Forum topic title is wrong and I can't figure out how to fix/edit it. The problem is with the LOW beam, not the high beam.

 

Meanwhile, back at my 1995 Legacy L wagon that I just bought last week.

 

Investigating a problem with ONLY the left headlight. High beam works, low beam doesn't (both work fine on the right). When I pulled the plug off the back of the bulb, an obvious problem appeared. The plastic casing around the one plug at the top (of the 3-prong plug) was all melted and crispy. The bottom two plugs seamed fine. So I'm guessing that top plug is what activates the low beam (duh) and something shorted it out?

 

Before I go buy a new plug and splice it on there, I wondering what could have caused that meltage? Will it just happen again? The problem is, I know ZERO about electrical testing and unless it's incredibly EASY to test (like one or two steps), I'll probably be lost. Any assistance greatly appreciated. 

Edited July 31, 201510 yr by xntryk1

Very common problem with Subes. The connector gets hot then turns brittle. Rock auto has new connectors.

Corrosion or loose fit of the connector. Electricity creates heat when too much amperage moves through too small of an area. Eventually the heat gets high enough to melt the plastic. Same thing happens when a fuse blows. Too much amperage through the fuse causes it to heat up and burn. Only the connector problem is much slower.

Ah, so a loose connection could have caused that. That makes me feel better about simply replacing the plug. And it just so happens that I already have a Rock Auto account, so off I go! Thanks guys (and/or gals ;-))! 

I kindly suggest you to install a High temperature, Ceramic plug.

 

So you'll get rid from problems regarding Headlamp bulbs on that Subie.

 

Kind Regards.

Thank you for the suggestion! As long as they're not outrageously expensive, I will consider it. :-)

Ceramic sockets handle a lot of heat but leave the terminals exposed to weather and moisture.

I installed High temp sockets on my 96.

Dorman 84790

The wires are larger guage and the terminals are heavy duty. Plus the connector is housed in a rubber cover to keep out water and dirt.

That's what I was looking at on Rock Auto - Dorman 84790. $8.92 (including shipping) for a PAIR of sockets. Seems like a good deal to me. 

I think I've had them in my car for about a year along with upgraded larger guage wiring and relays, and running Silverstar ultra headlamp bulbs for about the same amount of time. They've held up fine so far.

So meanwhile, back at my left headlight. I replaced the socket this morning and it didn't help. In fact, things have gotten worse. Whereas before (with the old socket), I had high beam, but no low beam, now I don't have EITHER. All I've got is a very dim glow from the bulb in either high beam OR low beam. I should have known I couldn't get off that easy (by just replacing the socket). What should I try next? Thanks. 

OK, just taught myself how to use at least one function of my digital multimeter. And with that knowledge, I've learned that one of the wires coming to that LH socket is not getting the same voltage that the equivalent wire is getting on the RH side. Whereas both of the bottom wires were showing about 9.85 volts on the RH side (the battery is obviously getting weak, since I've been playing with it so much), only ONE of the bottom wires was showing 9.85 volts on the LH side. The other wire (red w/blue stripe) was only showing about 1.85 volts. So I'm afraid to ask what that means... 

Red/blue is the power for both filaments.

 

You should be testing with the black probe connected to the battey negative.

 

Red probe to the blue/red, black on battery -, see what voltage you get. Compare that to battery voltage to determine voltage drop through the circuit. Should be less than 1 volt difference. So assuming 10.5 at the battery, you should have no less than 9.5 at the headlight plug.

 

 

This is probably a problem with the ground wire for the low filament which should be the yellow/blue wire. Possible the wire is corroded somewhere further back in the harness.

 

To check that you're going to test voltage drop on the y/b wire with the lamp plugged in and turned on. You need to pull back the rubber cover on the plug to get access to the back of the terminals. Black probe to battery -, red probe to the y/b wire terminal.

Compare that voltage to that on the other two wires.

Now switch from low beam to hi beam and check all three again.

 

When a circuit is properly grounded and is in operation the voltage should disappear after the load (the lamp) indicating a good ground path. If there is a large voltage remaining after the load this means the ground path is poor, meaning there is a loose or corroded connection or damaged wire somewhere between the load and the ground point, which is the headlamp dimmer switch in this case.

 

But since we know the lamp on the other side works fine, the dimmer switch is not the problem. The yellow/blue wires join just a few inches back in the harness on the left side. Likely the problem is going to be between the lamp connector and the splice where the wires join, or at the splice.

Thank you! Just wondering - would it be feasible to simply bypass the possibly bad ground wire by using a different wire? Since I've already got the wires stripped (at the ends) and the old, bad socket removed, it would be a simple matter to run a GOOD ground wire from the new socket to somewhere else. Or will that not work for some reason? 

You just need to follow it back to the splice in the harness which should only be a few inches away from where the three wires go into the rest of the harness.

Cut the bad out and wire in new from the splice to the connector.

But what if the bad part is farther beyond that? I guess it's not just a simple matter of running a good ground wire directly from the socket. That would be too easy...

It won't be. The yellow/blue wires are spliced together, as in two wires become one. From that splice the ground path for both low beam bulbs is the same wire.

One bulb works fine, which says that wire (from splice to the dimmer switch) is fine. The problem is between the bulb that doesn't work and the splice.

OK, good. I'll try all that out (next time I feel up to tackling it) and report back. Thanks. :-)

... wondering - would it be feasible to simply bypass the possibly bad ground wire by using a different wire? ...

 

On old school Subarus, the wiring has a permanent Positive (+) always live on both headlamps, what interrupts the Circuit, is the Ground, so the Stick on the steering sends one Ground for Low Beams, and another Ground for High Beams; I bet that your modern Subaru is wired likewise, so, in case you place a direct ground, you'll end with a Headlamp permanently on duty.

 

Kind Regards.

Ah, that makes sense. Thank you for explaining.

OK, just tested the voltage on all 3 wires leading to the LH headlight socket (while the engine was running). Here are the readings:

 

With headlights off

Red (High Beam) = .14V

Red w/Blue Stripe (Power) = .02V

Yellow w.Blue Stripe (Low Beam)  = .02V

 

With LOW Beams on 

Red (High Beam) = 14.14V

Red w/Blue Stripe (Power) = .09V

Yellow w.Blue Stripe (Low Beam)  = .04V

 

With HIGH Beams on

Red (High Beam) = .04V

Red w/Blue Stripe (Power) = .07V

Yellow w.Blue Stripe (Low Beam)  = .09V

 

Note: these are just the readings from the bare ends of the wires, without the socket/bulb attached. Electrically-clueless me is wondering if that's an insufficient test or what? Should I try it again with the socket/bulb attached to those wires? 

"Likely the problem is going to be between the lamp connector and the splice where the wires join, or at the splice." This prompts another question. How exactly does one actually access this "splice" - by dismantling the entire car?? It seems to be in a very inaccessible location... :-(

More possible clues (for those who know what they're doing - unlike me). I went ahead and connected the new Dorman socket and bulb to the bare ends of the LH headlight wires. Here's what happened - the high beam indicator on the dash came and stayed on (whether I switched from high beam or low). Also, the voltages at the back of the socket were ALL negligible (.02-.05 or so) and the bulb did NOT light up at all, not even dimly. Wonder what all that means...?

Appreciated xntryk1,

What is really happening, is that you are mistakenly / involuntary "Mixing" the wires in a bad way, let me Explain: What is inside any Halogen bulb, is a Resistance that in fact, makes a short circuit, joining together the Positive (+ or Power) and the Negative (- or Ground), and thus makes the Resistance to get terribly Hot, making Light and Heat, so you got a Permanent "High Beams" indicator light on the instrument cluster, because you are joining together (Using the bulb itself as a bridge), two wires that are not intended to be joined, and that could be pretty Dangerous for the health of the electrical system, specifically for the relays.

What you Really *Need* to do on a Test, is to Not Test the Wires among them or against each other only.
 
So, Connect the Negative (- or Ground) terminal from the tester, to the Bare Ground (any clean bolt) on the car's Body or directly to the Negative (-) Terminal on the Battery, and then, you use the Positive (+ or Power) Terminal from the Tester, on the Headlamp's Wirings, in order to find wich one is the Positive (+ or Power) and then mark it using a small portion of Masking Tape; then, such permanent Positive goes to the Center pin of the H4 Bulb, usually the Middle wire on the Socket.

The other two wires belongs one for the Low Beams' Ground, the other to the High Beams' Ground.

So, once you find the Positive, then you do the Test on the Negative Wires...

 

Kind Regards.

Thank you for responding, JesZek! 

 

First question: you say, "use the Positive (+ or Power) Terminal from the Tester, on the Headlamp's Wirings, in order to find which one is the Positive"

 

How will I know it's the positive (or power)? What will tell me that - the reading on the multimeter?

 

And I was worried that the wires might be getting mixed up. The fact that the NEW wires on the new Dorman socket are different colors wasn't helping me keep track. But after researching around various Subaru forums, I THOUGHT I'd managed to find and define each wire (by its color). But I was probably wrong.

 

Can someone please tell me if this right or wrong?

 

Red = High Beam Ground

Red w/Blue Stripe = Power

Yellow w/Blue Stripe = Low Beam Ground

 

And if that's correct, can someone please tell me where each of those wires should connect on the socket (left/center/right)?