Electronic throttle – So where’s the cable?
In 1988, the BMW 7 series was the first vehicle manufactured with
a fully electronic throttle. By 1997, Chevrolet had their "Throttle
Actuator Control" (TAC) on its latest 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 it work?
There are three basic components in electronic throttle control
1 - A dual position sensor on the accelerator pedal called an
APP “Accelerator Pedal Position” 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 - 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 they’ll use the throttle itself to position the throttle valve for idle. The throttle body will also have two TPS (Throttle Position Sensor) units mounted as part of the housing.
3 – Some manufacturers use a separate ETC “Electronic Throttle Control” module that is wired to the throttle body and PCM, while other manufacturers have their throttle body wired straight to the PCM “Powertrain Control Module”. 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 TPS’s receive a 5 volt signal from their controller and each APP and TPS receive that 5 volts on separate wires and at separate positions on the circuit board. Keeping the two signals separate also means less chance of a voltage interference and adds a bit of redundancy between the two operating sensors.
On both the gas pedal and the throttle plate (mounted in the throttle body) they use a heavy spring to control their movements. The APP (gas pedal) springs are there to return the pedal to the idle position as well as to simulate the feel of cable operated system and to add resistance to the gas pedal movement. The heavy spring that closes the throttle plate is there to return the throttle plate to the closed position in the event the system should fail as well as adding some resistance for the electric drive motor and reduction gears.
When the gas pedal is depressed, the APP sensors 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 3 APP sensors, APP-1 is a positive signal voltage that increases as the pedal is depressed. APP-2 and APP-3 are positive signal voltages that decreases as the pedal is depressed.
For the Ford 3 App sensor system the 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.
With the two APP sensor type of gas pedal, they use an opposing voltage and resistance. As one goes up the other goes down. A good reference check to see if they are functioning correctly is to add the two voltages together. They should equal 5 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 on. Typically on the Ford systems, if an electronic throttle control fault code is set a little yellow wrench warning light will be illuminated on the instrument cluster as well.
When a fault does occur the electronic throttle drops to a predetermined throttle angle and stays there. Commonly referred to as the “Limp Home” mode. Fords, generally will go to their idle position. Some GM’s will limit the vehicle to a maximum speed of 30 mph until the fault is corrected. Most imports will drop to and 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 one lead of one of the APP’s or the TPS that has broken off causing the entire limp home scenario. Another common issue is dirty throttle plates and throttle chambers. Before cleaning a throttle body check for any warning labels and the service information on that particular vehicle for cleaning instructions. Some of the throttle bodies have a coating lining the air passage ways and cleaning it with any harsh chemicals could remove this coating.
A code reader or scan tool is required for diagnostics. However to check the actual signals out of the APP’s or the TPS I prefer an O scope 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 from 0 to 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 no voltage or ground to the drive motor, faulty wiring or connections, or 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 OBD II trouble code reader, you can possibly 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 than the basic OBD II code reader.
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 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, I’ve been asked more than once by a frugal customer; No 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 quite impossible. The electronic throttle control systems may seem silly or overkill on the manufacture’s part to some people or you may be saying, “Mechanical throttle control systems worked so well for so many years, why make it more complicated?”
It comes down to the technology we have today. In order to have things like the modern cruise control that senses the distance of the car in front of you, to a more precise idle control which promotes better fuel economy just to name a few. But, for the die-hard-hot-rodding lead foot that temptation to stomp on the gas pedal and bark the tires has been programmed out of the car. But for those aficionados of the drag strip, on some of the high performance models the manufacturers have added a “line-lock” switch effectively locking only the front brakes and allowing you to warm your tires up and shred the rubber off just like in the good old days. Even the line-lock is integrated into the PCM and affects the operation of the throttle body. All of which is now controlled by the software and computer systems in the modern car.
Basically, a lot of the features incorporated into today’s cars require the computers to evaluate and somewhat control the operation of the car to a degree that isn’t humanly possible without the help of a computer making thousands of calculations a second. Another way to put it is; when you step on the gas pedal, you expect the car to go, but what is really happening is that you’re asking the computer if it’s OK to open up the throttle. The computer examines all the various system inputs and calculates the reference signals from the sensors and then makes the call whether or not it’s safe for you to move. All in a split second of course. Anymore, you don’t have that much to do with the actual movement of the throttle a computer is doing all the thinking.
For all those old salts that believe the modern car has gone way too far with all of this technology, well, the same thing was said when we switched from saddling horses to cranking the model T. So where’s the throttle cable at these days? It’s gone to the same place as the buggy whips have. No more tinkering with the idle, or adjusting the throttle cable, it’s all done electronically now. Get used to it fellas, cause advanced technology isn’t coming… it’s here.