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Automotive Industry

Driveability Corner

Motor, Jun 2005 by Warren, Mark

As throttle-by-wire and other proactive systems gain influence, safety and emissions will improve, but putting the pedal to the metal may no longer produce parallel black lines.

Throttle-by-wire (TBW) systems are slowly replacing conventional hardware-based throttle systems, for several reasons. There are many advantages to a TBW system. For example, one less firewall penetration simplifies production and reduces noise and vibration transfer. Also, assembly is simplified with the elimination of the throttle cabling or linkage system needed with hardware-based systems. TBW also replaces the idle air control (IAC) assembly and cruise control units, and assists in traction control.

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While these advantages are undeniable, the driving force behind TBW is emissions reduction. To achieve super low emissions levels, the PCM can no longer "chase" the throttle in a reactive mode. With a TBW system, the PCM takes control over the throttle action and the accelerator pedal becomes a request for throttle, not a demand. Now, rather than being reactive to the throttle, the PCM takes proactive control.

Many other control strategies are also moving to proactive control. For example, in the past, most vehicles reacted to evaporative system purge enrichment by feedback from the oxygen sensor. Now, many systems anticipate the enrichment and make presumptive adjustments before the O2 sensor feedback. Faster, tighter fuel control equals better mileage and performance, and a reduction in emissions levels.

What does TBW mean to the repair community? For one thing, more money. These systems are safety-critical and monitored continuously, and must operate within precisely defined limits, TBW systems are generally straightforward to diagnose, and components are replaced as complete units. When needed, repairs to TBW systems can't conveniently be postponed because the vehicle may be undriveable without them.

Let's take a look at the physical makeup of a throttle-by-wire system on a 2004 Toyota Camry with a 2.4L engine. The primary components are the accelerator pedal assembly with attached dual accelerator pedal position (APP) sensors, the throttle body assembly with integrated dual throttle position sensors (TPS), the throttle blade, the housing, return springs and the throttle actuator motor.

Using OBD II generic mode for diagnostics, the key parameters are the accelerator pedal position sensors, the throttle position sensors and the throttle actuator command (TAG). The Toyota service information labels the TPS's circuits as VTAl and VTA2. The APPs circuits are labeled as VPAl andVPA2.

On this Toyota, the APPs are standard 5-volt reference potentiometers. This assembly has six wires; both APPs have their own dedicated power, ground and signal wires. With all of the wires sharing a common harness, manufacturers use various strategies to failsafe the system in case sensor signals should happen to short together.

In Fig. 1 on page 18 you can see that VPAl starts at .8 volt and VPA2 is offset with a starting voltage of 1.6 volts. Also note that in the usable range, VPA2 can hit 5 volts at the top and VPAl is somewhat less than 5 volts. Both sensors are linear and offset; therefore, they should never be at the same voltage (unless they short together) and they retain a linear relationship to each other at all times.

Some manufacturers use the offset voltage strategy and some invert the voltage output from one sensor (one is O to 5 volts and the other is 5 to O volts). GM, on some models, periodically pulls one sensor to ground. While strategies may vary among different automakers, the result is the same: TBW systems self-test for harness shorts, opens and out-of-range sensors.

This Toyota uses Hall-effect sensors for the throttle position sensors, and the output voltages are shown in Fig. 2 (also on page 18). VTAl is .69 to .95 volt at idle and VTA2 is 2.25 to 2,58 volts at idle. VTA2 peaks at 5 volts and VTAl peaks short of 5 volts. Once again, the output voltages are offset for diagnostic purposes.

What happens when the system detects a fault? This is a duty-cycle-controlled conventional motor twisting against two throttle springs. One spring closes the throttle and the other opens it. The effect of these springs in opposition is that the default/rest position of the throttle is about 16% open, or 1800 rpm. When a failure is detected, the actuator motor is shut off and the throttle goes to the default/rest position. The engine will still respond to throttle request (APP) by varying injector pulse width and ignition timing to provide a safe limp-in mode.

Fig. 3 above is a scope recording of the APPs and TPSs. APPWPAl starts at .8 volt and peaks at about 3.6 volts. APP2/VPA2 starts at 1.6 volts and peaks at about 4.3 volts. TPSlA7TAl starts at .8 volt and peaks at 4.1 volts. TPS2/VPA2 starts at 2.38 volts and peaks at 5 volts. Comparing these readings to the information on system characteristics in Figs. 1 and 2, it looks like all four sensors are in good operating order.

Automotive Industry

Driveability Corner

Motor, Jun 2005 by Warren, Mark

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