It's not that it doesn't recognize the throttle position sensor. It's that it doesn't yet know what the lowest signal voltage is that it will send out at idle. All throttle position sensors are different and not adjustable. For training purposes we always say they are fed with 5.0 volts, and there are mechanical stops inside it that limit the signal voltage to 0.5 volt at idle to 4.5 volts at wide-open-throttle. In reality, you may find 0.38 volts at idle with one sensor and 0.46 volts with a different one. The exact value isn't critical.
If you install that second sensor that provides 0.46 volts at idle, the computer already has that 0.38 volts in memory from the previous one. When it sees the new 0.46 volts, it will "know", (incorrectly), that you have your foot on the gas pedal so it will leave the engine speed up to you. It will not try to maintain idle speed until it knows your foot is off the gas pedal, and it knows that when it sees 0.38 volts.
The same thing happens after disconnecting the battery. There is no default voltage value because half the time it would be lower than what the TPS actually delivers at idle, so it would still have to relearn the new value. There has to be a way for the computer to know when your foot is off the gas pedal. It knows that by a high vacuum reading from the MAP sensor. The problem is you could snap the throttle open briefly in the garage. When you release it, vacuum will go real high as the engine slows down, but that still doesn't prove you fully released the throttle. To address that, the computer needs to see that high vacuum for at least seven seconds along with a steady voltage reading from the TPS. The only way for that vacuum to be real high for that long is by coasting, and coasting with a TPS reading that is steady and not bouncing around proves your foot is off the gas pedal. THAT'S when the computer looks at the TPS voltage and puts that value in memory. From then on, whenever it sees that same voltage, it knows it has to be in control of idle speed.
If you install a new TPS with a lower voltage at idle, the computer will recognize that and learn that as the new minimum throttle. If the new TPS voltage is higher than the old one's, it won't relearn it until you do the coasting procedure.
That 0.5 volts to 4.5 volts becomes important when talking about diagnostic fault codes. The only way for the signal voltage to go to 0.0 or 5.0 volts is with a break in a wire, a break inside the sensor, a loose or corroded connector terminal, or a wire is grounded. The throttle position sensor is strictly a mechanical device so it can't really send out the wrong signal voltage that is still within that acceptable range. A MAP sensor sends out the same range of signal voltage but since it has a lot of circuitry inside, and since it has a diaphragm that can have a pinhole leak, it IS possible to send out the wrong voltage that is still within the acceptable limits. Suppose at a certain load on the engine the voltage corresponding to the manifold vacuum is supposed to be 1.3 volts. The computer uses mainly that reading to calculate fuel metering. If the sensor reports 1.6 volts instead, the computer will act on that and deliver the wrong amount of fuel, but it will not set a fault code because 1.6 volts is an acceptable value, even though it's the wrong value. That's how you can have a running problem with no fault codes. Eventually the oxygen sensors will pick that up but all they will do is report a lean or rich condition. You still have to diagnose why that condition is occurring.
Thursday, November 22nd, 2012 AT 11:21 PM