifier and the other more extensive vacuum stuff I have no idea how those effect the shifts.

[tyl604]

Wow; that is the best explanation of how the tranny works that I have seen.

Should be a sticky.

[Diesel911]

Originally Posted by tyl604

Wow; that is the best explanation of how the tranny works that I have seen.

Should be a sticky.

It is part of a Mitchell's Manual. I did not print all of it.

In the past Mitchell's manuals used to be what people with a sizable auto repair business used. As you see their writings are simple and too the point.
You did not find them at Auto Part Stores as they were expensive.

There would also be a section that had how long it would take to do specific jobs.

[dennislarock]

I believe you have an '85... does it have the vacuum amplifier/transducer...i.e.,
*The* “Blue Flying Saucer”


The ’85 transmission vacuum system does have a positive effect on shifting. When properly working, the shifts should be both precisely smooth and firm at varying acceleration conditions compared to the earlier simpler vacuum system.

The earlier turbo models had basically a direct vacuum source from the main line to the trans vacuum modulator, with a Vacuum Control Valve (VCV) - #65 on the diagram, in the middle to leak down the pressure. A high vacuum pressure around 10-15 Hg leaked towards zero at a different pressed throttle position with a full throttle position at zero Hg.

The vacuum modulator with high vacuum softens the shifts at a low throttle response and a modulator with lower vacuum firms up the shifts with more throttle. This is predicated only on throttle position and no other variables.

On the 1985 models, Mercedes engineered a trans vacuum system that didn’t just rely on throttle position to control the softness or firmness of shifting. It also relies on turbo boost pressure which becomes dependent on engine load and temperature.
The vacuum transducer (amplifier) (#123) works in a system. It works in relation to turbo boost pressure and engine temperature.
Unlike the prior years, the system now starts off at a high vacuum of 12-17 Hg to the modulator. It now leaks down five inches when full throttle travel is given. When the turbo boost pressure goes up, the boost applies an input to the vacuum transducer at connection PRE on the diagram. This produces an upward deflection of the transducer diaphragm, lowering the amount of full vacuum applied to the modulator valve to zero. The hydraulic modulating pressure in the transmission is increased which firms up the shifting.


At a coolant temperature below 50º C, a temperature switch (#20) cuts off vacuum signal from the VCV at a Switchover Valve (#125) on the diagram. The vacuum in the circuit produces a higher vacuum to the transducer from a direct line to the vacuum pump (#76). Then this higher vacuum is sent to the modulator softening the shifts while the engine is cold.
When the coolant temperature rises above 50º C, the switchover valve is powered on and the VCV controls the transducer. The vacuum circuit takes over as describe above which an increase in throttle/power load produces less vacuum to the transmission modulator, thus firming up the shifts.

This is kind of a long winded statement and hopefully it will give some info to make a decision of what direction you want to head towards.
One other note, If you make the vacuum circuit simpler by using the 1981-1984(Fed) model, the modulator will have to be adjusted/tweaked to shift in an acceptable manner.

The '85 300sd and 300D came with 722.416 transmissions for the California version and 722.315 & 722.303 transmission for the Federal version. They then used a version of this vacuum setup on the 603 powered w124 with the 733.4 transmission and the 603 powered w126 with 722.3 transmissions on the later years as well as some other vehicles. So Mercedes thought the improvement was worthwhile to carry it over to the newer models.

With that in mind, theoretically you could use this system on an earlier model and get good results. But only if your able to tweak it to work properly. What you lose is the simplicity of the old system in which more items can now fail over time.

dennis (vis-a-vis Delivery Valve)

[dennislarock]

Breakdown on w/s/c w123 & w126 Diesels

1977 - 1979 240D (ALL) = 722.117A (Mechanical)
1980 - 1983 240D (ALL) = 722.117B (Mechanical)
1977 - 1979 300D/CD/TD = 722.118A (Mechanical)
1980 - 1981 300D/CD = 722.118B (Mechanical)
1980 300TD = 722.118B (Mechanical)
1981 300TD = 722.303 (Electronic)
1981 - 1984 300SD = 722.303 (Electronic)
1982 - 1984 300D/CD/TD = 722.315 (Mechanical)
1985 (Fed) 300SD = 722.303 (Electronic)
1985 (Fed) 300D/CD/TD = 722.315 (Mechanical)
1985 (Calif) 300D/CD/TD = 722.416 (Mechanical)
1985 (Calif) 300SD = 722.416 (Electronic)

Please add, edit or delete if there are any errors... and, always double check the number on your transmission.

dennis

[dennislarock]

Vacuum Transducer (Amplifier) *Blue Flying Saucer* #123 on Diagram

[Frank Reiner]

Originally Posted by AndrewTaller

When I got on the road, the transmission would not shift into 2nd and kept revving.
The car has 223k miles on it with a 722.315 transmisson.

I was also able to get all my gears back if I revved the engine high enough, but the transmission only shifts at 3.4 k rpms now, while it was shifting at around 2.0 k rpms before.

Originally Posted by Diesel911

WILL NOT UPSHIFT

Incorrect governor pressure. Defective governor assembly.

Valve body dirty or valves sticking. Repair or replace valve body.


"UPSHIFTS AT HIGHER SPEEDS THAN SPECIFIED
Check governor pressure. If regulator pressure is too low, replace centrifugal governor. Ensure control pressure regulating valve."

The above from D911 is the essence of what appears in the MB FSM as probable causes for the symptoms described by AT.

It all points rather strongly to a governor problem.

[AndrewTaller]

Hi all,

Sorry it's been so long since I responded. I moved into my college dorm a a few weeks ago and have been busy with school related things.

Luckily I figured it out on my own . The problem ended up being a stuck kick down solenoid (the little thing that bolts to the back of the tranny). I simply unplugged the solenoid and everything went back to normal. Transmission shifts better than it ever has now as well, but I suspect that is because of the fluid change and the two quarts of trans x. I don't plan on changing the solenoid anyhow, as if I want to downshift, I just use the shifter (D to S or S to L).

From what I can say, if you ever have this issue, make sure you disconnect both the kick down switch (at the pedal) and the solenoid (at the tranny). I knew my kickdown was engaged, but was confused why it was staying on even with the switch unplugged. Turns out the solenoid can be powered even with the switch unplugged.

Best regards, Andrew