2000 Honda Accord No power to headlights

Boy, it's amazing how complicated the insane engineers can make such a simple circuit. There's at least two computer modules involved in turning the headlights on, but from the diagram I can't tell how they operate or when a certain point should or should not have voltage. We should be able to figure it out though if you can get to the terminals in the relay sockets and you have that test light.

There's four completely different circuits. You have to tell me if your car has daytime running lamps and whether it has "auto-off" headlamps. I guess that's for people who don't know how to turn them off or can't be bothered to do so!

I'll be here for at least another hour waiting for your reply, otherwise I'll be back tomorrow.

Do you have the "auto-off" feature?

Okay, ... Turn the headlight switch on, then find the two relays under the hood. Remove one and plug it back in. While doing that, feel if it clicks when you pull it out and put it back. Do the same thing with the second relay. If you feel the clicks, that will tell us that part of the switch is okay, the 12 volt feed is okay, and that circuit's ground is okay.

I'm going to keep studying the circuit while you do that.

Next step. Try the "flash-to-pass" feature. I'll bet that works. If it does, most likely part of the headlight switch is bad.

Does the flash-to-pass feature work?

The flash-to-pass is activated by pulling the dimmer switch to the "high" position, then releasing it. The high beams will light up as long as you hold the lever that way.

There won't be any voltage on those fuses until the relays turn on. There's two switches that will turn those relays on. One is part of the head light switch and the other one is half of the pass-to-flash switch. If the flash-to-pass doesn't work, that leaves out the head light switch as the culprit. There's either going to be a broken wire between those relays and the head light switch assembly, or there's no voltage to the relays.

I have to get to a different computer to access the wiring diagram again, but for now, remove one of the relays, then test for 12 volts in the socket. Two terminals should have 12 volts all the time.

Each relay has four terminals, and two in each socket should have 12 volts all the time. If they do not, there is only a 100 amp fuse feeding them, and if that is blown, lots of other things won't work either.

Touch the test light's probe to the battery's positive terminal to be sure it's working and has a good ground. If it lights up, touch it to all eight relay socket terminals. It should light up on four of them. If it does not, look for that 100 amp fuse. Fuses that large are usually bolted in so those provide nice test points. If you have 12 volts on both sides of that fuse, there is only one other thing between there and the relays. That is something listed as "ELD". I have no idea what that is but by the way the diagram is drawn, it is not something that is simply plugged into the fuse box. It appears to be built as part of the fuse box. If it comes to that, I'll have to try to figure out what that is.

So if I understand correctly, ... Uhm, ... I'm confused.

"on relay 1 I get close to nothing"
"I put the test light on both relays and they both have power"

Two of the four terminals in relay one's socket should have 12 volts all the time. Two of the four terminals in relay two's socket should also have 12 volts all the time. If that is what you have, ... Well, even if you have something different, I think we've established the 125 amp fuse is good. You said previously that you replaced the headlight switch assembly. That just leaves the wire between the relay box and the switch. That is a blue / red wire. There is a good suspect to look for. That is the inside fuse / relay box listed as being on the passenger side of the dash. There's no fuse in there for this circuit, but there are two connector terminals. Typically they will do that when there is some optional device that is used on some models. On yours, when that item isn't used, there's just a jumper wire in there. That would be a good place to look for corroded terminals.

If you can remove the headlight switch and get to the wires, use a paper clip and piece of wire to ground the blue / red wire. As an alternative you can test there for 12 volts. If you find 12 volts there, check again with the headlight switch turned on. If you still have 12 volts there, the new switch is defective or the wrong part for this application. At the same time, if you ground that wire, the headlights should turn on. If it's easier, you can do the grounding tests with the switch unplugged. The voltage tests have more validity with the switch plugged in. If necessary, I can explain that later.

If you do not find 12 volts there, and / or grounding that wire doesn't turn the headlights on, you'll have to work backward to that under-dash fuse box or to the relay box with those two headlamp relays. Ground the blue / red wire at the points where you find it and see where you can turn on the relays. You'll know the relays turned on by the headlights turning on.

If nothing has worked up to this point, we can approach this a different way by grounding the blue / red wire right in one of the relay sockets but I'll have to describe how to determine which terminal to ground. Lets stick with just one relay, because they're both tied together and will respond to any testing together. I'll need to know for sure that you have 12 volts on two terminals and 0 volts on the other two.

Hold on. You only have a 12.6 volt battery. There's no way to find 17.60 volts anywhere. You either have the meter set on too low a scale or you have an auto-ranging meter and you didn't notice that it selected a lower scale, then you're finding 17.6 millivolts which is essentially the result of interference.

Just pick one relay and stick with it. Once we find the solution to get it working, the other one is going to work too. There's four terminals in its socket. Two of them must have 12 volts all the time. I prefer to use a test light because there's no misinterpreting the meter and you don't even have to look at it. Also, I have over a dozen digital meters for radio repair but none of them are auto-ranging. I don't like them because they trick me too often.

If you don't find 12 volts on two terminals, look for that large fuse and check on both sides of that.