adjusting 300D auto shift points

[Jake]

Hi everyone :)

The car ('76 300D) is very revvy , goes a long way before changing gear, in my manual 240D I would be changing up 10mph earlier and still have good driveability.

Is there an adjustment I can make to shorten the revs that the auto changes at under modest throttle application .

Thanks.

edit ; the shift point can occur earlier if I back off the throttle a bit , when I want it to change. Wife doesn't drive like this .

[Hendrik]

Mercedes do publish the figures as to where the shift points should be, however you really have to hunt around to find them.
Shift points are dictated by throttle position, as such you should have a linkage going from your throttle to the gearbox. me things in 76 they where actual linkages, as opposed to a cable.
You tighten it to get later shifts and loosen it to get early shifts but I would be leaving it alone.
Higher rpms is better for your box and engine and you will not be gaining fuel efficiency, these old style Diesel engines really need to rev to get the best out of them.
Question is how heavy are you on the go fast pedal? They are set up so the shift gets delayed more if the pedal is pressed down further, so if you start messing with stuff instead of not pressing so hard on the go plate, you could have all sorts of drive ability problems.
Once again, leave it alone unless it is really out of spec, you do not know more about these cars than the people who engineered them.
If you are really concerned I would be trying to find a mechanic who knows these things and can offer an opinion as to whether your car is normal or if there is a problem/misadjustment.
These engines are not big old truck motors that have a pile of torque, these engines love to rev and should be revved.
Did I mention that you should not be messing with things unless you know what you are doing.

[Jake]

Thanks for your enthusiastic response :)

Yep , they love to rev.

When I give it a slipper full , it happily revs out to the shift point , changes firmly and revs on............ ok.

When I drift lazily away from the lights , MODEST throttle , it still wants to rev close to those upper shift points , with little load , I am aware it can drive fine at lower revs , my manual one does. But driving down the road at 28mph in third is too busy for me.

So ...... instead of leaving it alone :boohoo: I might just give it a WEE tweek , thanks.

And then maybe put it back , lol. Yes , this is mechanical / rods , easy turn a thread adjustment . One turn at a time , eh only one rod to the g/box , the kick down lever , I guess it could be a little looser. You are right I know little about these , that's why I ask here .

[Hendrik]

Hey it's your car and you can do what you like with it but we have somewhat of a duty to let people know that messing with the factory settings can have issues.
Sure in the nearly 40 years your car has been around, things may have worn and or other owners have fiddled with it.
I am not going to go into a long winded explanation of torque versus power and it's relationship with clutches in the gearbox.
However I will say that at one end of the gearbox you have a motor trying to turn the back wheels at the other end of the gearbox you have some wheels that need a fair bit of force to get them to turn.
Between them is a bunch of gear ratios that make it easier for the engine to turn the wheels.
OK a 'little' tweak is not going to do much damage, some cars in the late 80's early 90's came with a two setting gearbox, I think it was called spots and economy mode but how it worked was on shortening or lengthening the control cable, thus altering the shift points.
What I am trying to warn you about is that if you overdo it, you can cause premature wear because the engine is trying to turn the wheels at low revs in a high gear, this means that a lot more torque is being applied to your clutches, which especially in a cold box can lead to slippage.
Think of the effort required to ride a bicycle, you select a higher gear too soon and you need a lot of effort to turn the crank but select the right gear at the right speed and effort is reduced.
A good way to think of torque and power is that torque gets you there and power keeps you there.
Without driving the car I could not say if you are being over dramatic as to the engine revs but I do know that to drive a 240D auto you go pedal to metal until cruising speed is attained, the 300D less so but it don't hurt them to rev a bit, actually does em good. There have been many instances of problems occurring because the engines have been babied, these things where designed to visit the autobahn now and then.

[Hendrik]

Actually thinking about it may the other way around, loosen to get later shifts and tighten for early shifts, been mucking around with a website editor all day and the brain is kind of fried.
However the principle of operation is that when the pedal is to the metal the control cable is fully on and you get late shifts (at the red line if everything is right)

[TheMadRacoon]

But driving down the road at 28mph in third is too busy for me.

Ummm.... that's 45 km/h in 3rd gear of a 4-speed transmission. I don't think that's rev-ing it out, even for a diesel. Or have I missed something?

[Jake]

But driving down the road at 28mph in third is too busy for me.

Ummm.... that's 45 km/h in 3rd gear of a 4-speed transmission. I don't think that's rev-ing it out, even for a diesel. Or have I missed something?

That situation is when I am in slow moving traffic , not accelerating , just drifting along , no power being applied , it can handle being in top ,

Hendrik I do understand what you are saying , I enjoy immensely the tutonic engineering that enables us to drive the old diesels at full noise all day . I am really just after a little tweek to have the car shift up a little earlier, when under negligable load.

Thanks for your thoughts..

[Hendrik]

Yeah well as long as you keep a note of what has been done, so you can undo it, everything be sweet bro.

[TheMadRacoon]

That situation is when I am in slow moving traffic , not accelerating , just drifting along , no power being applied , it can handle being in top

Some autos need to get over a certain speed even at light throttle to change up. I've experienced this in a lot of automatics. Not to say yours is not out of adjustment with age. Hope the adjustments give you what you're after.... sometimes little things like that can be irritating.

[pastelgrey300D]

Hey Jake, just a thought, how long has it been since you've had a trans service done? Maybe a new filter and fluid may help? If the filter is a bit clogged and the fluid not flowing as well as it should maybe this drop in pressure could be delaying the upshifts? Also there was mention of adding a transmission seal conditioner from an old post I dug up:

viewtopic.php?f=6&t=9730&p=62078&hilit=sealer

I had Rowell & Searle do a service on the transmission in my 1982 280SE with 292000 on the clock(so it is worn). They put a conditioner (Valvoline Auto Transmission Sealer in saying that it would soften the hardening seals a bit)
8000 kilometers later, the trans is now better than it has ever been. Which is not to say that it is as good as new but I am very happy with R&S.

[Jake]

Hi guys , update on the auto shift issue.

I ordered a new kickdown lever bushing as the original had gone to mush.

weeks later ........ it arrived and is installed :dance: I have the kick down rod fitting onto the ball joint in a relaxed mode , no stress either way.

However , it has not changed anything for the better , in fact wife says it hangs up even more now , got to lift the foot right off at 35mph to get it to shift into top.

In the interim , I did change the fluid.


There is a complicated arrangement on top of the valve cover with several ball joints and a slip joint.

I cannot find a spec for the slip joint adjustment . Can someone please help me with that detail.

Thanks.

[bahnstormer109]

Sometimes if you have a vacuum leak, such as in the central locking this can interfere with the shift points on the gearbox.

I had a 300TD and when a central locking diaphragm leaked, the car held onto its gears for too long and revved out too much. I noticed it went back to normal when I drove with the doors locked, and held the gears too long when I drove with the doors unlocked. So I put the two together and replaced the locking diaphragm and the gearbox was fine again.

It was holding onto 3rd gear until over 60kph though, which sounds a lot worse than yours.

[New123]

But driving down the road at 28mph in third is too busy for me.

Ummm.... that's 45 km/h in 3rd gear of a 4-speed transmission. I don't think that's rev-ing it out, even for a diesel. Or have I missed something?

That situation is when I am in slow moving traffic , not accelerating , just drifting along , no power being applied , it can handle being in top ,

Hendrik I do understand what you are saying , I enjoy immensely the tutonic engineering that enables us to drive the old diesels at full noise all day . I am really just after a little tweek to have the car shift up a little earlier, when under negligable load.

Thanks for your thoughts..

Here's my 0.02...

My 230E, while just drifting along and not accelerating, won't shift into 4th until approximately 47kph. Always has. Seems normal to me. It will hold in 3rd until I give it just enough to push the speed over 48kph, then a nice smooth shift into 4th.
Like Hendrik says, personally, I would leave it alone.

[Jake]

Thanks for the low vacuum suggestion , but this trans only has a single rod to tell it what to do.

As an aside , I did renew the vacuum pump when I bought the car and the central locking operates perfectly.

[Tony From West Oz]

Adjusting accelerator linkages on 300D.
With accelerator pedal on the floor, the IP must be at max fuel point.
The slip joint just allows some pedal movement before increasing fuel.

Regards
Tony

[Jake]

Thanks Tony . I will check the max travel limits.

The thing is , the kick down lever is firmly connected to the Accel pedal , both being solid rods attached to the same bell crank on the block, so any application of throttle means the kick down lever is moving also even before the IP is engaged , such is the slip joint engagement . So I was thinking the amount of slip preset is important.

[Jake]

I have adjusted most of the slip (8mm) out of the slip joint , it now has about 2mm slip.

The gear shift points under light throttle now seem more normal , that is , still a bit on the high side for me, but as reported by my wife this morning , ALL GOOD . So that's a good result as she is commuting in some awful traffic and needs to stay relaxed , eh.

Thanks for all your thoughts .