testing this car I experience that the auto transmission slips at a brisk start.
What is the reason for that and what is the cure?

You may want to increase the modulator pressure. Slipping at a fast start sounds kind of odd. I know it sounds funny, but you did check the transmission fluid with the engine hot and idling, right?

David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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Fluid is checked.
Odd or not...slipping it is a fact especially when engine is cold.
Typically the transmission is very solid on these cars.

What is the set point for modulator pressure. Any manual?

Firstly, I please note that I know next to nothing about automatic transmissions! POST BELOW EDITED TO AVOID CONFUSION

From the little I have read, the modulator affects the quality of the shift - hard or soft. Doesn't sound like that is your problem. The pressure control affects when the transmission shifts.

For a start, if it was my car, I would double check the fluid level. It is harder to do than it seems. Make sure you do it according to the instructions in the owners manual.

Good Luck and hopefully someone here with transmission experience can help.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

If transmission is slipping under 'energetic acceleration' (I read) it must be that modulator pressure is too low.
This could have various reasons..
1) modulating valve binding (.,...where is this valve?)
2) Vacuum box binds
3) working pressure regulating valve binds (where is that?)
4 modulating pressure wrongly adjusted

Who can help? Who has done this?

From another thread Martin said "Transmission model is: K4 A 040, 4- speed" That would be a 4-speed with a fluid drive instead of the W3 X XXX 3 speed with torque converter.

Just a starting point so I don't confuse things again by talking about wrong transmission as I did in a post here yesterday

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Martin:

When the K4A trans is in 1st gear, two elements are transmitting the torque; band B2 and a sprag clutch known as the "freewheel". The freewheel is also used in reverse; if reverse is smooth and free of noise, the freewheel can probably be eliminated as a source of any slipping complaint. Typically, when a sprag clutch fails, it fails completely and will not carry any load at all. That leaves band B2 as the friction element that is slipping. Before chasing pressures it may be in order to insure that the mechanical linkage from the throttle to the lever on the trans is in good condition (bushings intact, locknuts tight, etc.), and that the band has not started to disintegrate (remove pan and check for pieces of the band). Bands B1 and B2 are adjusted by changing pressure pins (of varying length) inside the trans, and with the vale body removed.

The working pressure is not externally adjustable, although it can be measured. To change the working pressure independently of the modulator pressure requires a change of a spring within the valve body. As you note, the modulating pressure can be adjusted at the vacuum modulator; a change of modulator pressure does affect the working pressure also.

Edit:

Or more correctly, acknowledgement of blunder.

The above description is for the K4C and W4B transmissions, not the K4A.
The K4A does not have a mechanical throttle pressure link, rather that function is accomplished by a three position solenoid mounted atop the trans which controls a pressure range valve. The vacuum modulator then controls pressure within those three ranges (although in the lowest, coastdown range, and in the highest, kickdown range, the modulator has little effect). Low pressure faults originating with the modulator usually are associated with early, soft shifts. An early 1-2 shift in a low pressure condition could very well produce a slip in a friction element (either band or clutch). A check in the pan for debris would still be in order, I think.

Thanks Frank,

I recently changed AFT: No signs of contamination or chips or anything.

Disconnected vacuum line. I was measuring modulating pressure today: 3.2 bar instead of 3.8 bar (set point).

I was trying to remove vacuum pot but AFT came out, so I left it in place for the time being.
You are saying to adjust modulator pressure at the vacuum pot...but HOW?

I read in the manual that with screw driver pressure can be adjusted but I don't see any screw/slot inside the pot.
Can it be that the screw disappeared? Does anyone have a detail of the vacuum pot?

Martin

Attached Thumbnails

   

Found the adjustment screw it is an ALLEN key, not a screw driver thing!
Hard to see anything down there.
Here we are. I adjusted pressure to 3.8 and go now for a test ride.
Keep fingers crossed.

Martin:

If the pictures which you posted are of your trans, it is of the K4C/W4B/W3B type. (The K4A modulator is at the middle of the trans case, yours is at the back.)
Expect some ATF to flow out if you remove the modulator; also expect to find a spring and a pressure pin in the modulator and the hole. The pressure pins are available in different lengths to set the initial adjustment. Again, check the mechanical throttle pressure linkage and arm at the right front of the trans case; if that linkage and arm are not moving far enough forward with full throttle the shift points and working pressure will be low.

And one more edit: There are two K4A transmissions, of two different architectures. The one that you have, Martin, is the second one to bear the designation, and is also known as a 722.2xx. It is in the series 722.0xx, 722.1xx, and 722.2xx. The K4A040, which is yours, was used only with the 3.5L M116 engine.

yes the picture is my transmission.
I was driving yesterday. No slipping so far and faster shifts. Setting the higher mod. pressure probably did the trick. But I am still cautious not to be too happy. But maye I am lucky that I don't need to rebuild the transmission.

Thanks for your great insight:
1) "Expect some ATF to flow out if you remove the modulator; also expect to find a spring and a pressure pin in the modulator and the hole. The pressure pins are available in different lengths to set the initial adjustment."

I think I'd need to drain the AFT entirely in order to remove the modulator- Is the modulator = vacuum pot?? the little metal can where the vacuum line attaches with the banjo fitting?
What exactly can you adjust with the various pressure pins?

2) "Again, check the mechanical throttle pressure linkage and arm at the right front of the trans case; if that linkage and arm are not moving far enough forward with full throttle the shift points and working pressure will be low."

This is the pressure control rod? The rod that is connected to the throttle linkage below the air filter? I adjusted this pressure rod in a way that with throttle closed (engine idling) the rod is in a fully backward position.
I am not sure if this is the correct adjustment.
You are saying that I should rather adjust from the other end meaning that
I use full throttle position and see if the rod at trans is fully forward at trans?
Can you give me instructions and I will be very happy to adjust accordingly

Martin

Hi Martin,
I am a bit confused because I thought your car did not have the pressure control rod from the throttle linkage. If you have that rod, what you did seems correct to me. Does your linkage look like this one (from a 107)?


There is a bushing and pin on the transmission lever. When that wears, the bushing sometimes even falls out resulting in very poor adjustment!. Best to first make sure that the bushing is there and not worn. To change the bushing, you may have to remove the lever. Then you push the lever all the way back (once bushing is known to be good, it is easier to push rod from engine compartment) and adjust the throttle linkage ball joint so it just goes on.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Thanks Graham,
I think this is how it looks at my transmission. I have the pressure control rod and I think the lever for the rod looks like in your picture.

As I said I did adjust this rod (position controls the shift points meaning at engine speed does the transmission shift) in fully closed throttle position.
With removed rod (at ball joint) and throttle linkage fully 'closed' I push the rod all the way back to its stop and adjust ball joint to exactly the length, then connect.

Is this the way it is supposed to be?
I feel that shift points are a bit 'low' according to this adjustment.
Therefore I kind of liked Franks version...adjusting from the other end- fully open throttle and lever at transmission fully open.

Which method is correct?

I went with method in MB manual (your method). Should be fine if the bushing where the rod connects to the lever is not worn. As mentioned in previous post, that bushing does wear and even falls out. Best to check it out.

Frank's method could work equally well if lever is supposed to be fully forward when throttle is fully open (as opposed to MB method - fully back when throttle is fully closed). Maybe Frank can confirm that.

Either way good bushing with no slack is a must.

Another source of error, is adjustment of the ball socket end. Manual says rod socket end should be placed alongside linkage ball, not above when adjusting. Can't see it makes too much difference. I adjusted rod length until gear changes up and down took place at close to spec for transmission (several trial runs). Never quite got it right!

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Martin, Graham:

Re: Throttle pressure rod adjustment

You are both "right"; the MB method is with the lever all the way back, and the throttle on the idle stop. As Graham has suggested, that is for openers; once that is done, small adjustments to the ballsocket to shorten the rod are appropriate to raise the shift points to your personal liking. A caveat is in order here: if the lever is too far forward in the idle position the coastdown downshifts will be somewhat harsh, and the trans will downshift all the way to 1st, rather than coming to a stop in 2nd. The 722.2xx transmissions are arranged to stop in 2nd, and then shift to 1st with only a small movement of the throttle linkage, just before the throttle plate itself is opened. The objective, I think, was to minimize creeping while at a stop.

On the matter of the pressure pins for the modulator (aka, vacuum pot), they were supplied in varying lengths to compensate for manufacturing variations in the machining of the case and of the modulator valve itself (inside the trans). A gauge plug was available which screwed into the modulator hole, and which had a small hole through it into which the pin could be inserted. The pin was the correct length if it was flush with the end of the gauge plug. If all of the original parts are still installed, and the desired modulating pressure can be achieved by adjusting the modulator, then there is no need to change the pin.