The light has 3 settings: On, Door open, and Off.
When switched to 'On' the light works and stays on.
But when switched to 'Door open' it does not work for any door.
Also, it does not come on when removing the key from the ignition.
I looked at the owner's manual fuse locations, and checked each one I thought might be the culprit, but nothing.
I've opened all 4 doors and pushed the plunger switch in the door frame repeatedly, but it does not light up.
Surely all 4 door plunger switches can't have simultaneously failed.
I haven't pried the light cover off, but it looks like there's only 1 light bulb.
Any thoughts?

 

I will try to remember to look this up in the repair manual when I get home. In the meantime take the dome light out (if it isn't too hard) and use an ohmmeter to verify that all the connections are sound. It could just be that the connection on the switch at the center position is corroded and needs to be cleaned.

Also search around a bit. There are a lot of threads on related dome light issues, including some that say that installing an aftermarket head unit somehow or other disabled the dome light in the manner you describe.

 

The wiring diagram shows that each of the door open switches are wired into one big harness, they don't have a separate plug that I could see which would have disabled all of them. The way the door switches work is that they short a single wire to body ground. So if you take one out there should be 12V between the wire and the body.

The cable from the dome light goes to a plug under the dash by the driver's left knee (at least it looked like that in the drawing). There are only a few wires on that plug. There are a bunch of other plugs down there and I don't know how to tell you which is which beyond that. However, if you set the dome light to "on" and unplug them one at a time when the light goes off you will have the right one. Check it for corrosion.

There must be some other device involved, since the light fades out slowly, but I could not easily figure out where or what that was.

For future reference, fuse 3 (10A) under the hood is for the ceiling light, dash lights, trunk light, and license plate lights. Just in case you happen to short something and blow it while working on this problem.

 

Well, I took the dome light out and checked it with the multimeter. Everything looks fine up there.
I'm at a loss to figure out what's causing this problem.
It's never happened before.
The car is completely stock and I have made no mods save for adding a grid charger 2 years ago, which went without a hitch.

 

Define "fine". Is it getting power on the line that leads to the center switch position at the appropriate times? Or do you just mean that the resistance from the input pins all make sense for the various switch positions. Presumably the latter or this really is mysterious.

There is something called the "multiplex control module" which does the master logic for most of the interior lights, including the dash and the dome. It seems to be built into one of the fuse boxes, probably the one under the dash (I don't have the manual with me now ). Perhaps the logic there is bad or the plugs going into it are not seated right or are damaged.

There were some threads about similar issues in accords where they said the dome light warped such that the center position did not make an electrical connection. Presumably your multimeter tests ruled that out.

 

The wires are getting the appropriate amount of juice.
I read the Honda diagram of the underdash module. To correctly test this, one must unbolt it from the dash and unplug all connectors to move it. Everything is so tightly connected under there that it seems more trouble than it's worth.
I did look for other threads with similar problems as mine, and most of them deal with LED mods and other stuff that I didn't do. But I'll keep looking for a solution and hopefully find one.

 

I had my car in for a service at a dedicated Honda specialty shop and remembered that it had this problem, so I asked them to check it out for me.
They checked the dome light, door switches, fuses, etc., and determined the culprit was the Multiplex Control Unit (Module).

The bad news is that, according to the tech, this part is integrated into the under-dash fuse box and was only used in the '04-'05 models.
It's also pretty hard to find, even new (about $400), and he was only able to find me a used part for about $100 ($170 installed in an hour).

The seller guarantees it works at install, but the tech admitted that there's no guarantee the used part won't have the same problem I have now.
Also, if the part fails a week after install I'm screwed again.

I told him I'd consider it, as the only problem I'm experiencing with it is the dome light doesn't come on when the doors are opened.

Any thoughts, anyone?

 

Live with it? Seems like a lot of money for an iffy outcome on an old car.

One option would be to leave the dome light in the on position and then route the +12V (or ground, whichever makes the most sense) across the ceiling, down a pillar, and hook it up to the "close" switch on the driver's door. The existing wire would probably have to be shorted to ground or the car would think the door was always open. Check the rating on the door switch, it may not be big enough to handle the current Anyway, then the dome light would come on whenever the driver's door was open. Not quite the way it should be but perhaps better than what you have now. Still, a lot of work.

Perhaps a magnetic mount flashlight, something like this one:

https://www.harborfreight.com/househ...ght-63601.html

has a strong enough magnet to hang on through the ceiling lining? You could put it just inside the driver's door and easily turn it on before entering the car. Really easy to do if you find one with a sufficiently powerful magnet. (Or zip tie it through the header, really ugly though!) Downsides aplenty. Be sure it is nowhere near where an airbag deploys or it might be propelled across the car. It would almost certainly come free in a crash. The batteries would need to be replaced periodically.

 

Another possibility, if you don't mind punching a hole in the ceiling liner. Mount a small flat switch near the driver's door and splice it into one of the lines which enables the "on" position for the dome light. Leave the dome switch in the on position and you can turn the light on and off easily by feel while standing outside the car. Only the liner needs to come off to do that. Basically this just moves the "on" switch from the center of the roof to a more convenient location. Choose a switch which isn't going to punch a hole in your head if you bounce into it during an accident!