What are the symptoms / observations / tests / fault codes you're pursuing? If you already did continuity tests, you have an ohm meter and used it to measure between the two ends of one wire. That's what you use to measure between two different wires to see if two are shorted together. My guess is you're reading the steps in a diagnostic manual. We don't exactly follow those steps in that manner. Instead, we measure the voltage on the various wires. For example, you're probably working on a sensor circuit. The majority of sensors, like throttle position sensors and map sensors are fed with 5.0 volts, and there is a ground wire that will actually have 0.2 volts, then the signal wire will have between approximately 0.5 and 4.5 volts. A break in a wire or a connection inside the sensor can cause the signal voltage to go to 0.0 or 5.0 volts. THAT'S what triggers a diagnostic fault code, ... Anything outside that 0.5 to 4.5 volt range.
On '96 and newer cars, fault codes get pretty specific. If the voltage on the signal wire goes to 5.0 volts, the code would be "voltage too high" but it won't tell you how much it was or how much too high it was. The same code would be set if you applied 12 volts to that signal wire, or if the signal wire and a 12 volt wire were shorted together. The computers are designed to withstand that higher voltage, but the computer will not try to respond to it or act on it.
When we get that code we just take a voltage reading or connect a scanner to view live data to verify it. About half of the time the voltage is normal and correct. The code was set because of a momentary glitch, like from a corroded terminal in a connector or a spec of dirt in the sensor.
The troubleshooting steps in the diagnostic manuals get pretty tedious and we tend to skip over most of them and go right measuring the voltage on the wire in question. If you were to find 12 volts on a wire that is supposed to have between 0.5 and 4.5 volts, THAT'S when you'd typically measure between that signal wire and the positive terminal of the battery to confirm your findings. Even that can be misleading because there is a difference between a short that PUTS 12 volts onto the wrong wire, vs. A break that ALLOWS 12 volts to appear there. I know that doesn't make sense, but it would if you understood electrical theory.
The only thing that would allow 12 volts to appear on a signal wire is a defect inside the computer, and that is extremely rare. Those power supplies are very well-regulated and protected, and they will shut down to protect the computer.
The same is true of a short to ground, which is more common. A short to power, (12 volts), has to involve two adjacent wires or connector terminals touching, so that can only happen in a harness where they run side by side. A short to ground can occur the same way with two adjacent wires, AND anywhere the harness runs over the sharp edge of a metal bracket, falls down onto hot exhaust parts, or rubs back and forth as the engine rocks during acceleration and shifting to reverse. With a short to ground you'll find 0.0 volts on the signal wire. Since that is below the minimum acceptable 0.5 volts, it will trigger the fault code, "voltage too low".
When you do have a code for voltage too high or too low, just take the voltage reading on the signal wire of the sensor the code refers to. It's going to be 0.0 volts, 5.0 volts, or the correct voltage somewhere in between. When it's correct, the code was set when the computer detected a very brief spike to 0.0 or 5.0 volts. Mechanics use scanners and watch the voltage as the computer sees it, then they wiggle on harnesses, tap on sensors, and do stuff like that to try to make the voltage change. If they can't cause it to change and they've ruled everything else out, then they will suspect the sensor itself has an intermittent problem. You can do the same test with a voltmeter, but if you measure right at the sensor, your reading won't take into account the possibility of a break in the signal wire. When that happens, the scanner will show 0.0 volts or 5.0 volts for that sensor, but you'd find the proper voltage with your meter, (between 0.5 and 4.5 volts). That's why you're much better off using a scanner when diagnosing this type of problem.
Also be aware, as I tell everyone, diagnostic fault codes never say to replace parts or that they're defective. They only refer to the circuit that has the defect. Don't assume a sensor is bad until you do the couple of voltage measurements.
A signal wire will only have 12 volts on it in perhaps one out of a million cars with a fault code for high voltage. The troubleshooting steps in the diagnostic manuals always include the step to measure for continuity, (a short), between the 12 volt wires and the signal wire in case you have that one in a million car. That's why we skip over that step and many others. We know the chance of finding the cause of a problem during one of those tests is so extremely remote that it just wastes the customer's time and money.
The exception to all of this are the oxygen sensors. They generate a signal voltage of between about 0.2 to 0.8 volts, and it switches up and down within that range a couple of times per second. Most O2 sensors also have a 12 volt wire to run an internal heater inside each sensor. Those sensors don't work until they reach around 600 degrees. The heaters get them up to that temperature sooner after starting the engine, and it keeps them there during prolonged periods of idling when the exhaust temperature can go down.
Two things can trigger a fault code for voltage too high with an oxygen sensor. The heater element can bend and touch the sensing element. That will put 12 volts onto the signal wire. Also, given that O2 sensors are screwed into a very hot exhaust pipe, if the wiring harness falls down onto it, the wires can melt together. Most O2 sensors have four wires. There are two ground wires. If the 12 volt heater wire touches metal or either ground wire, usually a fuse will blow, and it may affect other systems like the injectors or ignition coil(s). The 12 volt wire can also touch the signal wire. That would set the "high voltage" fault code.
Saturday, October 5th, 2013 AT 12:21 AM