I got to thinking in the wee hours of the morning and decided I might need to post this to help those of you that may not have a lot of experience working on electrical systems, especially computerized engine management systems, regardless of the application(foreign, domestic, gas, diesel). Electricity can do some funny things, that's why it's called the Theory of Electricity, there are only two absolutes when working with it 1)Dead short it to ground and you can make FIRE!(YES!) 2) Touch the wrong place at the wrong time and it will kill you, or at least make you smack your head on something sharp so you have to go get stiches(Murphy's Law-I'm gonna slap him!). Everything else with electricity is engineered managment controls: fuses, switches, reostats, or, as in our case, a relay. A relay is nothing more than an electrical switch that uses a small amount of current to control a large amount of current(amperage flow). To kinda simplify this-you can have voltage on a circuit, but not have the ability to carry amperage, the "push" that makes electricity work for you, when you build into it a resistance factor(a light bulb). Manage the path of electricity and you make it do stuff for you(play my radio!), interrupt the management of electricity and the world goes dark and this fun stuff stops.
With that being said I want to clarify something VERY important for you guys that NEED to fix your truck yourself, or at least better understand why it's so time consuming or blasted hard for a mechanic to do it for you, hence the high repair bills, and sadly too many times don't fix the real problem, but I digress. Knowledge is power, and wiring diagrams rule. Wiring diagrams are simply a picture of how an engineer built it to go boom, but they can be intimidating till you learn to understand them, and math is involved(Durn it, knew I should have stayed in 8th grade!)Here's the important part-DON'T go get a 12v test light and start sticking it into computerized wiring circuits. Bad thing to do, lots of computer circuits use very low voltages, 5 volts or less, with high resistance circuitry. What you need to get, at SEARS, is a Digital Volt Ohm Meter(DVOM)with high impedance, NOT an AUTO-RANGING one, but one with selectible controls. They also sell a good book titled A Simple and Comprehensive Guide to Multitesters and Their Use For Electrical Testing. For the most part it's not hard to understand, but will help you to figure out how to check your circuits if something isn't functioning right. Sadly, that takes time, and who has any extra of that? However, pay me or pay yourself.
It has been my experience over the years, sometimes expensively because I've changed some parts trying to see if that fixed the problem, instead of really solving the problem, that the vast majority of drivability problems or electrical problems turn out to be something simple; a bad connection, a faulty or poor ground, a bad battery or alternator problem that's not charging properly, a frayed wire. It goes back to the old engineering acronym of K.I.S.S. Keep It Simple Stupid. The key to fix it is to figure out the proper circuit(wire colors) that the components are controlled by, what components are in the loop(switches, sensors, lights, solenoids, starters,etc.) and does the component let electricity back through it(electricity is lazy, it always must go back home-the battery, through grounds-and will seek the easiest path it can to get there-a short if it's available, or an "open"-blown fuse/cut wire-stops it) where does the circuit start(source) and where does it end(ground) and what controls it being on or off(switch)?
Make yourself a check list, even a mental one, and ask yourself these questions:
1) What's not working or happening that is/isn't supposed to when I power up the system?
2) Isolate the system or component circuit that you need to focus on. If it won't start you don't need to be checking the headlights.
3) Any engine, in it's simplest term, is a controlled vaccum pump with fire in the box. Suck, squeeze, bang and blow.(Diesel's don't really make vaccum, that's why they have a seperate pump, but they do suck, so to speak) Am I getting air?(Rare to cut it off completely) Am I getting compression(super heats the air to ignite the diesel) and IS IT SPINNING FAST ENOUGH(Batteries and starter-cranking system) Do I have fuel? If not, why? Do I have an ignition source(glow plugs for improved cold starting/electric ignition for gassers) Is it getting rid of it-clogged exhaust/turbo ain't spinning/wastegate stuck open.
4) What extra stuff is added to the basic system that might/can control or interfere with it functioning? This is where the brain power kicks in and the grunt work begins. Hang on it a lot of junk/get a lot of junk. Unless it's obviously broken, before you replace a part ask yourself if it makes sense that it's not working as designed, then test it, test it again, and check it some more. Don't assume anything, keep an open mind. Ask someone to help you, two heads are better than one as long as one isn't Fred Flintstone. What threw me when I checked the relay on ours was it showed battery voltage through it to the Fuel Injector Module, but when I cranked it over the extra working current draw across the relay broke the connection and cut the power off. It could have just as easily been the control circuit to the relay, controlled by the Powertrain Control Module, that failed on it. I just had to isolated the circuit and watch it work while somebody else energized the controls. If it's a switch or relay, do I feel it clicking?
5) Get a good component locator chart, Chilton's and Haynes are ok for most stuff, but for really technical detail you need factory(they built it) Mitchell's or AllData. If you don't know where it is and what it is, you can't test it or fix it. Match your wire's colors to the wiring diagram to be sure, then check it again.(A note here-manufacturer's mid year changes, revisions and recalls can change this equation, use some common sense too.)
6) A strong word about Fuel Injection Control modules is due here, along with an associated warning about hydraulics and their dangers. FICM's have electronic voltage amplifiers inside of them that boost the voltage to the injectors to impressive amounts, 115+ volts. Don't probe them with a test light and if you have to check with a DVOM, make sure you are on the right scale or you will ruin your meter, or knock the tar out of yourself, see above about stiches(No, I don't have any). Each one is turned on/off/on/off by the FICM and PCM according to the cylinder firing order algorithim programmed into it. That's why a Cam Position/Crankshaft Position sensor is critical to start the firing sequence. By passing a small magnet over the sensor it generates a brief AC voltage sine wave pulse, this tells/triggers the PCM/FICM where #1 cylinder is relative to the mechanical position of the pistons and valves. That triggers the sequence for when to turn on and off, by controlling the grounds, the fuel injector solenoids, popping off the mechanical valves(pressure differential) to let fuel pass into the cylinders.(WHEEEEE! We're having fun now!) Under my Quest for Knowledge thread I posted that to mount the FICM in the fender cavity was a dumb@$$ place to hang a computer, but I know why they did. The power amplifiers generate an incredible amount of heat, heat kills computers(see below), it's hot under the engine compartment, low to the frame rail exposes it to road splash, and the cabin is a fire hazard. That fender space flows cooler air around the FICM, and the body/fender acts as a heat sink to draw heat off of it. Except for putting fans/coolers on it(your home computer has them, keep them clean), it's really the best overall place to put it.
7) Caution is due here when working on high pressure hydraulic circuits and components on any machine. The HPOP and fuel injection system is not to be trifled with(and NEVER spray water or Brakleen or anything on a hot pump of any sort, it will ruin it by locking it up-study metallurgy) or handled in a reckless or careless manner. They are very precision components that demand the utmost cleanliness and care. You get dirt in your fuel filter housing or system and you can kiss that puppy bye-bye. Hydraulic systems, I don't care what they are-power steering, HPOP, fuel injectors, etc, compress petroleum fluids to extremely high pressures, which equals force, and will cut you or inject fluid into your personal appendages. Keep your hands clear and don't feel for leaks on a pressurized system. You get hydraulic or diesel fluid injected into you under high pressure, you best have a will and good life insurance, that AFLAC duck ain't gonna help you. At best you will lose that limb, at worst you will die a slow, agonizing, expensive infectious death as your organs shut down when that hyd fluid courses through your body. I'm convinced the majority of HPOP's replaced on no starts probably didn't have a thing wrong with them, they just weren't tested properly, and are a high profit item. That being said, most folks ain't gonna have the proper gauges to test HP hydraulics and DON'T NEED TO MESS WITH THEM. Those of you that do, God Bless You and this doesn't apply, you already know the deal.
8) And I'm going to stop here-all any computer really does is turn on and turn off circuits and components in a timely and controlled fashion, regulating when and how much electricity to flow back to ground in order to accomplish a task or command required of it(programming), change the data-change the task. It's a matching numbers game. However, you have to be able to "look" into it and see if it's getting data in(sensors) and sending commands out. That's what a scanner does and there are different levels of quality to achieve different end results. You get what you pay for. Just because you get a trouble code DOES NOT mean that the component referenced has failed. It just means the computer identified that the input/output information of that particular circuit fell outside of the tolerances, or parameters, programmed into the microprocessor chips on that memory board. You still have to figure out why it did it, sometimes it's a failed part, sometimes that part is just dirty and needs cleaning. That's where the math in computer circuit calculations comes into play. Heat is the bane of any computer system, with voltage surges, fluctuations, and unregulated currents running a close second. It kills them, dead! Keep them clean and cool, with lots of air and no liquids around them, and keep a good set of batteries, clean grounds and connections(fall in love with dialectric grease, heat sink compounds and anti-seize,learn how to use them properly). On dual battery applications swap their positions every 6 months-religiously. Closest to the alternator always charges first. REMOVE the grounds first, then the positive cables. RE-INSTALL the positives first and grounds last. This keeps system voltage surges to a minimum and protects the computers, regardless of car, truck, gas or diesel. More PCM's/ECM's and tranny computers have been ruined by bad batteries, failing alternators(thermal runaway) and improper jump starting(a huge risk due to a condition known as thermal jolt-too much electricity all at once)than anything else that can take them out(save for dumping a BigGulp! on your PCM). Try not to jump start. Disconnect your batteries and charge them thoroughly. Someone else posted to speak to him in IDIOT, I don't do that, I educate and respect, as others have educated and respected me. You are a human being with a brain, you may go kicking and screaming, but you can go. As Christopher Robin told Pooh-Promise me you'll always remember: You're braver than you believe, and stronger than you seem, and smarter than you think. That's about it, good hunting!
And remember kids: If you let the factory smoke out of the wire you have set it free from it's captivity, it shall not return unto thee. Possession is Irrelevant.
I see that my status has been upgraded from rookie to member-I am truly humbled.