You haven't provided any details, symptoms, history, or anything else to go on. Until you do, the only thing I can do is explain there should be 12 volts feeding the injectors, ignition coil(s), oxygen sensor heaters, alternator field circuit, and the fuel pump or pump relay at two times. The first is when the ignition switch is turned to "run". That lasts for roughly one second, then it goes back to 0 volts. That 12 volts is switched on by the Engine Computer through the automatic shutdown, (ASD) relay. The next time that relay is turned on is during engine rotation, (cranking or running).
You also didn't specify the engine size. When you have an engine running problem, we need to know which engine you have as they use very different circuit diagrams, usually with different diagnostic steps.
Assuming this is a crank / no-start condition, one of the first steps is to read and record any diagnostic fault codes. If you did that already, did any codes show up?
The best way to test the ASD circuit is to back-probe the wire that is the same color at every injector and ignition coil, or either smaller terminal on the back of the alternator, with a test light. Attach the test light in such a way as you can see it from inside the truck, or have a helper operate the ignition switch for you. You can try a digital voltmeter instead, but most of them don't respond fast enough. You should see the test light turn on full brightness for one second when the ignition switch is turned to "run", then it will go off. That proves the ASD circuit is working and the Engine Computer has control of it. Next, what is important is the light should turn on again when the engine is being cranked. If it does not, the best suspects are the crankshaft position sensor and the camshaft position sensor. If you're lucky, there will be a fault code related to the missing signal, otherwise we'll need a scanner to view live data to see which signal is missing.
Let me know what you find up to this point.
Friday, January 9th, 2026 AT 12:30 PM
