Gear box oil change.

[Tannaton]

Originally Posted by drjarmin

Sorry to ask a newbie question but why would changing the fluid cause major issues? Is it due to it being a 'shock to the system' or simply getting the level wrong?

Could it be boxes that need to have fluid changed have large miles on them and problems were just around the corner anyway?

There are two parts to that theory:

1 - The old oil is so saturated with deposits that it will not absorb any more and hence debris accumulates in parts of the gearbox. When the fluid is changed, the new clean fluid penetrates the debris accumulations and loosens it in lumps which can lead to blockages and sticking valves. This can be mitigated by doing it in 3 or 4 partial changes (aka sump dumping) with a few hundred miles between them.

2 - The difference in viscosity between the oil and new fluid is so great, the gearbox electronics do not adapt quickly enough and an old fragile box is further damaged by shift shock etc. as it re-adapts. This can be mostly mitigated by clearing the adaptation memory or (even better) re-adapting the gearbox after changing the fluid.

Where a gearbox is worn and has very old fluid, changing it can exacerbate certain faults due to the thinner fluid leaking through out of tolerance parts but this is really only a risk if the gearbox is knackered anyway.

Even those owners with faulty gearboxes who change the fluid usually report noticeable improvement in spite of the underlying problems not being fixed.