Hi, I am a professional certified mechanic, with an engineering background, and 35+ years experience. I will do my best to assist you. Also keep in mind I don't know if you are a pro or a novice, so feel free to add any additional info at any time.
You need to start off by getting to the "dead " devices and see if there is power to them. If you have worked with AC a lot DC can be a little strange. It sounds like you might have more than one problem. Honestly at this point we have no way to tell. The thing to keep in mind on DC systems that you don’t see on AC is, DC systems almost always have one wire to power a device, with some exceptions, like electric window motors. The body of the vehicle is the conductor on the negative side for most circuits. Just remember a power wire and ground, is all that is needed in a DC circuit. Another thing is: because automotive wiring is so complex, and interconnected if a circuit looses a ground, the power will back feed through any other circuit it can. Electronics techs work with this all the time when repairing circuit boards, but in the automotive industry I have it isn’t hardly ever understood, virtually no mechanics knew about this problem. It is called GF (ground feedback). You need to look out for this one, because I suspect it might be at least part of your problem. When GF is present, some devices will not work, and they will be getting power. They have lost their ground connection, and are feeding power back through other circuits, remember electricity takes the easiest path, this can take it places it shouldn’t go. GF can make devices come on that shouldn’t be on, and when GF backfeeds through a device that shouldn’t be on it will normally do screwy things.
You HAVE to get a good electrical schematic if you are going to take this one on, it isn’t optional, or you will be chasing your tail. When you get a schematic you need to make a list of all the devices/systems that are not working, and then start tracing them on the schematic. What you are looking for is common ground, or power feeds that the malfunctioning devices have in common. Because I suspect GF, I would go to and clean all the chassis grounds especially the ones connected to the devices that acting up. I like to clean the terminals with a wire brush or sand paper, and solder the terminal to the wire where it grounds. The terminal can loose electrical connection where it crimps to the wire. If you want to use a test to check the connection there is a test called voltage drop can spot this problem, I use it a lot on DC circuits, I can walk you through it if you haven’t dealt with it before. The schematics will show you the ground points. You might consider subscribing to Mitchell on line, it is a service manual, and they have the best electronic schematics in the industry.
Check power to malfunctioning circuits
Check/clean the grounds, this can work miracles, well almost.
Don’t assume a circuit has power if the idiot lights come on with the ignition on, because sometimes an electronic module will be testing the idiot light function, that is independent from actual system function.
Keep an eye out for a group of circuits/devices/systems that are controlled by a module that are not working. Always go to the power and ground of any control module if there is a module involved in a failure. You have to use voltage drop tests to get an accurate readings here. Voltage drop tests spot high resistance connections, these can cause low power and GF. Nothing picks up on a bad connect like a voltage drop test. I can point you at a good video, A LINK HERE, a basic video HERE. The first video gets a lot deeper, the second is so much easier to understand. I would watch both. Watch the basic video first, and watch it again to clear your head again after watching the more complex vid. Go over all of this and if you have any more questions at all, I am here to help. Good luck with it, have a great day, and Thanks for using Just Answer.
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