Why is it often difficult or even impossible to select 1st gear on the Mercedes box?

RONS'S TECHNICAL TIPS No 5 speed box

I have just been asked (again) why is the pre 95 Mercedes gearbox so difficult to get into 1st gear. Fiat owners will take comfort that just as they have an occasional problem with 5th, the Merc 1st gear is also an awkward child.

This is a long read, so if you have no interest in the intricacies of a Mercedes gearbox then get on with your life now!

QUESTION - Ron I am finding 1st a little tough sometimes. Not crunchy just seems a tighter gate than the rest. It's getting better as I get used to it. It doesn't jump out or anything. I have also found a "false gear" to the left of 1st. Is this something you have fixed or got used to? Cheers, Julian.

ANSWER - 

Yes it is exactly as you describe. First is stiff and awkward, but not temperamental - you can master it. The reason it is stiff is that it is a very low gear and spinning very fast even in relation to the engine speed at tickover, so the synchromesh rings have a lot of work to do to spin the gear up to speed before it can be engaged, and so are manufactured bigger and tougher on 1st gear for this reason, and so need more pressure to engage. It shouldn't be a problem because 1st is a crawler gear and usually always engaged when stopped. You just have to maintain pressure on the stick a moment longer to allow the gear to spin up, and then it should engage smoothly. The false gear you refer to is no mans land between 1st and reverse. !st is slightly to the left and back, reverse is fully to the left and forward. The stick naturally rests between 2nd and 3rd. If you push the stick towards the left you will feel 2 resistances, the first is where you pull back to 1st, and the second, further over, is a full stop, and will only go forward into rev. The trick is to know these two positions, and not to make the mistake most people make, which is to ram the stick all the way to the left and then pull back expecting 1st. This reason and this reason alone is why people accuse the Mercedes box of being a bitch. This is a bit unfair as it is actually a bullet proof box, it just has "character"! The most you can criticise it is that it is old fashioned compared with modern boxes, which are silky smooth, and have well defined gates (the layout of the gears). 

However on a very steep hill, you may find yourself wanting to do a moving change down from second to first - it is a rare situation, so doesn't get much practice. Basically, as others will testify, it is an almost impossible change. It has to be done quickly, or the van will stop, so it get's rushed and fluffed. As I said before, if 1st gear is spinning fast on tickover, then with the engine revving highly climbing a hill in 2nd then it is spinning even faster, too fast even for the synchromesh rings to have any chance of spinning the gears up to a speed where they match each other and can engage smoothly. So it refuses to go into first and the van stops on the steep hill and you have to engage the handbrake quickly, then select 1st and smoke the clutch to pull away again, and then remain in 1st and crawl up the hill at 10mph as you are afraid to change gear!

The answer to this is to master the ancient and lost art of double declutching. Baby boomers (and their parents) should remember the days before synchromesh when all boxes were "crash" boxes. But before I describe double declutching, I have to describe what synchromesh is. Most people know that a gearbox has cogs inside of different sizes to allow the same speed of engine revs to deliver a range of different road speeds. But when you want to change gear, the cog that is connected to the engine, is spinning much faster (or slower) than the next cog of the gear you want. If you try to push two cogs together that are spinning at different speeds, then of course you get that familiar grinding sound. The only way to get a smooth change is to match the speeds of the two cogs before pushing them (engaging) together. This used to be a matter of pure skill. Drivers had to be intimately aware of the speed and sound of every gear in relation to road speed and to skilfully use the accelerator (and the clutch) to match the speed of the engine cog to the speed of the next gear cog. They did this by either letting the engine revs fall, changing up (easy) or increasing the engine speed, changing down (hard). The trick was to gently let the gears just touch each other with hand pressure on the stick and feel the vibrations of the gears and as soon as they matched up, push in the gear smoothly - pure skill! After WW2 and through the 1950's as cars became common for non professional drivers, Porsche and others eventually came up with a much better system, known as "synchronised shifting" or synchromesh. This is an extra mechanism in the gearbox that does mechanically what the driver used to do by skill - it matches up the gears perfectly before engaging them, using an interlocking system of spinning rings and cones. When you change gear in a synchro box, the slight resistance you feel as you enter a gear, especially on older cars, is the synchro doing it's job. Nowadays it's all computer controlled, but not on Classic Merc Hymers!

It should also be said here that the selector mechanism - the system of rods and joints that connects the bottom of the gearstick to the selector arms on the side of the gear box - is also an important part of this setup, and should be kept well lubricated and maintained - 90% of the time when people think they have a bad box - it's just an easy fix to the selectors. If your gear change is very stiff, or sloppy like a bag of spanners, then your selectors need sorting out - an easy job.

So, back to double declutching! Synchro can only achieve so much, usually the differences in speeds between gears are not so much, so changes are easy and smooth. But in the case we are discussing - down in 2nd and 1st where the engine is revving highly, the speed difference is too much to allow the change in the time needed, hence the resistance, and difficulty in getting into 1st. To get round this and effect a smooth change - yes it is possible - you have to use a trick from the old pre synchro days, known as double declutching. You depress the clutch, pull the stick back from 2nd into neutral, and just hold the stick with gentle backward pressure right at the point where it should go into 1st, then lift the clutch, blip the accelerator, which revs up the engine, and then depress the clutch again and slip into 1st when you feel the resistance drop - all in one easy fluid motion that takes only slightly longer than a normal change down. This has the effect of spinning up 1st gear and making the change much easier. In fact it is basically the only way to make a moving change into 1st gear on the Mercedes box (pre 95). And this is the main reason, if not the only reason, the box has a bad reputation, because all the other gears are pretty much silky smooth. 3rd to 2nd is occasionally similar, especially when flogging up a hill, and this change can also be made smoother by ddc'ing.

It is a difficult manoeuvre to perfect, but not impossible, it is similar to a dance move - coordination of hands and feet movement. This is why you often hear trucks (in the old days) making that familiar sound as they came to a hill - the engine revs fall, then he changes gear with that familar blip of the accelerator, and then the engine revs resume at a higher note. Almost impossible to describe in words, but instantly recognisable if you hear it (and are old enough).

It is a vey satisfying technique to master, and it is essential if, like me, you want to really push your Hymer up the steepest of hills and hairpins, like Hardknott pass in the Lake District, and the even more spectacular minor Alpine passes. A fully loaded S700 or S660 just doesn't have the guts to get up a 1 in 3 in 2nd gear, especially if there is a hairpin coming, so you have to be able to get into 1st quickly and smoothly and with the minimum of loss of speed. The only other alternative is to know the road and what's coming, and select first from the bottom, and grind up in 1st without changing gear. Also there is no shame in having to stop to engage first on a steep hill - you have a right to be there so the guy behind in his Audio Quattro paddle shift computer controlled 6 speed auto box in sport mode can basically sod off!