Park-by-wire systems consist of input from the driver, such as a knob or button (more on this later), an internal mechanical piston attached to the park rod assembly, and a solenoid to hold the piston in the neutral or “car wash” mode. In this edition of Fun With Transmissions, we’ll check out a few examples of how car manufacturers assemble the Park by-wire systems.
NOT REALLY A NEW IDEA
The transmission industry is very familiar with lock-up clutches, which are standard on all transmissions these days. The lock-up clutch first came out in the early ’50s in Studebakers and Packard’s.
The same can be said with remotely shifted automatic transmissions. In the late 1965 into the 1966 model year and well into the 80s, there were several Rolls Royce models and the Bentley T-Series that had an electric motor bolted to the tail housing of the THM-400 that moves the manual lever through the gear changes (Figure 1). This rather crude setup used contacts in the column shifter and micro switches in the motor housing to move the shifter to the selected range.
NOWADAYS
Nowadays, the Park-by-wire systems use hydraulic pressure to push a park pawl valve attached to the park rod. We are going to look at the Ford 8F57 park lock valve first. The hydraulic diagram in Figure 2 shows the park lock valve at rest or in the park position. This is the failsafe position. That is to say, when the car is turned off, or power is not going to the transmission, the large spring on the park linkage forces the unit into Park. This is true for all Park-by-wire systems. In the hydraulic diagram in Figure 3, we see that the solenoids are on, the valve has moved to the right, and the transmission is ready to be shifted to any Drive or Reverse range.
The GM hydraulics look different (Figure 4), and just like Ford, while at rest, it’s in Park, and when a gear is commanded, a solenoid turns on and directs the pressure behind the park lock valve, and it goes out of Park.
The Chrysler systems on the 845 and 850 RE are exactly the same. These units, like those previously discussed, use hydraulic pressure to shift the park pawl valve out of Park.
IT’S ALL COMPUTER CONTROLLED
All of the manufacturers use a network of controllers and sensors to achieve the desired Park, Drive, or Reverse range.
To keep things short, we will look at the Ford 10R60 in a 2020 Explorer with a 2.3L motor. This system is like many others. It has park rod, a solenoid to hold it out of Park, the park pawl piston, and the transmission range sensor (Figure 5). It also uses five different modules!
They are:
- The Gear Shift Module (GSM)
- Restraint Control Module (RCM)
- Powertrain Control Module (PCM)
- Instrument Panel Control Module (IPC)
- Body Control Module (BCM).
GSM
The GSM’s rotary dial has Hall-effect sensors that are used as inputs to indicate the dial has been rotated. When Park is selected above 5 mph (8 km/h), Neutral and a message in the IPC are provided. The Brake Transmission Shift Interlock (BTSI) CAN signal is used to shift from Park (for normal function).
RCM
The shift-by-wire system uses the driver’s safety belt buckle as one of several inputs to determine the driver’s presence. If the buckle is deemed faulty for any reason, the system reads this as unbuckled so that any time the door transitions to open below a set speed, the PCM commands it to return to Park.
PCM
The PCM actuates the transmission in place of a conventional selector lever and cable. It receives commands from the GSM over the high-speed CAN.
IPC
The IPC receives signals from the PCM and displays the current range of the transmission on the PRNDM. Under some circumstances, messages from the shift-by-wire system are displayed on the multifunction display for the customer. For example, “Press Brake to shift from Park” is displayed when an attempt to shift out of Park when the brake pedal is not pressed.
BCM
The BCM provides Brake Transmission Shift Interlock (BTSI) information, GSM LED intensity status, electronic door lock commands, and ignition status to the GSM and PCM. The BCM is also responsible for customizing BTSI and other signals based on local regulations and vehicle features. Whew! All of that is going on to hold the transmission out of Park and select the proper drive range.
CAR WASH MODE
All of these systems have a “car wash mode.” This involves a solenoid that, when turned on, mechanically locks the park pawl valve in the neutral position. Say you want to get a car wash and turn the motor off. With the key on, the solenoid energizes, and the transmission stays in Neutral. Here is an example of Ford’s solenoid and valve (Figure 6).
Note that on the 845RE, the solenoid has arms that grab and hold the park pawl valve when energized (Figure 7). The 850RE park lock solenoid has a snout that fits into the park pawl valve. When the solenoid is energized, an internal rod pushes the balls in the solenoid out to grab and hold the inner diameter of the valve (Figure 8).
Now, with all that going on, a person might ask: What happens if the battery fails and there is no power to all the systems?
The Ford Explorer has a Park-bywire manual release cable (Figure 9). In the center console, a lever under a cover can be pulled and latched into place to set the vehicle in neutral (Figure 10). Caution! The vehicle is now in neutral and will roll freely. The funny part is you have to use an external power source to apply the parking brake.
CHRYSLER, JEEP, AND RAM TRUCK
The 845 and the 850RE units used in these vehicles have the same components as the Ford, and the manual release cable is bolted to the side of the unit (Figure 11). This is a Jeep Grand Cherokee, and like the Explorer, It has a release lever in the center console (Figure 12) that you can unlock and pull up to put the transmission into neutral so the vehicle can be moved when in fail-safe.
GM
GM took a different approach to the manual release procedure. They decided that building the release lever/ cable into the vehicle and making it permanent was too expensive. A special tool is needed to get a GM into neutral (Figure 13). Yes, it’s a cable that latches just like the rest, except in this one, the tech must bolt it to the transmission and remove it when finished.
WARNINGS AND CAUTIONS
All manufacturers have pretty much the same handful of warnings and cautions when it comes to putting the transmission in neutral. A warning example from GM reads: “The transmission will be placed in N (Neutral),” when the manual park release is pulled. The vehicle can roll on you, or others could be injured. Ensure the vehicle is on level ground, the parking brake is applied, and the wheels are blocked before the manual park release is installed.
Also, a caution from GM reads: “If the manual park release is pulled while the vehicle is on or if the ignition is turned on while the manual park release is pulled, the DIC will display a message to service the transmission.”
Keep in mind that this is just an example. Different manufacturers will have similar warnings for sure, just not the same words.
Park-by-wire systems are here to stay. Lucky for us, the manufacturers use the same general components. The signal device (a twisting knob or buttons on the dash) and actuators control a park rod and pawl system to shift the transmission in and out of Park. These systems allow the manufacturer to configure the vehicle interior without regard to where to put the shifter for the transmission. The shifter no longer needs to be in the center console or a lever on the steering column.
Keep an eye out for these Park-by-wire systems showing up in the shop. Don’t stress when it comes to moving the vehicle around the shop. Remember that when you manually release the park mechanism, the vehicle will roll freely. Don’t run anyone over, and keep having Fun With Transmissions!
