It turns out it’s a weirdie plastic twist-on bulb that apparently uses a notched hole in a printed circuit board as the “socket”.
I don’t have anything like that in stock. So I walked down to the brand new Advance Auto Parts here in town to buy one. They looked it up and said those don’t normally burn out in a car only a few years old, and in any case, they don’t stock them and I’d have to get one from the dealer.
This did not exactly thrill me, so it occurred to me that I could probably find the bulb elsewhere, probably cheaper. But what was it? I didn’t have a type number or anything. Some research on the web revealed that there were a lot of people looking for this bulb, and that it was a Ford part number F5VY-13B765-AA (or perhaps F5VF-13B765-A or maybe E83Z-13466-A). There were a few mentions in the pages I found to this being an 80mA bulb, and that’s a believable current for a bulb like that. The existing bulb is marked “2W”, which might mean that it’s a two-watt bulb or it might not. There’s a chance that the E83Z bulb mentioned above is a 1.2 watt version. There’s also a chance that one of those isn’t a green painted bulb like the original. I don’t know.
I decided to try out the parametric search for light bulbs available on the Don’s Bulbs site. I could make good guesses at the glass type (T-1½), voltage (12-14V) and current (.08A), but what was that base? Happily, Don’s Bulbs has pages where you can search for bases by drawings. After peering at the “automotive” category for a while, I decided it was probably a “neowedge” of some sort, but couldn’t figure out which variant. No problem, the site lets you search on broad base categories, and even a vague type of base narrows down the huge pile of results nicely. The closest match I could find was a GE type 91646, which doesn’t seem to be available much any more. It does sport a B8.4D base, but I wasn't at all sure it was the right one.
I went back to searching the web, adding “neowedge” to my search terms. I found many more pages of people trying to find this bulb, and a bunch of pointers to LED bulb vendors. However, many people had bought the LED replacements, only to have them not work.
This gave me an idea - I could convert the existing one to LED myself and have it fixed the same day! I rounded up a blue LED and calculated that an appropriate voltage dropping resistor would be about 525Ω. Some rummaging around produced some 560Ω resistors, which should run the LED at slightly lower current, for a little less brightness and longer life. I crushed the burned-out bulb so I could solder the new parts to the resulting leads.
I then assembled the parts into something that should fit into the space occupied by the original bulb.
I remembered the LED bulbs I had made for my pinball machine, which runs its bulbs on AC, so the polarity doesn’t matter. But my car uses DC, so any way I assemble it is likely to be backwards. Then I realized the base was symmetrical, so if it didn’t light, I could just install it the other way. I hooked it up to a power supply to make sure it worked.
Then I went to install it in the car. Unfortunately, it didn’t light. I pulled it back out to turn around, and saw that I had broken one of the fine wires. Rats!
I took it back inside, and attempted to solder the wire directly to the terminal, but without success. I tried every flux and solder I own, slowly melting the plastic base, but it was beyond me. I considered finishing building my tack welder, but that’s a biggish project and there’s a pretty good chance I would fry the LED in the attempt anyway.
Greatly annoyed, I called the dealer to see if they had the right bulb in stock. Naturally, they didn’t — and they wanted more than fourteen dollars for the stinking bulb! I reluctantly told them to order one and call me when it got in. They said it would be in by Saturday morning.
Back to the web. I noticed a lot of the references to LED replacements were to the same place, superbrightleds.com so I figured I’d have a look. They have a facility to look up bulbs by car make and model, but didn’t list anything for my clock. So I looked at their “automobile, instrument cluster and gauge, T1.5 Base - New-Wedge, B8 Type” page. After clicking around for a while, I saw they had some nice big clear mechanical drawings of the various bases, complete with 3D renderings. From this, it seemed that I had a B8.4D based bulb. They offer LED bulbs with this base in two brightnesses, and six colors, and they don’t cost much at all ($1.59 for the regular brightness). The original bulb had a green coating, but I thought red would be nice looking, and avoid impairing my night vision. I ordered one each of the red and green, in both regular and high power.
I went by the dealership on the way to Balticon on Saturday, but they had closed early for Memorial Day weekend. Fie on them.
The LED lights showed up in today’s mail.
I popped the regular brightness red one in, it fit perfectly, and lit on the first try. I’m looking forward to seeing it at night. As for the dealer, I still haven’t heard back from them, and at this point, I no longer care about their overpriced bulb.