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Martin Cutler
Prolific User
Username: martin_cutler

Post Number: 123
Registered: 7-2007
Posted on Sunday, 05 July, 2009 - 21:18:   Edit PostDelete PostView Post/Check IP

Hi all,

Can you tell me if the pump in the back of the gearbox pumps oil to the rear shocks all the time, or only when the lever is moved to "hard".

Cheers

Marty
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 355
Registered: 6-2006
Posted on Sunday, 05 July, 2009 - 22:24:   Edit PostDelete PostView Post/Check IP

Hi Marty,

It doesn't really 'pump' oil to the rear shocks, except when you bleed the shocks really.

It just varies the pressure. I think it should be something like 3psi at NORMAL and 30psi at HARD.

You need to jack up the rear so the wheels can spin (chuck the front wheels - H & S etc) and have it driving at about 10MPH or so.

What are you testing?

Cheers, Paul.
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Laurie Fox
Frequent User
Username: laurie_fox

Post Number: 63
Registered: 6-2004
Posted on Sunday, 05 July, 2009 - 22:58:   Edit PostDelete PostView Post/Check IP

Marty and all.

The pump does pump all the time and if there is a leak then it will remove oil from the gearbox. Here is an extract from an old topic on the BDC Forum which may be of interest.

Regards

Laurie

*********************************************
"Laurie Fox
User posted 11-03-2004 10:53 PM

Don.

It occurs to me that the poor brake performance may be more related to the gearbox failure than to any servo/brake shortcomings. If the damage to the gearbox was at the rear end of the 3rd motion shaft and some gear teeth had been stripped then it may be that although the car was still being driven the servo shaft was not actually going round. This would result in only the force from the brake pedal being applied to the rear brakes with no servo support and no hydraulic pressure to the front brakes. The speedometer might not have been working at that stage or may have been giving very funny readings. You might like to discover a bit more about why the gearbox failed so as to prevent it happening again. Was it insufficient oil in the gearbox? I ran into this problem some 20 years ago. Normally the gearbox only needs to be topped with small amounts of oil from time to time and there is tendency, particularly if the car is being used intensively, to leave checking until later on since it will probably be all right. So, in 1976, knowing that I had not checked the gearbox oil level for some time, I set off on a business trip to Italy with a spare can of oil in the back. Towards the top of the St.Gothard pass in Switzerland, fairly fast in third gear, the normally quiet gearbox started to sound as if something was beginning to run dry, so I stopped and topped up the gearbox which needed 4 pints (Imperial) to being it up to the right level. There was no more gearbox noise and when I checked again a little later the oil level was still OK. At the next check, about 10,000 miles later, it needed nearly 4 pints to refill, which seemed a lot but did not alert me to making further investigation. Some time later the "drying up" noise seemed to return near the top of the Hardnott Pass in the Lake District, shorter but steeper than the St.Gothard, on a 1 in 4 slope in bottom gear. I say "seemed to return" since bottom gear is noisy anyway and I could not be sure. Anyway I did nothing about it at that stage. The later on still I topped up again (another 4 pints) and the first drive from home was to the local car park just half a mile away to do some shopping. I was very surprised to see a trail of oil spots on the ground which had clearly come from B420EY. Where were they coming from and why hadn`t I seem them before. The answer was that the exhaust tail pipe had been vibrating against the copper pipe leading from the gearbox pump to the nearside shock absorber and had worn a hole on the copper pipe. So the gearbox pump was pumping as much oil as it could get hold of out of the gearbox quite quickly. But the pump does not operate until the car is moving and it does not pump out any oil if it can`t get hold of any more from the gearbox, so the fault is not easy to see unless you look specifically immediately after topping up. Having found the problem I rerouted the copper pipe to keep it well away from the exhaust pipe and all was well again. It is interesting to think about how much oil the gearbox pump can extract from the gearbox and whether what is left is enough to lubricate things. The pump is high up in the gearbox and the oil inlet is through a standpipe which points downwards. With the car on level ground there are about 2 pints of oil left in the gearbox when the pump has pumped out as much as it can. With only two pints of oil in the gearbox the frontmost gear (which is the largest) on the second motion shaft is still dipping into the oil and will throw it around to all the other parts. But if the car is on a upward slope the pump will begin to pump a bit more oil out and all the second motion gears may be above the oil level. This seems to explain why the "drying up noise" only appeared on long hills. However, if there are no long hills involved the 2 pints left in the gearbox after the rest has been pumped out seems to have been quite enough to keep things in good order. B420EY may have been running like that for years! When the copper pipe was rerouted I also renewed the flexible oil pipe from the gearbox pump outlet to the connection point on the chassis. It seemed to be in reasonable condition but I fitted a new one as a precaution since oil could be pumped out at that point. The time to get suspicious about oil being pumped out is when 4 pints or more are needed to top up since if less than 4 pints are needed then the pump standpipe will still have been below the oil level - the pump will have been applying pressure but not pumping oil out. I know of two cases where the gearbox has been pumped out to the 2 pint level but with no damage in either case. One was on a Mk VI single exhaust system, like mine and yours, where the exhaust pipe probably damaged the copper pipe (whether the twin exhaust system on the M series and onwards can have the same problem I do not know), and the other was due to a punctured bellows in one of the rear shock absorbers. It is interesting that on your car the failure was at the rear end of the gearbox which is furthest away from the large gear at the front of the second motion shaft and perhaps less likely that the other parts to benefit from the oil thrown around from there when the oil level is low. I cannot think of any reason apart from lack of lubrication which would cause that particular bearing to fail.

Laurie Fox"
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Martin Cutler
Prolific User
Username: martin_cutler

Post Number: 124
Registered: 7-2007
Posted on Monday, 06 July, 2009 - 19:16:   Edit PostDelete PostView Post/Check IP

Hi guys,

B319LH has "wet" rear shocks, they don't actually leak from what I can tell, and the oil level in the gearbox has remained steady. B256MD, on the other had, generally pumps out a fair amount of oil via the rear shocks when I put the control to "hard", but behaves fairly well when left on normal. As I feel I am not getting any change in driving feel from hard to normal, I was wondering if I block off the pipe coming out of the gearbox, would I damage the pump? While the pump is blocked off, I could remove and dismantle the rear shocks and overhaul them, but still be able to use the car. If the lever inadvertantly got moved to hard whilst the pipe was blocked, would that blow up the pump?
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Paul Yorke
Grand Master
Username: paul_yorke

Post Number: 356
Registered: 6-2006
Posted on Monday, 06 July, 2009 - 20:47:   Edit PostDelete PostView Post/Check IP

Hi Martin,

I'm fairly sure that the dampers just have the one pipe going to each damper.

Blocking off the pipe should cause no problems to the pump. As I mentioned earlier, oil does not (should not) be pumped through the pipe so it is (should be) constantly blocked off as it were, by the shock damper.

Personally I'd be more worried about driving a car with no dampers on. Although saying that, the springs are fairly self damping - especially dry ones.

Never though about it before now - but perhaps it's possible to modify the end cap so an adjuster screw is fitted instead, this would act against the bellows / spring so you could adjust and set the damping manually (from under the car), removing the worry of loosing your gearbox oil etc.
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Richard Treacy
Grand Master
Username: richard_treacy

Post Number: 1793
Registered: 4-2003
Posted on Tuesday, 07 July, 2009 - 00:03:   Edit PostDelete PostView Post/Check IP

Martin,

Just to elaborate on what I believe Paul correctly implies:

The dampers do not damp using gearbox oil. The gearbox oil is only pumped to the dampers as a control signal. The gearbox oil has nothing to do with the damper oil: it is fed by a control line, not an oil feed line. The ride control pump only modulates the damper control valve inside the damper through fully open to fully closed as set on the steering column ride lever. The damping, other than on fully Soft or fully Hard, also increases with road speed. Note that SC/S cars have an electric two-position ride switch and no pump instead of the oil control line.

There is practically nowhere for the gearbox oil to leak inside the damper on a MkVI/R unless your dampers have a very rare ailment. If gearbox oil is leaking from the damper area, then you may have an oil line leakage problem. It sounds strange that both dampers could suffer these problems simultaneously without warning.

More likely is that the dampersí oil may be leaking out faster whenever the dampers are in Hard as the dampers are working harder in that mode.

You will have checked the damper oil within the last 6 months. Best check the damper oil levels again. If they are already no longer full, then the chances are that the damper main shafts are leaking or just weeping badly. That ailment does come more-or-less in pairs over time, and left unchecked they will drain in Soft mode soon too. The main damper shaft seals are only rope affairs unless modified to lip seals, and consequentially slight weepage is acceptable provided that the damper oil only needs topping up every two years or later. Leakage or poor damping will deservedly fail the roadworthiness test automatically. Lack of damping is known to lead to broken spring leaves and suspension bottoming quite apart from unacceptable road behaviour.

Likewise at the front, poor damping can break coil springs and lead to suspension bottoming even on smallish bumps.

If the dampers are full, in acceptable condition, and are functioning as shown on the roadworthiness bump machine (they vary from state-to-state, but usually have electronic readouts), then blocking the ride control line will leave you in soft ride mode on the rear dampers. At least that is a safe status under most conditions. But why do it ?

RT.

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