Can anyone give me a bit of help please. I have a S5 GT with a misbehaving overdrive,(the switch is ok, I have checked that), the overdrive will not engage until the engine is up to running temperature. Sometimes it clunks in solidly and is ok, but subsequent engaging requires a peculiar sequence of pushing the stalk down (nothing happens) up as though to disengage then all the way down again (clunk, we're in!). Although this is annoying it is ok, the real problem is that if I get onto the dual carriageway before the gearbox is sufficiently warm for the overdrive to engage and am running at about the 60mph mark, even when the unit is warm if I try to engage the overdrive there is a noticed drop of revs but no satisfying clunk and as soon as I push the throttle the overdrive drops out. The only way I can get it to engage is to slow right down to about 35mph, (not good at 7am on the A38 with a Polish artic up your jacksie!)
The box is full of oil, SAE30 (good quality) I have tried 20/50 but it seems the same. Would flushing the box through with a very thin oil/paraffin mix help? Any ideas please.
Peter

Rootes were very insistent that you use 20W/50, but I doubt that using SAE30 is the problem here. I certainly wouldn't try any paraffin mix though.
Have you checked the pull-rod adjustment on the solenoid? The WSM tells you how, but basically you remove the small screws which retain the rectangular cover on the right side, then engage OD and check that a 3/16 drill goes through the lever into the casting. If it doesn't, you adjust the rod - you have to use a small spanner on the flats to stop it turning.
Incidentally, a 'clunk' might be satisfying but it puts a shock into the transmission. The easy way to avoid trouble is to dip the clutch, but at modest revs, opening or closing the throttle is enough.
Although it sounds as if the trouble is internal, you could also have a lazy relay - the special one on the bulkhead. If you are confident, you can temporarily just have a simple switch connecting 12v to the solenoid to test things out, but don't forget to disengage before engaing 1st, 2nd or especially reverse.
There two valves you can access in situ - with the oil drained - for cleaning. Details are in the WSM, accessible here:

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