Wiring logic is NOT rocket science. Most OEM harnesses get you confused because of where they are picking up grounds and power feeds. In a universal harness this is simplified, ALL power for the most part comes from the fuse block. So if your troubleshooting a new universal wiring install or a OEM harness, can modify the difficulty you experience in tracking down a problem.
What I try to tell all my customers and novices to wiring is this. Are you smarter than a 3rd grader..? Because wiring a car really comes down to a lesson learned in 3rd grade science class.
(+) --> SWITCH --> Light bulb --> (-) : That's it.. over and over and over again.
Diagnosing electrical problems is more mental work than anything else and while some electrical problems can be difficult to diagnose, many of the challenges can be eliminated by carefully following a well laid out approach.
The key to any successful troubleshooting is to have a plan and follow that plan in order. Skipping over steps, performing steps in a different sequence, will often result in missing an important clue to what the problem is.
- Check the basics
- Confirm the problem
- Identify any additional related problems
- Analyze the problem
- Isolate the problem
- Fix the problem
Occasionally you will think there is a problem, when actually the circuit is performing as designed and the bulb or bulb holder is blown or corroded. Often a ground is loose, or a plug or connector has lost connection due to corrosion, overheating, or simply it got loose.
So.. Before you get started chasing electrical gremlins..
1 – Make sure the battery is in good shape, has a good charge and holds it. If in doubt, go buy a new battery, after your done return the battery if you don't need it. That said, if you're in doubt you probably need a new battery.
2 - Find all grounds, clean and tighten them down (Scuffing the metal below the contact is always a good idea, and use a little di-electric grease too. Make sure the block is well grounded to the frame, and frame is well grounded to the battery. Make sure there is a good ground for the alternator and if the B(+) Connector on the alternator is isolated externally that it's not shorting to the alternator housing. GROUNDS ARE 50% of most problems.. !
3 – If a lighting issue, check bulbs, bulb holders, and housings.
4 – If a issue involving a switch (Starting, wipers, etc..) check connectors for tight connections, clean plugs (nothing melted or bent). Replace and fix connectors or plugs as you can. OFTEN (Say 25% of the time finding a repairing a loose, melted, hot or deformed connector solves the problem.
5 - Check the fuses.. Pull them out one by one and in good lighting check for any breaks.
6 – Last but not least check your fusible link.. bend it, and move it around.. often a partially fried fusible link will give you low voltage issues, which will cause problems.
OK, so now the battery is good, the grounds are good, plugs & connectors are checked, bulbs & holders are checked, and you know your fuses and fusible link are good.
Confirm the problem :
Before going any further see if the problem is gone? if it is then you don't need to trouble shoot any further.
Sometimes duplicating the problem can be the most challenging aspect of the diagnosis, especially if the electrical problem you are concerned with diagnosing is an intermittent problem. This is why you did the first step, to make sure it's not a loose ground or loose connection at the plugs. But that doesn't always fix the problem.
If you made it this far then one can assume it hasn't. So more information is needed and will be useful in duplicating the problem.
So, ask yourself the following :
- How often does the problem occur ?
- How long after the vehicle is started does the problem occur ?
- Does the problem occur only over bumps? If so, what kind of bumps ?
- Does weather have any effect on it ? Sunny ? Rainy ? Cold ?
- Does the car have to be idling, driving, in reverse, moving forward, be in gear, etc.?
- Is there anything at all, no matter how crazy it sounds, that you associate with this problem?
Now you can identify related symptoms:
I consider this step to be the most critical step in the troubleshooting process. It serves to help you isolate which circuit, and which part of the circuit, has the problem that needs to be repaired. This step requires no tools, no tear down, and can be done without under-dash contortions! Being focused and diligent during this step in the process makes your diagnostics fast, easy, and accurate.
What is meant by related symptoms?? You will want to identify any other systems, functions, or circuits, which appear related to the problem you are trying to diagnose. Using a wiring diagram, look to see what other circuits, or functions, are connected in any way to the circuit you are trying to troubleshoot. This will help you to determine points that the circuits have in common, and eventually isolate the problem to a particular section of the circuit.
Often you can use this step to determine if you have a power source, or ground circuit problem. For instance: when diagnosing a radio doesn't turn on problem, identify what the power source (fuse) for the radio is. What other components share that source? Do all of these components work, or do just some of them work?
If you discover that all the other components connected to a particular power source work, what does it say about the power source? It's working, right?! Now you don't have to bother checking it. You will want to do the same thing with the ground points. What circuits share a ground point in common with your problem circuit? Do they work?
By pushing buttons, flipping switches, and turning knobs, you can quickly determine whether or not the power and grounds associated with your problem circuit are good. You may also discover other circuits that share connection points, splice points, or wiring with the problem you are diagnosing. Recognizing that there are other systems, or circuits, that are also not working, you are able to narrow your diagnostics to certain sections of circuits. Knowing an approximate area in a circuit where the problem is gives your diagnostics a definite direction.
Analyze the problem :
We are still narrowing down where exactly our problem is likely to be, and by doing so are targeting our time and energy in the most likely place to find the problem. In your effort to determine related symptoms you looked for other systems, or circuits, that may have been connected in some way to your problem circuit. You then determined if these circuits were working or not. Now you need to decide what the results of your work mean for your problem circuit.
Analyzing symptoms is relatively easy if you discover a circuit not working which shares a power supply (fuse), or a ground, with your problem circuit. You would now have reason to head directly to the power supply, or ground, for testing. This would be a best case scenario.
What if you discover a system isn't working, only it doesn't show on your wiring diagram as related to your problem circuit? Do you ignore it and assume it has nothing at all to do with your problem circuit? Having had this experience, I would suggest that you investigate! Never assume something isn't related just because it doesn't show on your system circuit diagram. Since, many electrical problems are created by corrosion; you will often find this very scenario.
For instance, let's say we had a radio that was inoperative, and during our diagnostic process, just by playing around with everything else in the car, we discover the power seats don't work. The problem is that the seats don't show up anywhere on the wiring diagram for the radio! They also don't share a power supply, or a ground, with the radio. Although our job is much more difficult now, we still only need a good wiring diagram to continue with our diagnostics. Think about which components two circuits may have in common that wouldn't show up on a wiring diagram.
Depending on the manufacturer, often times junction connectors only show the location of wires associated with the system circuit diagram. That is, the radio diagram will show a junction connector with only a single red wire passing through it, but that particular connector may have a dozen or more wires that it serves to connect. In our scenario where the radio and seats don't work, we need to compare the two system circuit diagrams and determine if the two circuits utilize the same junction connector. If we were to discover that the two circuits shared the same connector, this is where we would concentrate.
Isolate the problem :
The next step, Isolate the Problem, is where we get out tools out, and do some testing. So far we've done all of our analysis by using a wiring diagram, turning systems on and off, and looking for commonalities between circuits. Now we will use our Digital Multi Meter (DMM) to test the circuit and identify exactly where our problem is.
Start with identifying the path that the electrical current should be taking through the circuit. As mentioned before wiring logic is 3rd grade science. Every circuit in the automobile needs three things to work: a source of electricity (usually a fuse), a load (something to do the work, a light bulb, motor, solenoid, etc.), and a ground. Your job is now to identify these three things in your circuit and link then together.
If you haven't already, on a printed copy of the wiring diagram you've been using in your analysis, trace the power flow for the circuit in question. Begin at the top of the diagram with the source of electricity for the circuit. Trace the wiring with a highlighter from the source, through the load, and on to the ground. Using a highlighter, rather than a pen or pencil can help you to visualize what should be happening in the circuit without making confusing marks on your copy of the wiring diagram. Taking a few seconds to highlight the circuit will save you valuable time as you begin to test. It will also help to ensure that you have a good understanding of how the circuit works.
Now using your knowledge of what is working, and what is not working, select some test points on the wiring diagram where you will measure voltage available and voltage drop. You have already determined, in the previous step, what part of the circuit must be okay, and what part may be suspect. You are now focusing your efforts on the parts of the circuit that may have a problem, rather than the entire circuit.
When picking where to test the circuit there are a couple of things to consider. First, select points that are easiest to get to. Secondly, try to test large portions of your affected circuit at a time. If you can divide the part of the circuit you are testing in half and determine if that section is good or not, you've just eliminated a large portion of the circuit that doesn't need further testing!
As helpful as testing large portions of the circuit at once can be, it will take a secondary role to points of easy access. If testing a large portion of the circuit requires more time to access the test points, then I will sacrifice the benefits of testing a large portion for being able to easily access my test points.
Locations that make for good circuit testing points are connectors, switches, relays, and sometimes the component itself. Always choose the easiest to access first, before digging into the more difficult areas of the vehicle to test. If you've done enough homework, your actual physical work to find the problem will be minimal. Continue to select the next easiest test points until you have narrowed down the problem to a specific area of the circuit.
To further isolate the issue you can diagnose by tracing voltage drops.
It's good to note that you are chasing very small resistances, often smaller than a single ohm. The resistance (ohm) scale on your DMM probably bottoms out at 200 ohms, making measurement of single-digit values tricky. Instead, use the voltage scale, which on most DMMs is accurate down to several millivolts.
Start by turning on the offending circuit. Now we'll measure the battery voltage. We need to know the exact number you see when metering across the battery posts. And I mean the lead posts themselves, not the clamps. It should be around 12.5 to 12.8 volts if the battery is fully charged.
For an example lets use testing a dim headlight.
First lesson: Electricity always runs in a circle, and the ground side is just as important as the hot side.
Second lesson: Use a little systems analysis as mentioned before.. If only one headlamp is dim, you can skip troubleshooting any part of the circuit that's shared with the one that's working.
Back to the Dim headlight:
Probe the connector on the dim headlight. The black lead on your DMM should go to a good ground. The voltage you meter at the low-beam lug, as it turns out, is about 11 volts. That's lower than our system voltage at about 12.5--but not low enough to explain the severe dim-out. Now probe the ground lug at the bulb connector. Surprise! The meter reads nearly 4 volts--it should read zero. This indicates a resistance in the ground side of the wiring, leaving only 7 volts for the filament.
As you're metering the ground side, suddenly the voltage on the meter jumps up. And it doesn't jump to the 11 volts we saw before--it jumps right up to 12.5 volts, exactly what we can meter at the battery. The bulb goes out at the same instant.
You're metering full battery voltage. That means there is lack of continuity--an "open" in the circuit somewhere between the DMM positive probe and the battery ground. If the open resulted from a burned-out filament or a broken wire on the hot side, you'd see zero volts. The open is on the ground side for sure. What used to be a resistance, around 1 ohm, in that ground circuit has suddenly become an open, with essentially infinite resistance.
Culprit ? It's a broken ground wire, replace the wire, problem solved; at least until you go around front to check the lights. Now they're both the same color. Perking up the dim one suddenly makes you realize they're both less than brilliant--which is what I'd expect when I meter 11 volts at the bulb socket instead of the 14 I'd expect when the engine is running.
There's still a resistance in the circuit, but this time it's between the battery and bulb. Back to the DMM.
Meter between the battery positive post and the clamp. You should see very little voltage there. With the lights up, the total draw on the battery is 15 amps or more. Any resistance between the clamp and the post will cause a measurable voltage drop. It shouldn't be more than a few millivolts. Chase the circuit toward the lamp, one metal-to-metal junction at a time. Probing between the input and output of the headlamp relay shows a drop of nearly a volt.
Popping in a new relay puts that reading down to a few millivolts. And both headlamps are blazing. There should never be more than a few hundred millivolts of drop across any connector. The total drop in any circuit shouldn't be more than 1 volt, whether it's a dome light drawing 500 milliamps or a starter drawing 200.
Fix the problem:
Replacing components is relatively straightforward if not always easy. Repairing wiring harnesses can be far more challenging, especially in an OEM situation, due to the wires being wrapped and hidden up amongst the dash.
Good thing is MOST problems will be at either end of the wire and rarely in the middle of the harness. Unless of course that harness is rubbing against a sharp screw or piece of metal, and in that case repair and protection will be required.
Where wiring harnesses can be repaired, soldering is the only acceptable method for such repairs, use of butt connectors, three way connectors, or other crimp type connectors should be avoided. There are two methods to do this, you can remove the plastic sheath of a butt connector, crimp the connector, and then solder the wire and connector or use the solder ring butt splices as noted earlier in the article.
Keep in mind that not only do you want to repair the problem keeping the circuit from working, but you also want to find out why the problem happened in the first place. If the circuit was open because a mouse chewed through the wire, there may not be much you can do about it, but if you found a component that appears to have overheated, or had excessive corrosion in a plug due to a windshield leak, or loose connectors you will want to repair the root cause.