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Automatic Transmission

Just When I Thought I’ve Seen Everything

OK, let me see if I’ve got this right. The Mercedes 7G-DCT 7-speed dual clutch transmission does not have a reverse idler gear. When shifted to reverse it uses 2nd gear, 3rd gear and 1st along with all four shafts. That’s correct, now let me explain how and why.

Just When I Thought I’ve Seen Everything

Souza Sezz

Author: Mike Souza
Subject Matter: Mercedes 7G-DCT
Issue: Four shafts

Souza Sezz

  • Author: Mike Souza
  • Subject Matter: Mercedes 7G-DCT
  • Issue: Four shafts

The Mercedes 7G-DCT can be somewhat mind boggling

OK, let me see if I’ve got this right. The Mercedes 7G-DCT 7-speed dual clutch transmission (Figure 1) does not have a reverse idler gear. When shifted to reverse it uses 2nd gear, 3rd gear and 1st along with all four shafts. That’s correct, now let me explain how and why.

This compact dual-clutch transmission was designed in 2012 to replace the 722.8 CVT transmission found in the later B Class 246 models. The 722.8 CVT is not found in the U.S. market. Mercedes vehicles equipped with the 7G-DCT are sold here in the U.S. To conserve on space and attempt to reduce noise, the 7G-DCT was designed without a reverse idler gear making it more compact. There are four shafts within the 7G-DCT: a solid internal shaft that is driven by the K1 external wet clutch; a hollow shaft driven by the K2 clutch; output shaft 1, which contains 4th, 2nd, 1st and 5th gear; and output shaft 2 which contains 6th, reverse, 3rd and 7th gear (figures 2-4).

The K1 and K2 wet clutches are external, located at the front of the transmission contained in one assembly like a torque converter (Figure 5). There is a cover in front of the K1/K2 clutch assembly like a Chrysler 45RFE (seen in Figure 1). The K1 (outer) clutch drives all the odd gears, and the K2 (inner) clutch drives the even gears. There is no single pinion gear to drive the differential, which is referred to as a pinion differential by Mercedes. Both output shafts have a fixed gear, which acts as a pinion gear that is constantly meshed with the differential ring gear (Figure 6).

The most unique feature is how reverse gear is designed. Reverse gear on output shaft 2 is constantly meshed with 2nd gear on output shaft 1 (Figure 7). No other gears on either output shaft have contact with each other.

The hollow shaft has two fixed gears, one is constantly meshed with 2nd gear but does not contact reverse gear (Figure 8). This is the key point to how reverse works. The other hollow shaft gear is meshed with 4th and 6th gear. The internal solid shaft has three fixed gears, one is meshed with 1st gear, another with 3rd gear and a 3rd fixed gear meshed with both 5th and 7th gear. All the shift forks are hydraulically controlled with solenoids by the Integrated Transmission Control Unit identified by Mercedes as Y3/14n4 which is part of the Electrohydraulic System (EHS) identified as Y3/14. The numeral identification used by Mercedes can be confusing when first dealing with the electronics and hydraulics controls of this transmission.

The EHS contains all the electronics including the electric auxiliary pump and hydraulic valve body as one component located under the side cover (shown in figure 1) on the transmission (Figure 9). Remember, everything within this transmission is designed to conserve space.

The Primary Hydraulic Pump is driven by a gear located on the back of the K1/K2 Dual Clutch assembly (Figure 10). The dual wet clutch assembly much like a torque converter is bolted to the flywheel. The primary pump is a very small simple vane type pump like a power steering pump. Nothing that we are not accustom to with today’s transmissions.

Now that you have an idea of how the transmission is set up, let’s get back to reverse. When reverse is selected the 1st/5th shift fork hydraulically engages the 1st/5th synchronizer sleeve onto 1st gear locking it to output shaft 1. The dual 6th/reverse shift fork engages the reverse synchronizer sleeve onto 3rd gear and attaches the reverse gear to 3rd gear as one (Figure 11).

Remember there is no idler gear used. At this point 3rd gear is not locked to output shaft 2; it’s freewheeling. The only time 3rd gear is locked to output shaft 2 is when the 3rd/7th synchronizer sleeve (17) is shifted onto 3rd gear. This dual shift fork also moves the 6th gear synchronizer away from 6th gear but does not contact the reverse gear. K2 inner dual clutch is applied driving the hollow shaft. The hollow shaft fixed gear 2nd/reverse is driving 2nd gear on output shaft 1, which is constantly meshed with reverse gear on output shaft 2. Keep in mind the hollow shaft fixed gear 2nd/reverse does not contact the reverse gear on output shaft 2. Second gear is driving the reverse gear, which is connected to 3rd gear through the dual 6th/reverse shift fork. Third gear is driving internal solid shaft fixed 3rd gear. Output shaft 2 is not driving just 3rd gear on the output shaft 2 is driving. The fixed 1st gear on the internal solid shaft is driving the engaged 1st gear on output shaft 1. The fixed gear on output shaft 1 is driving the differential ring on the pinion differential (Figure 12).

That was easy, right? Believe me, it took a lot of going over for me to get it straight in my head. Think about it, all four shafts are being used. First gear must be engaged for the output shaft 1 to drive the pinion differential. There is no reverse idler gear; it is substituted with the use of 3rd gear turning freely on output shaft 2 not driving the output shaft. The internal solid shaft is not engaged by the outer K1 clutch. It can turn freely to drive 1st gear driving output shaft 1 without a bind up being in two gears at once. The gears on output shaft 1 are used as an idle gear for reverse. No matter how you look at it, this design is very unique and somewhat mind boggling. If you think this is unique, wait till you see how the parking pawl is designed and how it works during failsafe situations.

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