This instructable will focus on one part of diagnosing an automotive lighting circuit; testing and (granted its not burnt out) restoring an automotive light bulb. I may do a follow up instructable on where to go after these steps if you're still not getting light, but for now, we'll start here.
Your methods may differ but, this is where I always start when I'm troubleshooting a lighting circuit that still works, but has bulb(s) that are not lighting. In this example I'm working on an automotive bulb from an older vehicle, but it should be noted that most of these tips apply to any incandescent bulb of the same style.
Things you'll need:
Step 1: Remove the Bulb From the Circuit
This step implies that you've already noticed a light not working. It also implies that you are familiar with your vehicle and how one would remove the bulb from the light housing. Due to the variety of different makes/models/years of vehicles I won't be giving specific information on how to remove the bulb from the housing. If you're not familiar, consult your owners manual. If all else fails, do a quick search on the internet of how to do this on your vehicle.
It should be noted that you should be careful removing the bulb because they are fragile and can already be broken due to vibration or some other cause. Also, if they have been in the housing/socket for quite some time, they can be stuck due to corrosion, which can lead to a broken bulb if you're not careful. Careful, precise wiggling may be required. Once you've removed the bulb, give it a good visual inspection. Things you're looking for:
-Corrosion on the contact points of the base
-Darkened (not clear) bulb
-How many contacts the bulb has
-Solder contact(s) "egged out"/flattened
-The number written on the bulb, if visible (write this down somewhere for future reference)
My bulb was a 1156, that wasn't working in the socket. It had a relatively clear bulb(after I cleaned the dirt off). Corrosion around the base was horrible (what I expect the actual problem was). There is one center contact on a 1156 and mine was egged out.
Step 2: Check Continuity
Okay, so you have your bulb in question in your hands and you can't quite tell if its burnt out or not by visually inspecting it. Filaments can be hard to read. That's okay because you are able to grab your handy multimeter, and test whether or not the bulb works or not; at least in theory.
Before I do this, I give the base of the bulb a good cleaning to ensure good contact for the multimeter probes and ultimately, for the bulb when it gets reinstalled. I prefer a wire brush on a bench grinder because I like to live dangerously. You may use a hand wire brush, or maybe a bit of sandpaper/steel wool, etc. Just make sure to remove any corrosion or dirt from the base and you'll be ready to check continuity.
In a lightbulb such as this one, current will flow through the center contact(s) and out to the brass base towards ground.
Once its clean, set your multimeter to continuity test and put both leads on the brass base to make sure you have a good connection with your probes. Then, while keeping one lead on the brass base, take the other lead and touch it to the center solder pad(s) to see if you have continuity through the filament. My multimeter beeps and the display goes from 1 to 0 when it gets good continuity. Depending on your model of multimeter it may just show a 1 as an open circuit and then approach 0 as it gets continuity.
Regardless of all that, if you get good continuity, the bulb should technically work. There are things that we can do to improve working conditions, however.
Step 3: Repair the Solder Pad
The base is now clean of corrosion on both the pad and the base. There is continuity through the bulb. But something is still not quite right.
If your bulb still looks new, (many used bulbs are still okay in this department) then you can skip this step and the next step and you're done! The bulb is technically usable and when you apply voltage, it should light up.
If your bulb is like mine, its been in service for quite some time and has developed a crater in the solder pad (see pictures). When you look at a new bulb, the solder pad is nice and round. Sometimes, this crater or flat spot can reduce the amount of pressure applied by the mating contact surface in the socket and cause problems like intermittent operation or just a plain old, non operational bulb.
To fix this, grab your soldering iron and carefully heat the pad and add the slightest bit of solder to it. I find this mainly just adds more flux and allows the pad to flow into a dome again easier and hopefully cleaning out impurities that may exist. I would be quick about it and don't heat too long, because this step could mess the bulb up if not done properly.
Step 4: Doublecheck Continuity
After soldering, double check the continuity with one hand like me. It makes the process more suspenseful and more fun.
If you get the beep, or your meter approaches 0 then congratulations! You've thoroughly troubleshooted and restored a bulb! When you apply voltage your bulb should light properly. If it doesn't light up, then the problem is somewhere else in the circuit which will require further troubleshooting. But that process is for another Instructable.
Step 5: Optional Step. Test With Voltage.
If you don't believe me that it still works, go on, test it. Hook it up to 12v and see it light up. I rigged up some test leads to allow me to take the picture while lighting it. The first picture is taken on HDR mode on my phone. You can kind of see the filament almost. The second picture is closer to the light output. I would say its every bit as bright as a new bulb of the same model.
Most of this may seem like common sense but I just thought I'd share a few tips that get me by and back to operational. Through all these steps, I enjoy saving a bulb of which was potentially deemed bad.
Just to give an example, I recently worked over the lights on a tandem axle truck and got them working again. Just in the "park/clearance" light circuit, there were 9 housings around the truck bed. Only 2 housings out of the 9 were lit up. There are 2 bulbs per housing. I decided to just go through this process for all the bulbs, mainly to clean them also. Only 1 out of the 18 bulbs needed for the circuit was actually bad.
I hope you enjoyed this Instructable and hopefully it will help out someone at some point or spark new ideas. I've entered it in a few contests. If you would be so kind, go ahead and vote it something.