Fun With Transmissions - May - 2017

845RE: 8 Speeds From Just 4 Planets!

The 845RE started life as a ZF8HP45, designed by ZF in Germany. Chrysler bought the license for the design and began building the 845RE at its Kokomo, Indiana plant.

First appearing in 2013, the 845RE is standard equipment in the Grand Cherokee, Charger/ Challenger, Ram1500, and Durango. All of these models use the 3.6L Pentastar engine (figure 1).

The 845RE uses just five clutch packs, two brakes (splined to the case via the pump body), and three driving clutches to get eight forward speeds.

Why eight speeds? The obvious answer is fuel mileage. With a wide range of ratios, the engineers can keep the engine running at peak performance during more of its operating cycle, letting them drag the best fuel economy possible out of their cars and trucks.

This transmission uses four planetary gearsets: a Simpson gear set with two planets, two ring gears, and a common sun gear; and two simple planetary gearsets (figure 2). Starting from the front, these gearsets are identified as P1, P2, P3, and P4.

In this edition of Fun with Transmissions, we’re going to examine this amazing geartrain and discover how these clutches and planets interact to achieve eight different ratios (figure 3).

To make it easier to understand the powerflow through this transmission, we’re going to disassemble the geartrain to show the individual components that are working in each gear range. This is really the only way to do it because, when the geartrain is assembled, all the components are nested into one another and held together with snap rings (figure 4).

Starting from the front we have the two brakes: the A clutch, and the B clutch. Brakes can be clutches or bands that, when applied, will anchor the rotating component to the transmission case. The A clutch is housed in the front pump housing, which also houses the B clutch piston assembly.

The B clutch stacks into the main case; the apply piston is part of the pump housing. The B clutch has no return springs; the TCM applies and releases hydraulic pressure to control the rate of apply and release (figures 5 and 6).

The A clutch holds the P1/P2 sun gear and the B clutch holds the P1 sun gear. With both of these clutches applied, the P4 ring gear is stationary, because it’s splined to the P1 planet by the long aluminum sleeve (figure 7).

Next is the P2 planet, which is permanently splined to the input shaft. One of the interesting things about the P2 ring gear is that it can drive the P3 sun gear, or, when the E clutch applies, the P4 sun gear (figure 8).

Another interesting thing about the P2 planet is that, when the A clutch applies, it holds the P1/P2 sun gear, which puts the ring gear output into overdrive. That’s always true of these gearsets: Whenever the planet carrier is the input and the sun gear is held, the output will always be in overdrive.

The P3 planet sits behind the P3 sun gear: This planet drives the D clutch hub (figure 9) and the P3 ring gear drives the P4 sun gear (figure 10). This hub drives the P4 planet. The cool thing about the P3 planetary gearset is that it reduces the overdrive ratio of the P2 planet when the transmission’s in 7th gear.

The other part of the very cool 7th gear is the C clutch is applied. This drives the P3 ring gear at input shaft speed. The combination of driving the P3 sun gear faster than the P3 ring gear — which is driven by the input shaft — creates a gear reduction to the P4 planet that offsets the overdrive of the P2 planet.

The E and C clutches are further back in the geartrain. The E clutch is splined to (and driven by) the P2 sun gear, and the C clutch is splined to (and driven by) the input shaft. These clutches drive the P4 sun gear at different speeds, depending on which clutch is applied (figure 11). This means that the P3 ring gear and the P4 sun gear can be driven in three different ways, to create three different gear ratios.

Finally we get to the P4 planet. This very stout, very busy planet has the D clutch built into it. It’s either in reduction or being overdriven in every gear range except 6th. In 6th gear, with the E and C clutches applied, the P3 sun gear and the P3 are driven at the same speed. This locks the P3 planet together.

Since the D clutch is applied, the back half of the unit spins at 1:1. Just like the P3 planet, the P4 planet is driven from three different inputs: the P4 sun gear, the P4 ring gear, and the D clutch hub, which is splined to the P3 planet.

Simple, right? Okay, maybe simple isn’t the right word, but it’s certainly intriguing. And it reveals how the manufacturer was able to get eight forward gear ranges out of a transmission with these four gearsets. In the next edition of Fun With Transmissions, we’ll look at the procedures to disassemble, inspect, and reassemble these geartrains safely. You’ll need some special tools, because the machined, cast aluminum drive shells are very fragile.

But, by following the procedures and using the proper special tools, you’ll find you can easily disassemble and reassemble these intriguing geartrains, and get them back on the road in no time. Remember when it’s easy it’s all about having Fun With Transmissions!