Fix a dead spot in a halogen headlight

Wasn’t satisfied with the amount of light being thrown by the headlights on our car, so decided to replace the OEM halogen bulbs with a new, whiter and brighter bulb**.

After wrestling to get the bulb in (c’mon Ford, give us a little room to work with!), I turned on the lights and saw this on the wall of the garage.

Huge weird dead spot in the middle of the halogen beam, and a big hot spot next to it

Took the car for a drive and, indeed, visibility was — poor — from the driver’s side headlight I’d just replaced.

Knowing that halogens are a little finicky, my first thought was that I’d smudged or somehow gunked up the bulb and was going to need to do the whole procedure again with a new bulb. Was not looking forward to that.

Did some more reading (thanks, Reddit!) and went down a rabbit hole on how the whole assembly for headlights works. The big thing in headlights is the reflector that, well, reflects and shapes the beam down the road, irrespective of bulb type. Those reflectors are designed and tuned for a particular bulb type, shape, and placement, and any variation from that can have negative effects. (This is why, for example, there are a ton of posts out there on why not to do a 1:1 replacement from halogens to LEDs, for example, without changing out the whole system — the placement of the light source on LED bulbs is different, and will likely reflectively throw light in all sorts of unexpected places, like oncoming drivers’ eyes.)

One of the threads I found noted that it was possible that if the bulb isn’t seated completely, it will have results similar to what I was seeing. This is because halogen bulbs have three tabs on them that seat into the housing, and if only two of the three tabs are locked in, the bulb will be on a slight angle and mess up the optics. The bulb will still stay in with only two of the three tabs seated, but it will be on a slight angle and completely mess up the way the beam is reflected and thrown.

All three tabs need to be seated correctly

I put the nitrile gloves back on, reached in and, sure enough, it sure felt like the bulb was on a little bit of an angle. I pulled the bulb out and felt around and then fiddled with the placement until I was relatively sure that all three tabs were seated properly.

I turned on the lights and…

Success! Totally glad I took the “before” photo so I could compare and see if what I was changing had any effect on the outcome.

Much better! Not only was the shape of the driver’s side beam pattern symmetric, it also totally matched the shape I was seeing from the legacy bulb on the passenger side that had not been touched.

Going to try it out again and road test things tonight, but this seems to be the winner.

** – Went with the Sylvania Silverstar Ultra (H11), but bulb brand is irrelevant for this issue.