What Was Wrong with a Cable?
As if a simple cable linking the gas pedal to the throttle plate wasn’t good enough, engineers have now made that design complicated too. Why do engineers do these things to us? That expensive throttle body can’t cost less than a cheap little throttle cable; or can it? Are there any advantages to the manufacturer or the consumer with such a design? Let’s examine the electronic throttle control (ETC) system and see.
In the Ford ETC system, the PCM directly controls the position of the throttle plate by use of a DC motor. A redesigned throttle body, coupled with a redesigned PCM, a redesigned throttle position sensor (the system has two position sensors inside of it), and the addition of an accelerator pedal position sensor (APPS) make up the ETC system directly. The ETC system replaces the throttle cable and brackets, IAC valve and its wiring, cruise control servo, plus some wiring for the cruise system. The only part of the ETC system that is truly new (excluding redesigning) to the gasoline vehicle is the APPS, and it has been in service on the DI diesel for years. So with that considered, what appears to be more complicated on the surface, is actually less elaborate when viewing the entire scope. It might even cost less to build.
The principle reason for using the ETC system is for improved driveability through its torque-based strategies, which take better advantage of VCT and transmission shift strategies. There are however, other benefits. It allows the PCM to limit vehicle speed and engine rev through throttle control, rather than inducing misfires. Also, adding factory cruise control to a vehicle that wasn’t originally equipped with cruise becomes a snap; Simply replace the steering wheel (in some cases replace the clock spring too) and “tell” the PCM it has cruise with a capable scan tool. Nice, huh?
The ETC system helps pave the way for future design changes to the vehicle’s passenger compartment as well; such as the consumer being able to slide the vehicle controls to the opposite side of the dash. ETC also makes it possible for the traditional throttle pedal to be eliminated all together by replacing it with hand controls on the steering wheel. That feature holds obvious potential for dealer or factory-equipped handicap driver applications.
Now that we have talked about the names of the parts, let’s see how they operate.
Ford ETC Theory and Operation
Based on the APPS input, the PCM changes the angle of the throttle plate by directly actuating the DC motor mounted to the throttle body assembly. The TPS, mounted on the throttle body, provides feedback to the PCM of the throttle plate position. The purpose of the TPS has been modified in the ETC system from that of the cable-type system. Rather than being the source of driver throttle demand information, the TPS is now simply for the PCM to be able to monitor its own actions with the throttle plate. The APPS is now what represents the driver’s throttle demand – which is something to keep in mind when reviewing fault code freeze-frame data.
The APPS mounts to the top of the throttle pedal. Inside of it are three position sensors and two return springs. The position sensors send an analog DC signal to the PCM, as you can see in Figure 1.
The PCM uses APPS1 as the primary indicator of the driver’s throttle demand. APPS1 uses a negative slope. In the at-rest position, APPS1’s voltage is high, usually starting around 4.0 volts. As the pedal is depressed, the voltage lowers. With the pedal fully depressed, the voltage value will typically read just under 1.5 volts. APPS2 is used to cross-check APPS1. The positive slope of APPS2 provides a more traditional position sensor behavior when viewed on a scanner.
The voltage readings of APPS2 start low in the rested position and rise as the pedal is depressed. In the event that APPS1 were to fail, the PCM then looks at APPS2 as the source of driver demand information. If APPS1 and 2 don’t agree, APPS3 is then used to cross-check APPS1 and APPS2. The two return springs inside the unit help ensure the pedal will fully return in the event one should break or weaken.
Trouble codes P2122, P2123, P2127, P2128, P2132 and P2133 are APPS sensor circuit continuity tests. Those codes all point to an open or short in the APPS circuits. Codes P2121, P2126 and P2131 are all related to the APPS performance or range. These codes are set when the PCM “sees” a signal from one of the three sensors that is present and active, but is not in the specified range. Codes P2138, P2140 and P2139 are APPS correlation codes. These are set when the PCM “sees” a discrepancy between two or more APPS readings. All of these codes will set an ETC lamp (wrench light or failsafe message), but not an MIL light. These circuits are monitored continuously. The PCM will indicate any one of these faults within one second from the start of the condition.
The ETC Throttle Body Assembly
The electronic throttle body (ETB) is very similar to the cable-operated throttle body, with a few notable changes. Among the most noticeable is the addition of the throttle actuator control motor (TAC). The motor is used to open and close the throttle plate based on direct command from the PCM.
On Ford products, the motor is currently located either on the side of the throttle body or underneath, depending on application. There are only two wires to the motor. As you can see in Photo 1, rather than simply bolting the motor to the side of a traditional throttle body, the throttle body is specially molded for mating with the motor. The TAC motor turns two reduction gears inside of the throttle body that link the drive gear from the motor to the throttle plate shaft. Although the motor can be removed from the throttle housing, it is not currently considered to be serviceable separate from the throttle body.
Another change to the throttle body is the absence of the IAC motor and the tiny drilled hole in the throttle plate. This is because idle speed is controlled completely by throttle plate angle in this system. The throttle body has two springs inside of it. One is for throttle plate return and the other is a “limp home” spring. The limp home spring is set to open the throttle about 8