Hi guys. Please allow me to stick my nose in here with a couple of comments. There must be 5.0 volts on one wire to the TPS. The ground wire is tied in with those from some other sensors and will have 0.2 volts on it. That's because that circuit goes to ground through the computer so it can be monitored. The 0.2 volts is dropped across that circuitry. It's always best to take sensor voltages with them plugged in, by back-probing next to each wire through the rubber seals. When you do that, you must find roughly 0.5 (closed throttle), to 4.5 volts, (wide-open-throttle) on the signal wire. Those values are for training and explanation purposes. In actual practice, no two sensors are alike. You may find.72 to 4.3 volts, for example. What's important is it can never reach 0.0 or 5.0 volts. Anything outside the 0.5 to 4.5 volt range is what gets detected as a defect and sets fault codes.
The problem is with the signal wire. If there's a break in that wire or inside the sensor, the voltage seen by the computer can "float" to some random value due to being interconnected with all the other circuitry inside the computer. That circuitry has no effect when everything is working properly. It's when there's a defect in that signal circuit that if that random voltage stays within 0.5 to 4.5 volts, the computer will accept it and try to run on it.
When a defect exists in that signal circuit, the computer has an internal "pull-up" resistor between the signal terminal and the internal 5.0 volt supply. That resistor is so big electrically that normally it also has no effect on circuit operation. When you unplug the sensor to take voltage readings, you create a break in the circuit. That's when the pull-up resistor does its thing and puts 5.0 volts on the signal wire.
Various sensor circuits in other models, imports in particular, use "pull-down" resistors for the same purpose. Their operation causes less confusion, but they're doing the same thing. Those are tied between the signal wire and ground. As with the pull-up resistors, when a defect occurs, they place their 5.0 volts or 0.0 volts on the signal wire to force a defective condition to be detected.
You can verify this by back-probing the signal wire and watching the voltage change from 5.0 volts to around 0.5 volts when you plug it in.
My second comment of value has to do with running an engine with a sensor unplugged. Often a defect in the sensor or in the circuit will cause one to report the wrong signal voltage, but one that is still within the acceptable range. This is where the computer will accept it and try to run the engine on that wrong value. When you unplug the sensor, that forces the defective condition to be detected. The computer sets the appropriate diagnostic fault code. It also turns on the Check Engine light if the defect could have an adverse effect on emissions. Third, since it knows it can't rely on the readings from that sensor, it "injects" an approximate value to run the engine on. For example, in is case, if it knows it can't use the TPS readings, it knows the throttle must be nearly closed since rpm is low. The hope is the injected value is closer to correct than the voltage coming from the defective circuit. That's why you sometimes hear someone say their engine runs better with something unplugged.
On Chrysler products, the MAP sensor has the biggest say in fuel metering calculations. The TPS actually has the least influence of all the sensors.
Friday, February 18th, 2022 AT 2:50 PM
