Fun With Transmissions - May - 2020

Ford’s New 8 Speed

In this edition of Fun with Transmissions we’re going to check out the new 8F35 from Ford Motor Company (figure 1). We’ll take a look at some interesting features like the hydraulically actuated sprag and many other points that make this a unique unit.

As with several other transmissions, Ford and GM have collaborated on the development of a series of 8 and 9 speed front wheel drive transmissions. In 2019, Ford introduced the front wheel drive 8 speed which is very similar to the 9T GM front wheel drive series. All the 8F series units use Ford Mercon® U LV. T he GM u nits require the use of Dexron® 6 fluid.

The eight forward speeds are achieved by using just two driving clutches and four brake clutches (figure 2). The brake clutches are splined to the case and hold a component from rotating.

One of the many things about this transmission that makes it interesting is the “selective one-way clutch” or SOWC. The advantages of a SOWC include increased fuel economy, lower emissions and lower production costs, all while offering a more compact design. This ultimately allows the transmission to fit into tighter spaces.

The SOWC is a hydraulically-controlled diode clutch that can operate in two different modes: First, it operates like any other mechanical diode; it freewheels in one direction and locks in the other. It can also switch so that it’s locked in both directions.

Hydraulic actuation is achieved using a servo piston mounted to a lever of the Controlled Mechanical Diode (CMD) and is controlled through the valve body with a solenoid and clutch-select valve. The servo piston then positions the CMD actuator lever into the desired position. In 1st gear the diode functions as a standard one-way clutch. In reverse, the diode is locked; preventing it from rotating in either direction (figure 3).

I have worked on a couple of these and there is a special tool that will work on both the Ford 8F series and the GM 9T series for the return spring on the A clutch.

Removing the snap ring is easy. Just pry out one end pull the snap ring out of the groove until the return spring jumps out of the piston. The issue is getting the snap ring back INTO the groove. The return spring needs to be compressed so we can get the snap ring into the groove.

The only method that I was able use to get the snap ring in is with this special tool.

We install the spring and then the snap ring into the housing. Use the press to compress the spring and the snap ring at the same time (figure 4). When the snap ring gets to the groove it pops right in! It takes so many headaches out of the operation.

This is a Kent Moore tool #DT-47797 (figure 5). Ford has the same type of tool which is #307-770. Check out the video in the 9T40 virtual training section of the ATRA website.

SOLENOID BODY

This transmission uses 2 types of solenoids: Casting-Integrated Direct Acting solenoids (CIDAS) and variable force solenoids.

The solenoid body contains 8 solenoids: 6 shift solenoids, SSA, SSB, SSC, SSD, SSE and SSF, 1 TCC solenoid and 1 LPC solenoid (figure 6). The TCC solenoid is identified by the green electrical connector. The LPC solenoid is identified by the blue electrical connector. The TFT sensor is located in the main control.

If the PCM is replaced and the PCM data cannot be extracted from the old PCM or downloaded to a new PCM, the solenoid body identification and solenoid body strategy must be downloaded into the PCM (figure 7).

SHIFT SOLENOIDS

This transmission utilizes six shift solenoid (A-F) that are normally low (CIDAS). Unlike previous shift solenoids they are mechanical in nature in that no transmission fluid passes through them.

CIDASs use an armature/pin assembly that moves a control valve in the main control valve body to control and apply hydraulic fluid pressure (figure 8). Each clutch (A-F) has a corresponding shift solenoid (A-F) that is directly proportional in that zero current equals zero pressure and maximum and current equals maximum pressure. Since there is no pressure with zero current if the power is interrupted to the shift solenoids none of the clutch packs are able to engage. This also means the vehicle won’t move if the case connector is removed.

TORQUE CONVERTER CLUTCH (TCC) LINE PRESSURE CONTROL (LPC)

The TCC solenoid is a normally low variable force solenoid. The TCC solenoid uses proportional operation. As the current from the TCM decreases, the pressure from the solenoid decreases. As the current from the TCM increases, the pressure from the solenoid increases.

The LPC solenoid is a normally high variable force solenoid. When current is high, no fluid pressure passes through the solenoid into the control circuits. When current applied to the solenoid is low, full pressure is passed through to the LPC control circuit and the main regulator valve.

I believe this is the first Ford transmission that has varying voltage type pressure sensors (figure 9).

The Transmission Fluid Pressure (TFP) sensors for clutch A (TFP A) and B (TFP B) have been added to improve diagnostic capability for non-electrical faults (stuck solenoid or control valve). The TFP sensors provide a voltage proportional to the absolute hydraulic pressure in the transmission clutch control circuit using a piezo-resistive silicon sensing element. Pressure readings are used to monitor the status of specified clutch systems during operation. The typical operating output range is 0.5V to 4.5V.

The pressure sensor is typically mounted on the transmission main control assembly with a dedicated feed port. The TFP A sensor measures A clutch pressure and the TFP B sensor measures B clutch pressure.

FLUID PUMP

The Ford 8F series and the GM 9T series unit do share some of the same components that much is true. However, the fluid pump assembly is not one of them. The 8F series uses an aluminum housing for the fluid pump while the 9T series is all cast steel. The 8F pump assembly does not have any valves in the aluminum housing.

The chain-driven vane-style pump is used to save space and to improve fuel efficiency (figure 10). The 8F series units will be showing up at shops’ doors sooner than later so keep an eye here for more information. Also keep checking Virtual Training Solutions section of the ATRA for ongoing training videos for the 8F35 transmission. Remember online training is an easy effective way to keep having Fun With Transmissions!