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Discussion Starter #1 (Edited)
Electrical diagnosis is usually a fairly simple process; Know the problem, time it occurs, track the problem, and correct the problem.

Let's assume that the headlights in your vehicle are dimmer than they normally are. They didn't slowly dim over time, it was rather sudden. The brights are dimmer than usual as well.

Now, we've got our symptom. Dim headlights can be caused by a few things: Corroded battery terminals, old lamps (bulbs do lose lumens over time, but not many to be abruptly noticeable), breaking ground/positive wire, corrosion on the switch, corrosion on the headlamp connectors.

We can always poke and prod around with a multi meter set on either volts or Ohms (Volts are generally easier to understand as resistance can vary circuit to circuit, but that doesn't mean it's a useless setting on your DMM, it's actually very informative for specific systems, anyway, onward!) or we can do a quick visual inspection of the circuit itself.

First off to the battery. If there is a lot of corrosion on the battery terminals themselves, this can certainly cause a low voltage/high resistance on the entire electrical system, but since the other systems aren't exhibiting any problems, this may not be the lead cause.

Next to the headlamp connectors. Is there high corrosion, oil, dirt, broken wires?

Since the visual inspection has passed, time to start poking around. Turn your ignition to the On position, but not running. Then set your DMM to V, 0 out the voltage by touching the prongs together, then find out the health of your battery both at the terminals and at the lugs (it doesn't matter, on most DMMs, if the red is to the positive and the black is to the negative, it will read either 12.65V or -12.65V in which case it gives the same reading in voltage, just negate the "-").

Your battery should be at a standard 12.65 at the terminals and at the lugs. 12.20+ is a healthy battery. Anything above 13 (after the battery has been sitting by it's lonesome for 30 minutes) means you've got a bad voltage regulator in your alternator (replace it soon!)

Next is to test the individual ground wires which are the black wires coming off of the connectors. Back probe the black wire through the rear of the connector (wire side) while it is still connected to the headlamp and touch the battery's positive. It should read roughly 12.65 as there is not a lot of voltage drop in this circuit.

After testing both grounds, you should come to the consensus that the grounds are not the issue. Time to check the positive wires. Back probe the Violet/Orange wires and touch the negative battery terminal. Does it read 12.65? Let's say it reads 11.00V. That's quite low, and definitely a start to finding the problem.

With the high beam switch clicked in the On position, check the high beam wires; Red/Orange. 11.20V is what the high beams are reading. Now we can infer that on two different circuits you're getting low voltage across four individual wires.

Since the low voltage across four individual wires will usually not mean that wires are the problem, we should start checking switches. Before we tear into the steering column to remove the high beam switch, let's check out the source for the lighting; the headlamp switch itself. This is much easier to remove and check both the high beam switch, the headlamp switch, and the wiring to the switch.

After removal of the switch, make sure it is still connected to the connector. With the headlights off, find a Known Good Ground (battery is usually best), then test the Red wire. We'll say it reads 12.65V. Next, turn the headlights on and test the Light Green wire. Reading 11.25V? Possibly found the problem right there. But let's be sure. Use a lead wire with the specified fuse (40A) in junction with the wire. Connect one of the leads to the positive terminal, the next end of the lead should be jumped to the Light Green wire. Are the headlights the proper brightness? Do you have 12.65V at the Violet/Orange wire? If you do have 12.65V at the headlamps, then the headlight switch is the problem.

Congratulations, you have successfully learned how to test a simple electrical system with your DMM!

Edit 1: This post will probably need restructuring, organization, and, in some places more/less in-depth. If more explanation, or less, is needed, feel free to let me know or correct me. I will put the corrections in bold.
 

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Excellent Troubleshooting Advice

You need to be a technical writer , that was probably the most concise instructions on headlight troubleshooting I have read. It would have saved me some time and frustration a year ago if I followed your directions. I will pass these on to others. Thanks and I will look for more of your advice.
 

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Discussion Starter #3
You need to be a technical writer , that was probably the most concise instructions on headlight troubleshooting I have read. It would have saved me some time and frustration a year ago if I followed your directions. I will pass these on to others. Thanks and I will look for more of your advice.
Now if only I could get paid to be a technical writer.

Thanks!
 
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