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-   -   6.9 transmission - no kickdown & early upshifts (http://www.peachparts.com/shopforum/vintage-mercedes-forum/63952-6-9-transmission-no-kickdown-early-upshifts.html)

mapab 05-04-2003 10:16 AM

6.9 transmission - no kickdown & early upshifts
 
I am having problems with the auto transmission on my 77 6.9, if you can provide advice I'd really appreciate it. I can do most repairs myself unless it requires specialist equipment:

The transmission works 100% in normal driving under light/medium pedal pressure. When transmission is in Slope/Sports mode (Position S) the upshift from 1st to 2nd happens later, and 3rd is locked out - all as it should be. Downshifts happen at the usual preset speeds.

When I try to accelerate hard, the transmission never goes into a lower gear even when travelling 30mph in 3rd. The upshifts come early (as under light throttle settings) and there is no kickdown when I floor the accelerator pedal. I can change gears down/up manually by using the selector lever.
It is hard to be certain but under hard acceleration when I move the gear selector from L to S or S to D the upshifts are too smooth when there should be a harder upshift.

I have looked at the kickdown switch but have not tested it. There is no physical restriction to pedal travel. I have checked the metal rod going from the engine to the gearbox and it is adjusted as per manual.

Oh, also there is the correct quantity of fluid in the transmission and it does not smell burnt or anything.

Any advice, especially from people who are pro or home mechanics that have had similar symptoms and have resolved them, would be highly appreciated.

Adam

psfred 05-04-2003 11:00 AM

Adam:

There are serveral possiblities. The kickdown solenoid could be stuck in the tranny, or defective, so that you don't get downshift. The governor could be stuck, so that you are getting mininum upshift speeds all the time (my 220D did this, it fixed itself with time, it had been sitting a considerable time), or the control rod valve is bad.

I'd change the fluid and filter and see what happens.

If the car has been sitting for some time, the problem may cure itself with use.

Peter

mapab 05-04-2003 11:13 AM

Peter,
Are you suggesting that these problems may be unrelated, i.e. due to multiple items being broken simultaneously?

There is the
a) kickdown solenoid
b) upshift governor for controlling the upshift moment
c) the control rod - which connects to some other part of the system.

Is there nothing common which links these 3 things? If there is then maybe I should look at it first.

The car has not been sitting but has had regular use. Although I do not doubt the benefits which can be derived from fluid changes, I do suspect that these problems won't cure themselves through an oil/filter change.

Adam

psfred 05-04-2003 11:35 AM

Adam:

All three things regulate what is know as the "control pressure" in the hydraulic circuit in the transmission. Shifting is accomplished by moving spool valves to open or close openings in order to direct hydraulic fluid to the apporpriate clutches.

What is happening to you is that either the control pressure is signalling fast upshift or a valve is stuck.

The kickdown solenoid causes a pressure change (I don't know if it is up or down) in control circuit to delay upshifts and to downshift the transmission if already in 3rd. Upshift will still occur, but at a higher speed.

The governor is a speed sensing device, either a spring loaded centrifugal valve that allows more pressure into the control circuit the faster the output shaft goes, or a shaft driven pump that does the same thing.

The control rod operates another variable valve in the same circuit to vary the control pressure relative to throttle opening.

One or more of these is not working properly, or there is a problem in the valve body (such as a stuck spool valve or a broken spring).

All I would do would be to verify the operation of the control rod, the vacuum line to the modulator and modulator (should only control shift quality, and it will be VERY hard if something is leaking) and the operation of the kickdown solenoid. A fluid and filter change will verify that you don't have a clogged filter and resultant low pressure.

If none of this fixes the problem, time to go to the mechanic.

Peter

mapab 05-04-2003 02:31 PM

Right then. I'll attempt to check those things you recommend. Thanks.

I don't suppose you know of a source of transmission manuals for the 6.9? I've got the W116 MBUSA manuals on CD but haven't seen the transmission manuals on sale anywhere except eBay. Do you know if they are available on CD?

Incidentally, the Haynes manual for the 250/280 W123 series has a fairly detailed breakdown of the procedure to recondition the shift housing but the 6.9, of course, has a different gearbox.

Adam


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