Hydraulic Gear Selector - Strip Down and Overhaul (Part 1)

Having bought the rebuild kit several years ago, I have eventually got round to rebuilding the bvh 'hydraulic gear selector'.

The 'hydraulic gear selector' of a hydraulic gearchange car

The first thing to say, is that I don't call this the 'hydraulic gear selector'. That's what Citroen and the manuals call it. I've always called it the 'gear brain'. It interprets the movement of the gear wand (yes - I call the gear lever the 'wand') and translates that into the selection of the relevant gear. So it's a brain - right? A hydraulic gear selecting brain. 

 

It's also worth noting that the unit is actually two devices - the main 'gear brain' and also a 'gearchange speed regulator'. This second device was introduced to deal with changes in fluid temperature and pressure that would otherwise alter how the gear brain responded. And I don't call it a 'gearchange speed regulator' either. I tend to call it a 'viscosity compensator' or 'viscosity regulator'. Take your pick....... In stripping and rebuilding the 'unit' I will deal with each of these devices seperately. Anyway, mine was (were) looking decidedly shabby. 

'Hydraulic gear selector' and 'gearchange speed regulator'

The paint had fallen off the aluminium body and the steel part had surface rust. I was keen to clean it out  - partly because it seemed like a good idea and partly because I'd previously had clutch problems and so wanted to rule out the gear brain (told you) as a possible cause as well. 

 

Workshop Manual Instructions - A Cautionary Note

English language instructions for stripping and rebuilding the gear brain can be found in manual 518 (which dates from 1966) and you will find the instructions there at step 25 onward of Operation DX.334-3. The instructions were not originally included in printed copies of English language manual 814, though you may find they are added into PDF copies from manual 518 - which dates from 1966.

 

Where the 'gearchange speed regulator' is concerned, you need to be aware that while the instructions in French language manual 583 (volume 3) are correct, the instructions (and photos) in the English language manual 518 contain several errors!  For a good few years these errors have been left as-is in digital versions of 518 (and 814 where added-in) but, at the time of writing (mid April 2023) have been corrected to mirror the French instructions. If you have a PDF copy of manual 518 or 814, then depending on when you downloaded it (and from where) the English instructions may (or may not!) have been digitally corrected. Unfortunately the one weakness of the digital versions is that here is no version control on the digital versions  - so you don't know what may have been added or updated. If you want to keep a downloaded local copy of the manuals, you really need to download the 'current' copy every time to be sure. Which kind of defeats the object.

Spot the difference: printed English copies of 518 have text and photo errors....

Last couple of point on the manuals: as the instructions are from manual 518 and 1966, they relate to cars with LHS fluid. The instructions tell you to clean and wash the parts in alcohol. For gear brains from LHM cars, the parts should be washed in petrol.

 

It's also worth noting that while the LHS gear brain instructions hold good for LHM carburetor cars, the gear brains of fuel injected cars are a little different. Mechanically I think the parts and construction are the same, but a noticable difference is how the 'viscosity compensator' is fitted to the top of the brain (more on that later). Anyway, overall there is enough common ground for the instructions to still be of use.

 

Initial Dismantling

I won't go into the details of removing the unit from a car. That is covered in the manuals. The only tip I will give is tthat's it's probably better to release the 'back' bolt of the double-bolt clamp connecting the gear selector lever to the slide valve of the brain (so that the clamp is left on the valve and gear brain - not the selector lever. The 'back' bolt is lined up better with the hole under the rubber bung and it just means that, when you come to connect it all back up, it's a little bit easier to tighten the bolt.

Double bolt clamp on the selector linkage

 When it comes to dismantling, the manual suggests you build a stand to hold the gear brain (stand 'MR. 3053-200').

Another Citroen 'Special Tool'.....

I just rigged something up with a piece of old angle. I just needed to drill a few extra holes for the bolts and to make room for the shaft that sticks out.

Home Made Stand

With the gearbrain mounted on it's end I first removed the gearchange speed regulator and the associated 'S' shaped pipes that join it to the gear brain.

Gearchange Speed Regulator Removed.....

With the viscosity compensator out of the way, the front end cover of the 'brain' can be removed. The 'front' being the end that points towards the nose of the car.

 

 

This cover holds a powerful spring for the syncro delay return pistons and the manual says each of the nuts and bolts of the cover should be released very gradually in turn - just one flat (of their hex heads) at a time. This is to ensure that the cover lifts off straight and doesn't snag.

Front End Cover

 I will confess that I didn't follow the 'one flat' rule studiously, but it soon became clear how easy it was for the case to jam on the studs and I did take my time on this stage. It was also clear how the stand helps to keep the unit steady and frees up one of yout hands - worth making one.

The front cover - released

With the nuts and bots removed, the innards (and the spring) were revealed. Also revealed was thick black goo and I was glad I'd tackled this job (remind me I said that later).

That is a BIG spring.....

 

.....and a dirty spring

Turns out the syncro return spring is actually two springs - one inside the other.

Second spring inside the outer spring

There are thrust plates on the ends. These are different shapes and it's important to note which goes where.

Syncro delay return springs

With the syncro delay return springs out of the way, the large main gear selector slide valve can be removed. You shouldn't need telling but be very careful not to drop or scratch this. Same goes for the shaft from which it was removed.

 

The gear selector slide valve

And with the base of the springs cleaned up you will see this....

Head of a syncro delay piston

It's the nipple end of one of the syncro delay pistons. On the end of the gear brain clamped to the stand, you will see the end of the 'auxilary clutch control slide valve' (or 'the auxilary clutch control slide valve' as I like to call it) sticking out. It has the hole through the end - for the hook of the auxilary clutch mechanism. This is it.

 

Tip of the 'auxilary clutch control slide valve'

Push this into the unit slowly and carefully and (with the end cover and springs now out of the way), you will see the first syncro delay piston stsrt to emerge from the top end. Again - be careful not to push this out and drop and damage it. Grab it as soon as you can and put it aside safely. Other syncro pistons will be removed and you need to make sure you do not muddle these up. they need to go back in in reverse order of removal. Citroen recommend that you make a little stand. I didn't do woodwork at school, so just carefully laid things aside.

Rack to store the pistons and other parts

With the first piston removed, you will find an 'O' ring seal in the circumference of the chamber. You need to CAREFULLY winkle this out. This is made all the more difficult by the seals being hardened over time. The technique I used was to skewer the seal with a dental pick and then pull it's edge in sufficiently to get something behind it. You need to be EXTREMELY careful that you do not scratch the surface of the chamber as this can ruin the gear brain. 

 

In this diagram, parts 12 and 13 are the syncro delay pistons and part 14 is the auxilary clutch control slide valve. The 'O' rings aren't numbered but are around parts 12 and 13.

 

Diagram showing the syncro pistons in situ

Right, with one syncro delay piston and seal removed, you need to do it all over again..... carefully push the end of the auxiliary clutch control slide valve a little further and another piston will emerge at the top end. (Remember not to muddle this piston with the previous piston). Now you need to remove the 'O' ring seal for the second piston. This is deeper in the chamber and less easy to get at. You need to take even more care with winkling it out. Things only get harder. 

It's increasingly hard to access and remove the old '' rings.

To remove the two remaing syncro pistons (yes - there are still two to go), you need to use a drift or similar to push the auxilary clutch control piston further up into the chamber. Once again, whatever you use, make sure you don't scratch the edges of the chamber. Removing the deep 'O' rings for the last two pistons is quite tricky and brings the most risk of scratching the walls of the chamber. Be warned! Finally, with the last piston and 'O' ring out, the auxilary clutch control slide valve can be removed.

I put the parts aside in seperate bags - marked so that I could make sure I put them back in the right order after cleaning and inspection.

Bagged and tagged......

With the stand having served it's purpose for now, I removed the gear brain and tackled the back end cover on the bench.This revealed another spring (not as big) and more goo (but not as much).

Rear end cover removal

Rear end cover removed

It's worth noting at this point  - and before atention turns back to the main body - that there is a 'secret' seal tucked away in the cover. I took a photo to remind me....

A note to myself....

Beneath the spring is a disc - a thrust washer - Under the thrust washer is the very tip of the automatic gearchange slide valve. (It's part 4 in the diagram earlier in this post).

Pushing the end of this valve SLOWLY into the shaft from the back end first pushes out a small plug from the other end (part 3 in the diagram). Don't lose this! Continue pushing SLOWLY and, one by one five small pistons will pop out in turn. NOTE: the slide valve will not, and should not,go all the way through the shaft

One of the small gearchange pistons starting to emerge

There are no 'O' rings to worry about this time but, again, Citroen say the pistons need to be replaced in the order in which they were removed - so don't muddle them up. Store them safely and carfully as, when the time comes, they will need to be reftted in reverse order. Finally, with the pistons out, the  slide valve itself can be pushed/ pulled out.

Removing the slide valve

Other than very carefully cleaning and preparing everything, that's 'disassembly'.

Gear brain parts

 

Rebuild kits are readily available. Some contain new springs, others do not. They all seem to contain a large number of small 'O' ring seals, but it turns out that these are just plate seals for the several hydrualic connections to and from the outside of the hydraulic gear selector. I mean 'gear brain'. 

There are a number of different rebuild kits available

Surprisingly, considering the complex job the gear brain does, it only contains five 'O' ring seals. But don't forget that the slide valves are manufactured to a fine tolerance interference fit. Plus you need a couple of replacement paper gaskets of course. My kit was simpler and cheaper and did not include springs.

 

Assembly is (of course) the reverse of disaasembly.....First the gearchange control slide valve was lubricated with LHM and carefully dropped into it's shaft.

 

Gearchange control slide valve

It should be a good tight fit but should not need any forcing.The thrust washer and spring can then be perched on top and a new gasket - brushed with LHM - was fitted. 

 

A new gasket was fitted

 And, of course, a new seal was put in the appropriate recess in the back cover (remember?).

 

A new 'O' ring for the back end cover

The new 'O' ring seal meant that the cover had to be lightly pressed (by hand) into place. The bolts and fixings were then tightened.

 

I put the unit back on my homemade stand.

Back on the stand

From the front end of the body, the five gearchange shaft was lubricated.....

  ....as were each of the pistons, that were then dropped in. These go in with the nipple end facing out of the hole.

Refitting the gearchange pistons

Lastly the small cap was replaced. If everything has gone back as it should, the top of the cap will be flush with the face of the shaft.

Now it's time to get back to those syncro pistons and tricky 'O' rings. This time job gets easier as you go along. The first thing to do is to refit the auxilary clutch control slide valve. This was lubricated and carefully dropped into the shaft. It slid in with a satisfying 'slurp'.The valve has a little circlip right at the end that stops it sliding all the wat through. It just sits at the bottom of the shaft, where it narrows, beneath the syncro pistons.

 

Next in goes one of the new 'O' rings. Lubricated with LHM, these can be manipulated in with a finger. Avoid the temptation to use anything sharp to get them to sit in the grooves. 

'O' ring for the syncro piston shaft

Next in goes a syncro piston. The last one you removed, should be the first one to go back in etc. Lubricated with LHM, the piston will slide down the shaft and sit on the rim of the new 'O' ring. It needs to be encouraged to go further.....I selected along depth socket that sat comfortably on the piston - but which was not so large that it would touch and damage the shaft walls.

Selecting a socket to help refit the syncro pistons

The pistons have a bevelled edge to help things along. I found that a generous brush of LHM on the fitted seal and when putting the piston in, meant that I only needed to give a couple of gentle hammer taps to persuade the seal to yield and let the piston past. It was obvious when this happened and I stopped tapping. I continued with another 'O' ring  and then another piston and gradually the stack built up - with lots of LHM to help things along the way.

Can you see the tip of the piston?

If everything has gone as needed, the rim of the last piston should be flush with the rim of the shaft. Like this.

The last piston refitted

The gear selector valve can be replaced.......

Refitting the gear selector slide valve

.....and then it's on to the home straight.

 

The last parts to fit are the springs and their thrust washers. An important point to note here is that, as the springs are compressed, the shank of one washer is presed into the body of the other. Citroen recommend that you check these for 'free play' to make sure they do not snag on one another and prevent normal operation of the gear brain.

The spring thrust washers fit together under compression

The other important point to note is that the shank of the thrust washer for the body end of the assembly (the 'male' thrust washer if you will) has a hole in it.

Note the hole through the thrust washer

The repair manual says that, when the spring assembly is fitted, it should be orientated so that this hole is 'towards the bottom' when everything is fitted back on the car. I took this to mean the hole should face down towards the ground and so took a few moments to work out what was the top and bottom of the gear brain when fitted. 

With that clear, the spring assembly was appropriately put in place and a new gasket added.

Nearly done......

The cover was carefully pressed-on - against the pressure of the springs - and the nuts wound on by a part turn. 

Refitting the front cover

I gradually added the bolts and gave each nut and bolt a small tighten in sequence - much as you would the nuts when fitting a wheel - until the cover was uniformly tightened down.

And that seems a convenient point to leave things. In my next post I will tackle the 'gearchange speed regulator' by which I mean the 'viscosity compensator' of course.