Jeep Once Built A Four-Speed Auto That Actually Had Five Forward Gears

Have you ever asked yourself the question, “Are there any five-speed automatics that are actually six-speeds but they used to be four-speeds?” If so, read on for the answer. About a quarter-century ago, Jeep introduced its very first five-speed automatic transmission, found in the 2000 Grand Cherokee and mated to the likewise-innovative overhead-cam PowerTech 4.7-liter V8. But believe it or not, that five-speed gearbox was mechanically and physically identical to the four-speed found on the Grand Cherokee, differing only by shift strategies and transmission programming. How is that possible, you may ask? Through the magic of planetary ratios (and some just plain weird engineering).

Two Second Gears, Depending On Which Direction You’re Going

As reported by The Autopian‘s resident Jeep lunatic David Tracy, the original transmission fitted to the new-for-1999 Jeep Grand Cherokee V8 was a four-speed, but it actually had two different second-gear ratios. When the gearbox was shifting up, it would use a conventional second gear, but when it was kicking down, it’d switch into “2 Prime.” This ratio was shorter than third, but taller than second, meaning the prime gear would take longer to upshift back into third, providing swifter freeway passes and merging maneuvers. Count all the available ratios up and you’ll end up with five gears – first, second, 2 Prime, third, and fourth – but since the transmission would only use four of them when stepping up on acceleration, Jeep marketed it as a four-speed.

However, a four-speed automatic transmission is limited to only two planetary gearsets, so the mere existence of 2 Prime necessitated its own sun-and-planet gears. Theoretically, this four-speed transmission had six different gears to play with, depending on which clutches were acting on the planet gears. And for some reason, Jeep chose not to activate one of those ratios when it debuted the WJ-generation Jeep Grand Cherokee for 1999.

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In 2000, however, the automaker reconsidered. It “replaced” its dated-sounding four-speed auto with a much more advanced five-speed – one of the first in an SUV from any automaker – and keen eyes would notice that its first four gear ratios would match exactly with those of the four-speed. Only the fifth gear was different, an overachieving overdrive with a skyscraper-tall ratio of 0.67:1 compared to fourth gear’s 0.75:1.

As you might suspect, this new gearbox was physically and mechanically identical to the old four-speed. Jeep reprogrammed the transmission controller to allow the internal clutches to make use of all available gear ratios, meaning that 2 Prime’s extra set of planetary gears wouldn’t go to waste. And speaking of, second gear’s unusual split personality persisted into the new transmission, meaning there were now six total forward gears to play with, although again, only five at a time.

Tracy also notes that it’s possible to reprogram a 1999-model’s four-speed auto to behave like the five-speed using cheap junkyard parts and an internet’s worth of shadetree-mechanic knowledge. Whether this information affects you, reader, is as rhetorical a question as the one posed at the top.

Source: The Autopian