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Why are early MB transmissions so weird and scary?


Why are early MB transmissions so weird and scary?



Hmm..... I think you all seem to be missing my point. I believe I understand how the transmission should function . You all mean to tell me that from the factory, the transmission would shift from second VERY early and harsh, and hang on to third until the engine blew??! I have read the service manual and checked all the "carpet in the way of the kickdown" solutions.

Theres something not right with my tranny... if thats normal, the DB automatic is a VERY poorly made piece of machinery! Does anyone have any experience with adjusting or repairing the vacuum controlled shifts?

I've been following this thread, and the others are right: that's the way these old D-B 4-speeds work. I only have the 108-109 service manual which doesn't cover the 250S, but the model K4A 025 was used in the 280S/8, 280SE/8, 300SEL/8 and 280SL/8 up to May 1969. The service manual states that this will start in 2nd gear except (1) if position "2" is selected, or (2) if throttle is completely floored to kickdown position from a standing start in position "3" or "4".

You should also keep in mind that this is basically a transmission designed in the '50s by a company that had never built an automatic before, and was at least a partial knockoff of a transmission that first appeared in the '39 Oldsmobile. We are not talking sophistication here. Fred Flintstone had smoother shifts. For example, you state, "Won't downshift with increased throttle, unless you're going slow, and it feels like it." Exactly. These transmissions will not downshift until their pre-set downshift speed, even at full throttle (accelerator to floor, touching but not depressing kickdown switch). They should downshift if you push through to the kickdown position. The lack of part-throttle downshifts is a PITA when you're going up a hill in an urban area, you have to manually downshift or kick it down. But that's what you get with a forty-year-old design.

They are designed to allow a somewhat variable upshift, depending on throttle position (i.e., light throttle-early upshift, heavy throttle-late upshift). According to the service manual (see disclaimer), the K4A 025 should have the following shift points (all in KM/h):

Position 4
Upshifts:
    Idle throttle  Full throttle   Kickdown
1-2     n/a             18 *1         25
2-3     25              45            75
3-4     45              120          120

*1 Applies only when first speed has been attained with kickdown first.

Downshifts: Idle throttle Full throttle Kickdown 2-1 n/a n/a 10 3-2 18 18 30 4-3 30 30 105

Position 3 Upshifts: Idle throttle Full throttle Kickdown 1-2 n/a 18 *1 25 2-3 25 75 *2 75 *1 Applies only when first speed has been attained with kickdown first. *2 This may be a misprint, but it's what the manual says.

Downshifts: Idle throttle Full throttle Kickdown 2-1 n/a n/a 10 3-2 18 18 64

Position 2 Upshifts: Idle throttle Full throttle Kickdown 1-2 15 45 45

Downshifts: Idle throttle Full throttle Kickdown 2-1 10 10 30

If yours doesn't kick down to first when you floor it to kickdown from a standing start in position 3 or 4 (or doesn't kick down when flooring it in a higher gear), the manual suggests the following problems: sd Kickdown modulating pressure too low.


1.  Double lift solenoid not moving into kickdown position<
    (a)  Kickdown switch badly adjusted or defective.
    (b)  Cables or their connecting points damaged or loose.
    (c)  Fuse blown
    (d)  Double lifting solenoid defective.
2.  Kickdown linkage disconnected, knocked out or broken.
3.  Floor mat not adequately cutout (you said you'd checked this).

The double lift solenoid is mounted on the top of the transmission, and actuates several rods and bellcranks leading down the side of the trans to the modulating pressure transmitter on the lower right center of the transmission.

At this point, I recommend that you take it to a trans shop that knows what they're doing. I do not recommend mucking about with automatic transmissions. As far as I'm concerned, automatic transmissions (and differentials) are magic. They might as well have a ball of pure white light in them, for all I understand them.

However, if you're crazy, the manual suggests the following procedure for adjusting the kickdown linkage (again, this is the 108/109 manual). For most of this, you'll need the car on jackstands or the front up on ramps, and a friend to operate the accelerator pedal. First, check that the kickdown switch on the floor has current, and that current is going through the switch (use a test light on both leads). Second, disconnect the two wires to the idling throttle switch on the throttle body (it's a round housing). Run a jumper from one terminal of the idling throttle switch to ground, connect one wire of a test light to the battery + pole, and the other wire of the test light to the other terminal of the idling throttle switch. With the hand brake pulled (and your friend holding the service brake down, and you standing on one side of the car, and the car pointing not into the back wall of your garage), idle engine in gear. Test light should light, and extinguish as soon as the throttle is moved slightly off idle. Select N or P, and increase throttle. Lamp should extinguish at LESS than 1,600 rpm (anything less is better).

The idle throttle switch controls one step of the dual position solenoid on the transmission. When at idle with the switch closed, it delays downshifts, so that as you come to a stop, the trans doesn't downshift abruptly. You might notice, as you're coming to a stop light and the light goes green, and you're moving about 5-10 mph, that as you touch the gas, the trans abruptly downshifts, with kind of a jerk. That's because as you coasted up to the light, you've dropped below the part-throttle downshift speed, but with your foot off the gas the idle throttle switch was closed, keeping the transmission from downshifting.

You didn't say that you were having problems with a jerky downshift on deceleration, so your idle throttle switch and double position solenoid are probably okay on that aspect.

Your next thing will be to check whether the double-position solenoid moves the kickdown linkage when the throttle is kicked down.

On the passenger's side of the transmission tunnel under the carpet, there's an access plate. Remove this. With the engine off, ignition on, your friend in the driver's seat, and the transmission in position "3" or "4", look through this plate with a flashlight. You should see a rod that kind of diagonals from 1:00 to 7:00. Have your friend engage the kickdown switch with the accelerator pedal. Does the rod move when you kick down? If it does, there are several possible problems: first, the bellcranks and/or ball sockets and/or balls on the actuating levers are worn out.

Check by wriggling the actuating lever through the access plate. Second possiblity is that the double-acting solenoid is not moving the lever enough (the first position of the solenoid gets a lot of work, since every time you open and close the throttle, it's actuated. The second position doesn't get nearly as much activation, since it's only used at kickdown, and may just be sticky. It's been reported here that removing and cleaning the double acting solenoid, all the electrical contacts, and cleaning and lubing the actuating linkage balls and sockets has cured the problem.

Finally, assuming everything's clean, tight, and working, the kickdown linkage needs to be adjusted by changing the length of the kickdown linkage by screwing the linkage in or out (you'll notice that it's threaded). You DO NOT do this yourself. In order to correctly adjust the kickdown linkage, you need to connect a transmission pressure gauge to a connection for measuring the kickdown modulating pressure. After adjusting the kickdown modulating pressure, the transmission basic pressure must be checked and adjusted as necessary. Ham-fisted amateur adjustments that affect the transmission hydraulic working pressures are a one-way trip to overhaul city. Which is why I recommended you take it to a trans shop about 8 paragraphs ago.

Enjoy the idiosycrasies. Or trade it on a ToyoHondaSsan. ;^)

Mark Moburg
Seattle, Washington