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Automatic Transmission

Going Bald

Curve-recognition programs in BMW's 3, 5 and 7 series with bald tires can make your head go even balder. The purpose of this strategy is to prevent shift business during turns for driver pleasability. Recently we had a call where a shop was distraught over a 530i with a 5HP18 that had no upshifts to 4th or 5th under light to medium throttle and no upshift to 3rd under heavy throttle. Much was done and replaced to remedy this complaint when all along it was one tire being a different size from the other three.

Wayne Colonna
By Wayne Colonna
Published:
Going Bald Technically Speaking Author: Wayne Colonna, Technical Editor Variation in tire sizes causes shift complaints in BMW 3, 5 and 7 series

Technically Speaking

  • Author: Wayne Colonna, Technical Editor

Variation in tire sizes causes shift complaints in BMW 3, 5 and 7 series

Curve-recognition programs in BMW’s 3, 5 and 7 series with bald tires can make your head go even balder. The purpose of this strategy is to prevent shift business during turns for driver pleasability. Recently we had a call where a shop was distraught over a 530i with a 5HP18 that had no upshifts to 4th or 5th under light to medium throttle and no upshift to 3rd under heavy throttle. Much was done and replaced to remedy this complaint when all along it was one tire being a different size from the other three.

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Now you might ask yourself, how could that be? Well, this car owner had three worn tires and a new spare. The tread on the new spare made this one tire significantly larger than the other three worn tires. This tire-size difference was enough to initiate the curve-recognition program while the vehicle was driving straight, resulting in no upshift to 3rd, 4th or 5th depending upon throttle opening. In fact, we have had complaints of no upshifts or late upshifts into 4th or 5th after kick-down on 7 series and 2nd-gear starts and no 4th on 3 and 5 series with the 4L30-E transmission. In other words, depending upon the severity of tire-size differences due to wear, underinflation, overinflation or several different tire sizes combined, the result could be a respectable variety of complaints of no or late upshifts – it’s enough to make you go bald!

Now, if you have a scan tool that provides you with data stream which presents the Curve Recognition PID, if you see this PID display “activated” while driving straight, you may keep a few hairs on your head and fix the problem without emptying your wallet on computers, speed sensors or throttle-position sensors. If you do not have data-stream info but you have a scan tool that does not report any codes, look at those tires!

There is shortcut in which you would know for sure that an initiated curve-recognition program is causing your loss of upshifts, but I cannot put it in print, for it entails unplugging a related part of the system that would be considered unsafe. If it were to be unplugged, proper shifting would resume, confirming that a curve-recognition program is responsible for very late or no upshifts.

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