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I get a scraping sound as my 2012 Mazda BT-50 shifts gears under low to normal acceleration from first to second to third. From there everything is normal. I'm pretty sure it’s the auto gearbox trying to change down as other people have commented. Under more aggressive acceleration I don't get the problem. I have only done 40,000km in the last eight years (bought brand-new) and the ute has had an easy life and never towed (no towbar fitted). Am I damaging anything by continuing to drive it? What does this sensor cost? Does Mazda have to pull the gearbox to fix it? Any help would be great.
This is indeed a known problem on these transmissions and one for which Mazda issued a Technical Service Bulletin (TSB; like a recall but not safety related) back in 2014. Apparently, a problem with the valve-body (that controls the gearbox’s shifts) caused a scraping or buzzing noise usually when shifting form Park to Drive or Reverse. That’s not exactly what you’re reporting, but it’s almost too similar not to be related. Depending on the build date of the vehicle, Mazda either replaced part of the entire valve-body assembly.
The other problem you’ve alluded to; that of a faulty output speed sensor is a different issue to the one that sounds like it’s causing your problem. In the case of the dud sensor, the gearbox could suddenly decide to shift back to first gear unannounced. If that happened at speed, it could cause the vehicle’s wheels to lock up with some very interesting consequences.
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