Hello all,

I recently purchased LED headlight bulbs for my 1999 mazda miata, as the old halogens were starting to dim. Upon installing them, I noticed only one the driver's side was working. I quickly swapped the headlight not working to the opposite side(driver's side) and it worked no problem, so I knew it wasnt a bulb issue. I than tried replacing the headlight fuses...., also no bueno. So im kinda stumped. I even checked if there was power getting to the electrical connector on the faulty headlight side, there is. So im left with 2 options, replace the current connector, or check the relay, or a ground issue maybe? Im at a loss. Let me know if anybody has any ideas, Thank you!

 

I've not personally dealt with this issue before, but I will still try to point you in the right direction.

A few questions:
1) Did you try the working LED bulb in the non-working side to prove that it's not a bulb issue? You mentioned trying the opposite, but this could still be worth doing.
2) Does your car have Daytime Running Lights? If so, the problem appears more complicated to diagnose, but if you do not have DRL, then there are very few "moving" parts in this system.

I'm sure you're already looking at them, but if you're not aware, the wiring diagrams on Mellens.net are incredibly useful for diagnosing electrical issues:
1999 Wiring Diagram

In either case, in the event it's a ground issue, you'd be looking at JC-02 (underneath the left headlight), or JC-03 (in the driver's side of the dash, behind the dash).
JC-03 and G202 seem pretty difficult to find on wiring diagrams, but the 2000 Wiring Diagram shows a vague picture. JC-03 will be a flat plastic box with multiple black wires coming out of it, I would hope you could trace the wiring out of that box to figure out where it's grounded to the chassis. But if it's anything like JC-02, it'll have a terminal lug connected to the edge of the plastic box, which is then screwed into the chassis.
This is what the 2000 Diagram shows for the location of JC-03.

In this case, JC may as well stand for "Jesus Christ", because that looks insane to get at. I doubt you'll be able to see it if you stick your head under the steering wheel, but you may as well try!


You mentioned that there's power at the faulty headlamp connector. How did you determine that? Did you hook up a multimeter to the power lead at the connector, and the other end of the multimeter simply touched to a bare metal spot on the chassis? If so, and you saw 12V there (I'm making an assumption that 12VDC is the correct headlamp voltage), then I'd have a hard time believing the connector is the issue. Double check the connector by probing the return pin of the connector to a bare metal spot on the chassis. If the connector return pin has continuity to ground, and the connector also delivers the proper voltage with respect to ground, then try cleaning the contacts of the connector. Deoxit would work, or isopropyl and a good bit of scrubbing.

After all this, I would have to blame the bulb itself. Again, if you have voltage at that connector when your lights are switched on, then everything else upstream in the system is working as it should.

To throw a hand grenade at that idea, and this is completely speculative, perhaps the faulty side is not getting enough current. I wouldn't think this is the issue, as I would think LED bulbs draw a lot less current than their halogen counterparts, and if your old halogen bulb works, I'd have to imagine the LED bulb would definitely work. If you have spade connectors or otherwise which would fit into the headlamp connectors, then make up some wires to go from the connector to the bulb, and insert an ammeter in the middle of the power-carrying wire between connector and bulb. You can then compare the measurements between your faulty side, and the working side, when the headlights are switched on, to see if there are any differences.

Good luck, man. I hope I've given you some ideas.

 

Good, informative post. Have a poscat.