Heavy Metal! - May - 2019

The Flat 500 L DDCT C635

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The Fiat 500 and 500 L are beginning to use a transmission that’s taking repairs to a different level. When this unit starts to break or act up, information and some knowledge of the product is key to performing repairs.

The aftermarket plays a big role in our industry and ATRA is committed to its role in helping you up keeping with the information for these vehicles. That’s important, because some of the information available is either discombobulated or incomplete. Our research has given us a better understanding of these units.

Start by recognizing that this valve body works identically to the 5W110W. How is that? Well, we have no moving valves in the valve body. We do have moving electro valves… or solenoids, to be more precise.

For this unit to operate at 100%, it’s imperative that these solenoids be in perfect working order. If not, you’ll end up with all kinds of weird shift patterns and bindups.

This units also appear in the Alfa Romeo 4C, MiTo, and Giulietta; the Fiat 500X, Tipo (2015), and Viaggio; and the Dodge Dart.

Let’s get acquainted with these units, so when they roll into your shop it doesn’t scare anyone.

The C635 DDCT is a six-speed, dual-dry clutch automatic transmission developed jointly by Fiat Powertrain Technologies (FTP), Magneti Marelli, and BorgWarner, and is manufactured by FPT at the Verrone, Italy plant. They’re produced by Fiat Powertrain (FPT).

The letters and numbers in the names describe the unit:

C = Cambio (Gearbox)
6 = Six forward gears
35 = 350 Nm of torque

There are two different fluids used in this transmission: The gearbox uses Mopar C Series manual and dry clutch transmission fluid. The hydraulic fluid used in the slave cylinders is Mopar C-Series DDCT hydraulic fluid.

Both components are filled for life, but if you make any repairs on the transmission, these specific fluids are imperative. Like most transmissions, the fluid level is correct when the fluid is at the bottom of the fill port.

Internally, the C635 DDCT and the manual shift, clutch-pedal-equipped C635 — its conventional mechanical counterpart — are the same. It’s a three-shaft design that reduces overall unit length, enabling easier vehicle packaging. The extra shaft is an output shaft.

Gears 2, 4, 5, and reverse are on one output shaft, gears 1, 3, and 6 are on the second shaft. Each output shaft has a different final-drive ratio, which is also different from most transaxles. All ratios are synchronized.

Here are the components that are different between the C635 DDCT and the C635: double dry clutches, double slave cylinders, dual mass flywheel, and a hydraulic power unit. These unique components allow the transaxle to shift gears mechanically without any input from the driver.

There is one clutch for the odd number gears (1, 3, and 5) and a second clutch for the even number gears (2, 4, and 6). The clutches operate in the same manner as the manual clutch in the C635, but the actuator arm is activated by the hydraulic power unit instead of the driver’s left foot. The C635 DDCT doesn’t have a clutch pedal.

Before you start working on this unit, you must depressurize the system using a scan-tool to tell the computer to drain the tank into the reservoir. This sounds weird but is how it was programmed. On the other hand, you could loosen a line and allow the fluid to depressurize and leak into a container. Once this is done, you can proceed to remove the accumulator and then the rest of the unit.

This unit uses C series DDTC SAE 75W hydraulic fluid.

The complete assembly of the dual clutch includes these components (figure 1):

  1. Flywheel
  2. Throw-out bearing
  3. Pressure plate
  4. Dual mass
  5. Pressure plate
  6. Both discs

The pressure accumulator (figure 2) always holds about 30 to 40 lbs. of hydraulic pressure in reserve. The electric pump to the left of the accumulator makes sure that this accumulator stays pressurized.

In many cases, the vehicle may show up shuddering or won’t engage, and the first thing that comes to mind is a gear destruction of some sort. That’s probably not the case. More likely the clutch disc cracked from heat (figure 3) or the K1 clutch bearing self-destructed (figure 4). When a bearing comes apart like this, you’ll lose the K1 clutch, preventing the vehicle from moving. The seal breaks apart and ball bearings fall out.

Moving on to the valve body, notice the dog where the shift rails go in (figure 5); on the other side you’ll see the solenoids (figure 6).

If the vehicle won’t move, check these two molded pressure hoses. They burst all the time and aren’t available from the dealer; at least, not yet. But you can have these hoses built at your local hydraulic hose supply company without an issue. In the event of not having a supplier in town, you’ll need to purchase a complete valve body / solenoid assembly from your dealership for a whopping $1400.

All solenoids are normally vented and measure 2.5 ohms. But the part numbers are different, so they probably have a different flow rate.

Till next time.