First let me tell you what I DO know. Any time you have a sensor that runs on 5.0 volts and its signal is a variable voltage, (as opposed to a square wave that goes between 0.0 and 5.0 volts), the acceptable range of signal voltage, for explanation purposes, is 0.5 to 4.5 volts. In actual practice you may find it going from 0.38 to 4.22 volts, for example, but it will never go all the way to 0.0 or 5.0 volts. Those are the conditions that trigger a fault code. If you understand electrical theory, you'll understand that to unplug a sensor would create an open circuit. If that's a two-wire sensor like for intake air temperature or coolant temperature, the 5.0 volt feed wire is the signal wire. Thanks to a dropping resistor inside the computer, with a complete circuit, meaning the sensor is plugged in, current flow will cause some of the 5.0 volts to be dropped inside the computer and you'll see what's left across the sensor. Regardless of the temperature, the signal voltage will never go outside of 0.5 to 4.5 volts. If you short the signal wire to ground, you'll set a fault code for "voltage too low" for that sensor. If you unplug it, you'll set a code for "voltage too high", meaning it went to 5.0 volts.
Things work a little differently for three-wire sensors. If we assume the 5.0 volt feed and ground wires are okay, it's just the signal wire we have to look at. (Also, to save time for our customers, if the signal voltage is okay, we know the 5.0 volt feed and ground wires have to be okay, so we don't waste time testing them). The throttle position sensor is easier to explain because it's a simple mechanical sensor. It works like the volume control on an older tv or radio. On one end the terminal is grounded. On the other end the terminal has 5.0 volts applied. The wiper, or movable contact inside, goes up and down to pick a point along that resistor depending on how far you open the throttle, however, there are mechanical stops inside it that prevent it from going beyond 0.5 or 4.5 volts. The only way you might be able to make it go too far is to remove it from the throttle body and turn it by hand beyond its normal range. On the car, if the 5.0 volt feed wire was missing, as in a broken wire, you'd see 0.0 volts on all three terminals, and that would set a fault code for voltage too low. If the ground wire were open, you'd have 5.0 volts all over and set a code for voltage too high.
The clinker is with the signal terminal. If that wiper inside the sensor makes intermittent contact, or if the sensor is unplugged, you would think you'd have 0.0 volts on that wire, and in fact, on a few car models you will, but on most cars you'll find 5.0 volts. That is because with an open circuit on that signal wire, we don't know what that voltage will go to. Because of being tied to all the other circuitry inside the computer, that signal voltage is going to "float" to some random value. If that random value is between 0.5 and 4.5 volts, the computer might accept it and try to make fuel metering calculations based on it. To prevent that, we use a "pull-up" resistor tied between the signal wire and 5.0 volts. That resistor is so big electrically that it has no effect if the circuit is working properly, but when there's that open circuit, it places 5.0 volts on the signal wire to force a fault code to set. That also tells the computer that signal voltage can't be trusted and it suspends any other self-tests that rely on the signal for comparisons.
If my sad story is correct, with your sensor unplugged, you should find 5.0 volts on the supply terminal and on the signal terminal, but only with a digital voltmeter. Those draw such little current from the circuit that the pull-up resistor won't drop any voltage.
That brings me to your EGR sensor. The first thing to consider is you aren't getting a fault code for that sensor. Under normal operation, that pintle valve might only open part way. Just like with taking the throttle position sensor off and turning it manually, you may be pushing it too far and beyond its normal range. A better test would be to monitor the signal voltage while the sensor is plugged in, then run the engine and apply vacuum to the EGR valve. You should see the voltage stay within the acceptable limits.
Now I'm going to tell you what I don't know. The fault code you got was for insufficient flow which means the computer was operating the EGR valve and not seeing the expected results. The sensor on the valve just reports that it opened, not how much exhaust gas was flowing, so that means the sensor was working. What I'm not sure about is how the insufficient flow is detected. That could be by watching the change in readings from the front oxygen sensors.
You said you checked for blocked passages too, but you might try opening the valve manually with a vacuum hose while the engine is idling. That might cause the engine to stall, but for sure it will run rough and slow down for a few seconds.
Sunday, April 26th, 2015 AT 1:04 AM
