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Gearbox starting in 3rd when cold??

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Age: 54
Zodiac: Sagittarius
Joined: 30 Apr 2018
Posts: 91
Location: Northumberland

PostPosted: Tue Dec 04, 2018 1:40    Post subject: Gearbox starting in 3rd when cold?? Reply with quote

..For the first couple of miles, or thereabouts, on these colder days, I'm finding that the Mini won't engage first or second gear (High range only affected) when driving, even when I'm pulling into traffic and pushing hard enough on the accelerator to operate the kickdown switch. In low range, it starts off in second which is how it's supposed to behave but driving it that way on the road isn't an option because changing from low to high can only be accomplished by stopping, selecting neutral and then moving the lever back all the way into high, it would also max out at around 22mph if driven in low range, plus doing that would cause massive understeer since there's no centre diff.

Once the temperature (presumably that of the transmission fluid since the heater starts to provide warmth after as little as half a mile) has risen sufficiently, the 'box behaves perfectly in high range and will change down and up as it should. The vehicle has only covered around 42,000 miles since it was new in 1997, the fluids, including that ATF were changed during the service carried out by the importer prior to my buying [the car] and that ATF is the correct fluid for the Mini's Toyota-supplied A131L/L gearbox, I.E. Toyota WS fluid and not the Dexron 2 that some folk use - inadvisedly - in these 'boxes. The fluid is as pink as it was when I bought the car, it feels silky smooth when squeezed between the fingertips and the levels, both when cold and when hot are bang on the lines, gearbox function is otherwise perfect with up changes only really being noticeable as a result of the rev counter needle dropping a couple of times between standstill and around 20mph, which is where it takes off just fine in (high range) top, that's to say third as the 131 has a converter lockup but no overdrive and that lockup comes in at a maximum of 35mph, allowing acceleration to continue up to the car's maximum 56ish mph, more if I'm gathering momentum down one hill to help me up the next. Laughing

This is definitely not "Morning Sickness" since the box doesn't slip, it's merely annoyingly blunt when taking off in 3rd and besides, Morning Sickness usually happens when there's some form of contaminant in the fluid. I did think that I may have taken on some water after going fairly briskly through a ford the other week, but the pressures at the check locations on the valve block are exactly at the middle of their correct range in each position and that also eliminates sticking valves.
I have taken these boxes apart before now but am reluctant to haul this one out and strip it just yet because I'm thinking that there has to be something I'm missing. There is a rudimentary ECU that functions for both the engine and the gearbox, but it contains only a handful of random passives and two Darlington transistors, so checking each out of circuit took all of an hour, including repotting the components and reconnecting all three of the wires that come from the unit.

Next thought I had was that the thermal coupling on the transmission fluid cooler may have been sending the wrong information to the 'box, allowing fluid to run colder for longer than it should from a start, but unlike the Toyotas that use this same gearbox, albeit transversely orientated, there isn't in fact a sensor in the cooler and nor is there any electrical connection between the gearbox and the coolant temperature sender for the engine.

I'm now thinking that maybe this is normal in cold conditions, after all it does start behaving eventually, but why would it default to its highest ratio from cold when every other automatic transmission I've ever met - and that's no small quantity - have only come with connections that allowed the unit to avoid changing up into an o/d ratio until up to full temperature, in the way that the U441EL in the bB does in fact, and many other 'boxes with 1:1 3rd and o/d 4th.

Your thoughts on this much appreciated unless they involve pouring STP into the ATF!  Shocked  (I kid you not; a USA-based "specialist" suggested that I should try that. Nooooo! Evil or Very Mad

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PostPosted: Tue Dec 04, 2018 1:40    Post subject: Google Ads keep the POCUK free to join!

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