w123 kickdown cable - lack thereof

[Hibbo]

Hi,
My 300D does not have a pressure control cable (bowden cable) aka kickdown cable, connecting the throttle linkage the the transmission. I was quite perplexed by this as I thought all the Merc mechanical/hydraulic autoboxes had these, indeed every forum/noddy guide I've read refers to there being one.

I have seen an (American) thread somewhere where the bloke said his 240D didn't have one, and someone replied saying 'yes, some don't have them'.

So I suppose my question is why not? I assume that the shift spacing on mine is determined purely by the vacuum presented to the modulator? I had always thought that the vacuum only modulated the internal hydraulic operating pressure therefore determining the 'firmness' of the gearchanges.

Why on earth is this omitted from some cars? Or do no UK cars with 722.3 boxes use the bowden cable and I need to stop reading yank websites?

It is not a major issue as my box is performing quite well since I sorted the vacuum system out, but the lack of pressure control cable did throw me somewhat.

 

[tom7035]

Hi,
My 300D does not have a pressure control cable (bowden cable) aka kickdown cable, connecting the throttle linkage the the transmission. I was quite perplexed by this as I thought all the Merc mechanical/hydraulic autoboxes had these, indeed every forum/noddy guide I've read refers to there being one.

I have seen an (American) thread somewhere where the bloke said his 240D didn't have one, and someone replied saying 'yes, some don't have them'.

So I suppose my question is why not? I assume that the shift spacing on mine is determined purely by the vacuum presented to the modulator? I had always thought that the vacuum only modulated the internal hydraulic operating pressure therefore determining the 'firmness' of the gearchanges.

Why on earth is this omitted from some cars? Or do no UK cars with 722.3 boxes use the bowden cable and I need to stop reading yank websites?

It is not a major issue as my box is performing quite well since I sorted the vacuum system out, but the lack of pressure control cable did throw me somewhat.

Can't quite understand this one. There HAS to be some connection between throttle and gearbox. How does your car acquire kickdown?
I think I would concur with your penultimate paragraph - don't pay too much attantion to yank websites. Or maybe I'm too old to understand modern systems of communication between components.

 

[Hibbo]

Thanks for the reply Tom,

There is a method of communication; vacuum.
There is a quantity of vacuum presented to the modulator that mimics the manifold vacuum one would find on a petrol engine. This is achived by a proportioning valve on the side of the injection pump that dumps more of the vacuum (supplied by the vacuum pump) the more you press the pedal, before it reaches the gearbox.
As I understood it, this vacuum then affects the hydraulic pressure inside the box, lower vacuum equals high pressure. Thus under hard acceleration the vacuum is low, more torque is being shoved through the box, the hydraulic pressure is greater therefor gearchanges are firmer. My box was flaring as the vacuum presented to the box was too much and the pressure upon gearchange was to low. The fluid was minging and stank - everyone told me the box was dead and needed replacing - I got a vacuum gauge, probed around, drilled the rivets out on the proportioning valve, adjusted it to take out the backlash on the hairspring, and now the box is working great.
Sorry, I am now waffling (I am just back from the house of the Stella Artois), but I thought the spacing of the gear changes was determined via a bowden cable.
The kickdown is actuated by the electrical kickdown switch.

My 190 (722.4 box) has a pressure control cable, and as such has a two position kickdown - go to three quaters throttle and it'll drop a gear, floor it and press the switch, and it'll drop two gears - revs permitting.

Thanks again for the reply, I am not a mechanic by any stretch, I am purely working on (slightly) educated assumptions, so any info is appreciated.