JR405E RWD 4-Speed: The Subaru FWD twin - Transmission Digest

JR405E RWD 4-Speed: The Subaru FWD twin

With the unrelenting proliferation of transmission models and today’s focus on extreme gears, it’s refreshing to see a design that crosses over from RWD to FWD applications. Having one basic architecture certainly makes diagnosis and rebuilding procedures much easier. Such is the case with the JR405E. The transmission is made by Jatco, and although the company is currently the king of CVTs, it also makes a variety of step-type units. Jatco makes transmissions for the global market as well, meaning that shops in the U.S. would never get a shot at repairing.

JR405E RWD 4-Speed: The Subaru FWD twin

Technically Speaking

Author: Mike Riley, Technical Editor
Subject Matter: Automatic Transmission
Unit: JR405E
Vehicle Applications: Isuzu, Mazda, Suzuki

Technically Speaking

  • Author: Mike Riley, Technical Editor
  • Subject Matter: Automatic Transmission
  • Unit: JR405E
  • Vehicle Applications: Isuzu, Mazda, Suzuki

With the unrelenting proliferation of transmission models and today’s focus on extreme gears, it’s refreshing to see a design that crosses over from RWD to FWD applications. Having one basic architecture certainly makes diagnosis and rebuilding procedures much easier.

Such is the case with the JR405E. The transmission is made by Jatco, and although the company is currently the king of CVTs, it also makes a variety of step-type units. Jatco makes transmissions for the global market as well, meaning that shops in the U.S. would never get a shot at repairing.

Although Jatco’s main customer would be Nissan, due to their financial arrangement, many other OEMs have used its products and continue to do so. Subaru, for one, is an OEM that relies heavily on Jatco for transmissions. Note the relationship between the RWD RE4R01A series and the first-design Subaru four-speed FWD, R4AX EL.

The JR405E fits a variety of vehicles inside, but mainly outside, the USA. Vehicles using the JR405E that could surface in the U.S. would be the Isuzu D Max and Suzuki Vitara as well as the Mazda RX8, with the transmission designation of RC4A EL. Even certain applications of the Chevy Colorado can have the JR405E.

The “kissin’ cousin” to the JR405E is the Subaru second-design four-speed AWD transmission, 4EAT, which was launched in late 1998. A key difference between the first- and second-design Subaru models is the omission of the band. One other change is the omission of the overrun clutch. These changes result in a clutch-to-clutch shifting (synchronous) scenario on the JR405E and 4EAT transmissions. Externally, the easiest way to distinguish between the first- and second-design Subarus is that the second-design models have a spin-on filter. The JR405E is a normal-looking RWD unit, with a bellhousing, case and extension housing (Figure 1).

Unlike GM, which has always been big on variable-displacement vane-type pumps, Jatco has been a little less inclined to use vane pumps, although it certainly has had its share. The first-design Subaru used a vane-type pump, but the pump was changed to a gear type on the second design, 4EAT. The JR405E also uses a gear-type pump for simplicity purposes (Figure 2).

The JR405E, as did the 4EAT, took a different approach for the reverse- and high-clutch assemblies from the R4AX EL. The early Subarus used two-clutch drum assemblies, one for reverse and a separate drum for high. In addition, the reverse-drum outside diameter (O.D.) is where the overdrive band rides. The JR405E and 4EAT have a different setup altogether. The main drum assembly splines to the reverse steels, and the O.D. of the drum is the spline configuration, with no band surface (Figure 3).

The high-clutch drum is actually the reverse-clutch piston. The high-clutch piston and bonded retainer ride within the reverse piston (Figure 4).

This makes for a more compact assembly. The connecting hub for the high clutch functions the same for all models. It’s splined to the high clutches and to the front planetary (Figure 5).

The front-planet sun gear for the JR405E and 4EAT is somewhat different from that of the R4AX EL. The R4AX EL sun gear meshes directly to the reverse drum. The JR405E and 4EAT sun gear splines not only to the reverse frictions at the small diameter hub but also to the 2 4 brake frictions at the large hub. The design is different; however, the outcome is the same (Figure 6).

Another distinction between the JR405E and R4AX EL designs has to do with the rear-planet assembly. The rear sun gear and planet are basically the same. It’s the rear ring gear that is different. The R4AX EL rear ring gear has the forward-sprag inner race attached, which is ultimately driven by the forward clutch. The JR405E and 4EAT rear ring gear has no race included. The outside diameter of the ring gear has the low clutch splines cut into it (Figure 7).

One main difference between the early Subaru and the JR405E is how the transmission applies going forward. The R4AX EL uses a forward clutch as well as a forward sprag. There is also an overrun clutch for engine braking. The setup provides a non-synchronous shift. The JR405E and 4EAT have a more-basic approach. There is only one clutch pack, which is called the low clutch but functions the same as the forward (Figure 8). The rear hub of the drum is for the low/reverse brake on all models. Early and late units all have a low sprag.

Because the forward- and low-clutch assemblies are different, the drum support at the back of the case was changed. The R4AX EL support has four sealing rings to accommodate the forward/overrun clutch, whereas the JR405E and 4EAT support has only two sealing rings (Figure 9). It sure cuts down on cost.

As a sign of progress, transmission manufacturers have been purging bands from transmission designs for years, to the point of non-existence. Clutch-to-clutch shifting is now the norm due to efficiency of apply. When the JR405E and 4EAT were designed, the band was not part of the layout. Instead, Jatco chose to use a 2 4 brake, which is anchored to the case. The 2 4 piston and housing are a regular style (Figure 10).

The output shaft on the Subaru R4AX EL and 4EAT is basically the same, whether 2WD or AWD. The JR405E output shaft is longer, of course, but the front end of it conforms to Subaru outputs (Figure 11).

Although the JR405E is a four-speed transmission, there is no shortage of electrical stuff in it. The valve body is rather basic; however, it does contain six solenoids and three pressure switches (Figure 12). As shown in the illustration, five of them are neatly in a row, with only one freak with wires. There is also an input-speed sensor and output-speed sensor, which are external.

In the age of electronics and increased precision of components, filtration has been taken to new heights. Several transmissions actually have redundant filter systems (more than one filter). What does the JR405E filter look like? It’s a basic screen-type filter (rock collector), which is a big departure from late-model filters (Figure 13). It must get the job done; so be it.

At least the JR405E is simple to work on, more so than the Subaru FWD counterpart.

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