Ahhh. You aren't measuring a ground wire. You're using your meter's continuity test to check for a complete path TO ground. Most meters will beep with anything less than 200 ohms. You're reading through the numerous "loads" on the circuit. You'll find the same thing if you measure the 12 volt feed wire to the heater fan motor, or the 12 volt feed wire to the head lights. All of those circuits have loads of less than 200 ohms, so your meter would beep, but that doesn't mean they're ground wires. If you did NOT have that resistance to ground, you wouldn't have any current flow when the circuit is turned on, and it would be dead.
Forget continuity readings. Those get people way too "wrapped around the axle" with misleading results. Mechanics never do these tests, except occasionally to verify their diagnosis, because it is a waste of time, and therefore, their customer's money. There are much more effective ways to diagnose problems.
Your understanding of electrical theory is also getting you into trouble. My head is spinning from trying to follow your logic with sensor readings. You're confused too as to what a sensor's signal voltage should be. That is to be expected before you get some help from geniuses like us!
12 volts and 5.0 volts is not a signal voltage. If that were the case, you wouldn't need a sensor. You could just connect that wire to the battery.
Sensors react to a condition that varies, and their signal is the result. The computer looks at that signal voltage to know what has taken place with that variable. 5.0 volts and 12.0 volts is not a variable and is not a signal voltage. The easiest example to understand of a signal voltage is perhaps the throttle position sensor. Imagine a garden hose connected to your house faucet, and the nozzle is open, so water is flowing. You have 50 pounds of pressure at the faucet, but 0 pounds after the nozzle. There's your 5.0 volt supply and ground for the throttle position sensor. Half way along the hose you would find 25 pounds of pressure. At half throttle, the movable contact inside the throttle position sensor would find a spot that has 2.5 volts. THAT is the signal voltage. The further the throttle is opened, the higher the signal voltage goes.
That 5.0 volt supply comes from a very carefully regulated supply inside the Engine Computer. It can't be allowed to vary even a few tenths of a volt. If it did, the voltage at any point along the throttle position sensor would vary a proportional amount, and that would produce an incorrect signal voltage.
The crankshaft position sensor's signal is totally different. For these position sensors, voltage is unimportant. Timing is the critical information. As a magnetic field is disturbed by something moving past it, the sensor turns on and off. WHEN those events occur is what the computer needs to know so it can time the firing of ignition coils and injectors properly. Digital voltmeters react much too slowly to be of any use with these "square wave" signals. If you could slow it way down, as in turning the crankshaft by hand, you might see 5.0 volts, then close to 0.0 volts, then 5.0 volts again, as you continued to turn the crank. Slowing this down gives the meter time to take the reading, analyze it, and display it while it takes the next reading. During cranking this happens way too fast for the meter to catch it. There are some high-class digital meters that measure frequency, (hertz), but all that is good for is to verify a signal is being generated. You don't know if there's some gaps, or "dropouts", if it's the right frequency, or if the signal voltage is high enough for the computer to see. Even engine performance specialists rarely use this test.
The advantage we have is the Engine Computer does all the preliminary testing for you. The first step is it is constantly monitoring numerous circuits, and it will detect a problem and set a diagnostic fault code. It is important to understand that fault codes never say to replace parts or that one is bad. They only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. I can go into how codes are set related to circuit problems if it comes to that. The second step is setting codes related to operating conditions. One of the easiest to understand might be the computer knows engine speed and throttle position. It also knows the engine can't be idling at 800 rpm if the throttle is 3/4 of wide-open-throttle. Those two don't correlate. An even better example is the computer knows if the engine has been off for at least six hours, the intake air temperature sensor and the coolant temperature sensor had better be reading the same temperature. If they are wildly different, the computer has ways of figuring out which one is wrong.
The third step, if the computer hasn't detected a missing position sensor signal, is to view live data on a scanner, then you have to determine what is missing or wrong. Very often cam and crank sensors won't set a fault code simply from cranking the engine. They might only set while a stalled engine is coasting to a stop. That is where we would observe those sensors on the live data screen. I use a Chrysler DRB3 scanner because it works on other car brands too. Those sensors are listed with a "No" or "Present" during cranking. Aftermarket scanners have some similar way to show the same thing.
Regardless if you find a signal to be missing or you have a fault code stating that, you still have to figure out why yourself. That is when it's appropriate to start with individual voltage readings.
Now to get more specific to your truck, you do indeed have a crankshaft position sensor and a camshaft position sensor that run on 12 volts. That is very uncommon, but diagnosing the circuits is the same. Each sensor is fed 12 volts through a different fuse. Given your original observation of a grounded wire, be sure to check ENG1 fuse and ECM1 fuse. They are both 20-amp fuses in the under-hood fuse box. Next, check for 12 volts at both sensors. To be valid, these readings need to be taken by back-probing through the rear of the connectors while they're plugged in. If you unplug the connector, then measure right on the terminals, if the 12 volts is missing, it's missing. If you do find 12 volts, it is possible it is a false reading and will mislead you. I can explain that too if it becomes necessary. The ignition switch must be in the "Run" position for these tests.
To add to the confusion, the ground wire for both sensors is purple, and should have 0.2 volts. Don't confuse those as the pink wires when looking for the 12 volts. If you have the 12 volts and 0.2 volts for both sensors, they will develop their signals as long as they're good and have no mechanical issues related to them. This is the time it is appropriate to replace a suspect sensor. Way too many people just throw in a new sensor when they see a fault code. About 50 percent of the time the missing signal is caused by a wiring or connector terminal problem, not the sensor itself, but they'll keep on buying sensors, and will become more and more frustrated. Replacing a sensor as a first step is okay if you already have it on hand, as long as you understand there's only a 50 percent chance it will solve the problem.
Wednesday, December 28th, 2016 AT 1:19 PM
