Focus Fanatics Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
4 Posts
Discussion Starter #1
My driver's side headlight won't turn on -- neither will the running light on that side. I've replaced the headlight bulb, that wasn't it. I replaced the fuse, and it's not that (the original fuse looks fine anyway).

But the driver's side turn signal works -- and that's the same bulb as the running light that won't turn on otherwise.

Any help here would be really, really appreciated. I'm stumped.

Mods, sorry if this is in the wrong forum.

JA
 

·
Registered
Joined
·
17,159 Posts
Could be connector on headlight and bulb on running light. You need to check for power at the headlight socket and the running light uses one of two filaments for its' lower light output. The turn signal uses the other filament with more light output, two lights essentially in one bulb there.
 

·
Premium Member
Joined
·
44,595 Posts
Moved it to General Tech Chat for you, better for most repair questions.

Voltmeter & test light are your friends, guessing only takes you so far & you're past that now.

Need to check for both power & ground at the headlight socket, running light has a proven ground from the winker working.

Should be more than one headlight fuse as well, for high & low beams.
 

·
Registered
Joined
·
4 Posts
Discussion Starter #4
Great advice, thank you both.

One thing I didn't mention is that the high beam works fine.

I'd love to get a voltmeter but I don't know what kind to get. Apologies, I'm very new to electronics (don't even know what a test light is).

Any further thoughts or advice would be so appreciated.
 

·
Premium Member
Joined
·
44,595 Posts
At it's most basic, a "test light" is just a bulb with two wires attached so you can touch power & ground locations - if good the light will go on.

Commercial versions have a handle with a bulb enclosed & a pointed probe, attached to a wire with an alligator clip on the end for the other end of the circuit.

Hook the clip to a "known" good ground, and you can probe for power at any location. Always helps to touch a known power source to check if your ground is actually good, such as touching the battery positive when working under the hood.

You can actually use it in reverse to look for a good ground by attaching the normal ground clip to battery "hot" or positive and probing possible ground locations looking for the light to turn on.

Fanciest version I've got is electronic and needs a hookup to both power & ground so it can sense either hot OR ground with LED's that show green or red to show what you touched. NOT handy most of the time to make that double hookup, so seldom used.

Basic test light is both cheap and not subject to false readings when checking lighting circuits. If the test light lights, you can be fairly sure a bulb hooked up the same way will as well. (voltmeter can show power in a circuit that won't carry enough current to light a bulb - poor contact somewhere in the circuit)

For mutimeters, an inexpensive one is good to start. "Analog" type with a needle is just fine for basic measurements, they often can tell you MORE than a digital unit that's best when more precise or small measurements are needed.

Initial use of one is usually limited to checking for voltage & testing dead circuits (disconnected ones) for continuity between two points with the "ohm" meter portion to test resistance.
 

·
Premium Member
Joined
·
44,595 Posts
Now, when testing lights you can use the "test light" to see if you are getting power at the connections for the light bulbs - takes the bulb & ground portion out of the circuit to check one portion at a time.

Multimeter can be used as well, with that comment considered that just showing power doesn't mean you have enough to light a bulb.

Multimeter is more handy for other purposes. You can test a bulb for resistance to determine if it's circuits are good, and once the assumed ground wire is checked for power you can switch to the ohm meter portion to check the quality of that ground - need to see minimal resistance between the ground contact & the location your meter contacts for a good ground (such as the battery negative or clean frame/engine block location).

Some of this testing can isolate a problem. For example, if the power/ground looks good at a connector, and the bulb tests OK, you know the problem is likely a poor connection between bulb & connector/socket - one of the most common issues.

No power getting to the connector - check further back in the circuit such as at a fuse to see where you DO have power to further isolate the problem.

No Ground? The ground wire only goes a short distance through the harness before going to "frame ground" at a bolt to the body. Poor connections there are common as frame grounds corrode.

All of this is to help you avoid hair loss from frustration arising from not being able to SEE what's happening. The tools give a window into what is happening electrically that your eyes can't see at all directly.

At the most basic, you need a complete circuit before anything will happen - battery through wires/switches/fuses to a load (bulb in this case) & back to the negative terminal of the battery through wires & the car body.

Again:

Break that down into segments and you can rapidly reduce the possibilities for where a problem lies. Good ground? Then that half of the circuit is fine. Power to the connector? You've just checked battery/wiring/switch & fuses and they're all OK.

Once you find something that DOESN'T check out, then you can move along testing it in part to isolate further.

Luck
 
1 - 6 of 6 Posts
Top