Electronic Throttle: Where's The Cable?

Electronic Throttle: Where’s The Cable?

Today, virtually every manufacturer has an electronic throttle system on board every make and model. That old throttle cable is a thing of the past.

In 1988, the BMW 7 series was the first vehicle manufactured with a fully electronic throttle. By 1997, Chevrolet had its throttle actuator control (TAC) on its newest C5 Corvette. Today, virtually every manufacturer has an electronic throttle system on board every make and model. That old throttle cable is a thing of the past.

What Makes Electronic Throttles Work?
There are three basic components in electronic throttle control systems:

1. There is a dual position sensor on the accelerator pedal called an accelerator pedal position (APP) sensor. Most manufacturers label them as APP-1 and APP-2. Some manufacturers use three pedal position sensors and label the third one as APP-3.

2. You’ll also see a throttle body with a gear-driven throttle ran by a reversible electric motor. (A set of reduction gears is used to rotate the throttle shaft.) The throttle body generally will contain an idle air control valve or will use the throttle itself to position the throttle valve for idle. The throttle body will also have two throttle position sensor (TPS) units mounted as part of the housing.

3. Some manufacturers use a separate electronic throttle control (ETC) module that is wired to the throttle body and PCM. Other manufacturers have their throttle body wired straight to the PCM. In both cases, information is sent over the CAN-bus or serial data lines for faster communication between the various components.

On most vehicles, each of the APP sensors and throttle position sensors receives a five-volt signal from its controller. Each sensor receives that five volts on separate wires and at separate positions on the circuit board. Keeping the two signals separate also means less chance of voltage interference; however, it adds a bit of redundancy between the two operating sensors.

On both the gas pedal and throttle plate (mounted in the throttle body), a heavy spring is used to control movement. The APP (gas pedal) springs are there to return the pedal to the idle position as well as to simulate the feel of a cable-operated system. They also add resistance to the gas pedal movement. The heavy spring that closes the throttle plate is there to return the plate to the closed position in the event the system should fail. It also adds some resistance for the electric drive motor and reduction gears.

When the gas pedal is depressed, APP sensor resistance and voltage values change. The control module notes this and checks the opposing APP sensor for verification. Then, the PCM or ETC module gathers the needed information from the other sensor inputs (such as engine RPM, engine load via the MAP sensor, MAF sensor, and the vehicle speed and transmission gear selection).

It now has to calculate how much throttle opening is warranted based on those inputs and how quickly it should respond. The module then sends a command to the electric motor on the throttle body to open the throttle to the calculated position. The throttle position sensors react to the change in throttle position and provide feedback signals to the control module so the module knows the exact position of the throttle and that everything is working correctly.

On the typical GM systems using three APP sensors, APP-1 is a positive signal voltage that increases as the pedal is depressed, while APP-2 and APP-3 are positive signal voltages that decrease as the pedal is depressed.

For the Ford three-app sensor system, APP-1 uses a signal voltage with a negative voltage slope of 5-0 volts. APP-2 uses a signal voltage with a positive voltage slope of 0-5 volts, and APP-3 uses a signal voltage with a positive voltage slope of 0-5 volts.

The two-APP-sensor-type gas pedal uses opposing voltage and resistance. As one goes up, the other goes down. A good reference check to see if these units are functioning correctly is to add the two voltages together. They should equal five volts.

Limp-Home Mode
If any of the position signals disagree, such as the TPS signals or the APP sensor signals, a fault code is set and the malfunction indicator lamp (MIL) will be activated. Typically on Ford systems, if an electronic throttle control fault code is set, a little yellow wrench warning light will also be illuminated on the instrument cluster.

When a fault does occur, the electronic throttle drops to a predetermined throttle angle and stays there. This is commonly referred to as the “limp-home” mode. Fords will generally go to their idle position. Some GMs will limit the vehicle to a maximum speed of 30 mph until the fault is corrected. Most imports will drop to idle or just above idle and remain there until the problem is repaired.

A lot of the faults will be related to the gas pedal or throttle body. Sometimes, the actual throttle plate motor can fail in the throttle body, which will then default to wherever the spring tension and predetermined fail-safe throttle angle is set at for that given vehicle. Wiring is also a big issue. On many occasions, I’ve had to find a lead of one of the APPs or the TPS that has broken off, causing the entire limp-home scenario. Other common issues are dirty throttle plates and throttle chambers. Before cleaning a throttle body, check the service information and warning labels for cleaning instructions. Some of the throttle bodies have a coating lining the air passageways that could be damaged when cleaned with harsh chemicals.

A code reader or scan tool is required for performing this diagnostic work. However, to check the actual signals out of the APPs or the TPS, I prefer an oscilloscope with at least a two-channel capability. BIAS control testing is another proven method of checking the input and output signals. A lot of aftermarket scanners will have a BIAS control for the throttle body. They are usually controllable in 10-degree increments ranging from 0-100% throttle angle.

One of the things to look for while doing the BIAS testing is whether or not the throttle plate is moving smoothly as you increase or decrease the BIAS signal. No movement could mean there is no voltage or ground to the drive motor, faulty wiring or connections, or there is a faulty drive motor. The motor should move freely and smoothly. If any jerking or erratic movements are noticed, chances are the gears inside the throttle body have worn or are stripped.

If all you have is a generic OBDII code reader, you may be able to see pedal position sensor fault codes such as P0120 through P0124 and P0220 through P0229. However, there are several codes that are manufacturer specific that will require a better scanner.

Service Tips
On some systems, a special relearn procedure is required when any of the parts have been replaced or the throttle control wiring harness has been disconnected for any length of time. The relearn procedure is necessary for the controller in order to understand the gas pedal and throttle position sensors’ range of movement. On some vehicles, this occurs automatically every time the key is turned on. But on others, this will require the use of a scan tool or a special manufacturer procedure.

And yes, although I’ve been asked more than once by a frugal customer, it is NOT possible to convert an electronic throttle back to an ordinary mechanical throttle linkage. The electronics are so integrated on today’s engines that modifications such as this are impossible. The electronic throttle control systems may seem like overkill on the manufacturer’s part to some people, but there are good reasons for it.

It comes down to the technology we have today. We need this system to have things like the modern cruise control technology that senses the distance of the car in front of you and precise idle control that promotes better fuel economy. But, for the diehard hot-rodding leadfoot, that temptation to stomp on the gas pedal and bark the tires has been programmed out of the car. However, for those aficionados of the drag strip, some of today’s high-performance models have added a “line-lock” switch that essentially locks only the front brakes and allows you to shred the rubber off your tires just like in the good ol’ days. Yet, even the line-lock is integrated into the PCM and affects the operation of the throttle body. All of which are now controlled by the software and computer systems in the modern car.

Basically, a lot of the features incorporated into today’s cars require computers to evaluate and control their operation. Another way to put it is that when you step on the gas pedal, you expect the car to go. The computer examines all the various system inputs and calculates the reference signals from the sensors before making the call on whether or not it’s safe for you to move — all in a split second, of course.

Article courtesy Underhood Service.

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