You're asking all the right questions. Here's the drawings showing the sensor locations.
A better approach is to check if the automatic shutdown, (ASD) relay is turning on. Look for the wire that's the same color at the ignition coil(s) and Injectors. The fourth and fifth diagrams show them. The standard color is a dark green / orange wire. You can back-probe any of those places, but it's best to use a test light instead of a digital voltmeter. Most meters respond too slowly.
You will see the test light turn on for about one second when you turn on the ignition switch, then it will turn back off. That tells you the ASD relay is working and the computer is in control of it. You may hear the hum of the fuel pump for that one second at the same time.
What's important is if the test light turns back on during engine rotation, (cranking or running). The Engine Computer will do that if it receives signal pulses from the crankshaft position sensor and the camshaft position sensor.
Your very last sentence suggests you already are aware of what most people don't understand right away. To back up for a minute, it is fairly common to not have any diagnostic fault code related to these two sensors after one has failed. They often don't set a code just from cranking the engine. They need more time to be detected, as in when a stalled engine is coasting to a stop. Even when that does happen, many people erase the code to see if it comes back, but then it doesn't set again just from cranking the engine, and the confusion ensues.
The next issue is when you are lucky enough to get a fault code, they never say to replace parts or that one is defective. As you eluded to, they only indicate the circuit or system that needs further diagnosis, or an unacceptable operating condition. When a part is referenced in a fault code, it is actually the cause of that code perhaps half of the time. First we have to rule out wiring and connector terminal problems, and mechanical problems associated with that part. An example of a mechanical problem with older sensor designs was the critical air gap. A perfectly fine sensor wouldn't develop the right signal if the air gap was wrong.
If you have a replacement sensor on hand, a quick test might be to just replace it and see if it solves the problem. When you have to buy one or wait for one to be delivered is when we do some tests first to rule out everything else first. Start by measuring the orange 5.0-volt feed wire at either sensor. If that is missing, that wire has a break in it, one of the sensors is shorted internally, or that wire is grounded. To check if a sensor is shorted, unplug them, but to see if the 5.0 volts comes back, you have to turn the ignition switch off, then back on to reset that power source. When the 5.0 volts comes back, plug one sensor back in, then the other one. The 5.0 volts will go to 0.0 volts when you plug in the shorted sensor. The Engine Computer turns that power supply off to protect it.
Next verify the sensors' ground circuit is okay. Expect to see 0.2 volts on that black / light blue wire. If there's a break in that wire, you'll find 5.0 volts on it at one or both sensors, depending on where that break is. The wires themselves actually don't cause a lot of trouble. After a failed sensor, the next best suspect is a corroded or stretched pair of mating terminals in one of the sensors' connectors.
Testing the signal circuits is a little more cumbersome. In this case you can try back-probing the gray / black wire with a digital voltmeter, then you'll have to rotate the engine by hand. If signal pulses are generated while cranking the engine with the starter motor, those pulses will come and go much too quickly to be seen by a voltmeter. This sensor design may not lend itself to testing this way, so don't jump to any conclusions based on what you find. The sensors have a magnet with a coil of wire wrapped around them. Generating any kind of voltage or signal this way always requires movement between the magnet and the coil of wire, but in this case the movement comes from notches, or holes in a tone ring moving past the tip of the magnet. That disturbs the magnetic field and is what induces the signal voltage. Two-wire sensors send that signal directly to the Engine Computer. Three-wire sensors like these have additional circuitry inside them to amplify and strengthen the signal. With some of these, you'll see the signal voltage drop to near 0 volts as you slowly rotate the crankshaft by hand, then it will pop back up to 5 volts. Other sensors need a minimum speed before they'll do that switching. That's why this really isn't a good test. A better test would be to use an oscilloscope to view the signal voltage as you crank the engine with the starter, but by far the best test is to just let the Engine Computer tell you if it is happy with the signals it's receiving. A scanner is needed for that. My Chrysler DRB3 lists each sensor with a "No" or "Present" during cranking to show if those signals are there. They switch to "Present" very quickly during cranking when they're working properly. Other aftermarket scanners have some similar means of showing that.
Be aware it is also real common for either of these sensors on any brand of car to fail by becoming heat-sensitive, then they work again after cooling down for about an hour. They usually keep working as long as you're driving. Natural air flow keeps them cool. It's when you stop briefly with a hot engine, as in when stopping for gas, that engine heat migrates up to the sensors and causes one to fail. Based on the time it takes to cool down and start working again is the best clue that this is caused by the sensor itself, and not wiring or connector terminal problems.
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Friday, July 23rd, 2021 AT 2:42 PM