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I would not condemn the headlight sw at this time until I verify by bringing power to the switch wire that goes to the dimmer sw. If headlamps work then we just eliminated everything , but the sw.
Take the color wire at the dimmer that comes from the lite sw and see if there is not a break in that wire. Use a piece of wire as a jumper and take power from a point you know has power and touch that color wire at the sw . If headlamps work now , you just verified wiring is goodWhat. If the headlamps still don't work you verified the wire is open or poor connection which can explain why power to some things drop out. Somehow I still feel you are losing power to the part of the fuse panel when another load is put on the system.
What are you using to ck for power ? Whatever tool, blow the horn and see if you get loss of power to lite sw.
The reason I asked if you have speed control because with it, your horn uses a relay, without it, no relay as shown below:
In your situation horn is the easiest to fix so let's go over that, some of your other devices can be fixed automatically.
The location of the relay is attached to the bottom of your steering column if can't locate it from the location mentioned on the top diagram above.
Look at the diagram above, w/o speed control your horn switch is positive trigger from fuse F16 and it's Y/LB (yellow/light blue wire) and always ground at the horns.
With speed control it's negative trigger from fuse F16 to the relay (hot at all times), if UK spec uses other fuse, change it for step (3) below, your horn switch sits between the slip ring and one end of the slip ring is always ground, when the horn switch is depressed it energizes the relay, look at the DB (dark blue) wire, and sends 12v to the horns, and the horns are always ground.
Therefore, let's check the horn relay first. Find a ground source inside your van, anywhere on the metal of your steering column or the tube or even the housing of your ignition switch and verify it's a good source.
1) DB wire at the relay (without removing the relay and when horn switch is depressed)
This DB wire gets 12v when the relay is energized.
So energize it and if you get 12v but the horns don't sound, check the connection to the horns, and check the grounding on your horns.
If you don't get 12v then continue to (2) and (3) below.
2) Y/LG wire at the relay (with or without removing the relay doesn't matter)
Ground - YES or NO (when horn switch is depressed)
Look at the diagram your speed control switch assembly is self grounded at one end of the slip ring to the steering column, so this Y/LG wire should get continuity if you check it with the ground source that you picked earlier
3) Y/LB wire at the relay (with or without removing the relay doesn't matter)
12v - YES or NO
this wire should be hot at all times, if not, check the fuse F16 and the Y/LB wire for short to ground if your F16 is blown.
If 1=NO, 2=YES, 3=YES, then your horn relay is probably bad.
Either bench test the relay (if you know how) or borrow an identical relay from your junction/fuse box for testing purpose.
Let's look at the door locking system. Locate C236 (door lock/unlock control relay) and check:
1) Circuit 517 (BK/W - Black/White) wire, check for 12V at the relay (without removing the relay unless you can trace and know for sure which female terminal for the BK/W wire)
2) Circuit 57 (BK - Black) wire, check for ground. Locate a major ground point G201 and check for corrosion, and clean using contact cleaner and wire brush if needed.
You should notice that your door locking system uses G201 as well at least we've found something in common between locking and the lighting system but not relevant to the horn. Look for a grounding problem when all of a sudden so many devices are malfunctioning at the same time. You know they use different fuses and on different circuit.
Locate at ground location below:
Headlamp diagram above. Look at circuit 38, the BK/O (black/orange) wire you mentioned previously. Still, it looks like you have grounding issue more than power distribution issue.
Locate F16 it's horn, speed control and DRL. Do you have DRL?
If you are not sure whether or not you have DRL, locate the DRL module (C167), terminal 4 at C167 you see Y/LB wire (same circuit as (3) mentioned previously), use voltmeter and get voltage, then press the horn button and see how much voltage drop there. If none, you might need to fix the horn issue first.
If you have UK-Spec fuse block layout please attach here and let us know which one is for horn other than F16 on US-Spec.
For the less important dome light issue check the pdf attached below.
We do appreciate the diagrams and info and thank you.
Instead of going through a lot of work that may not be necessary consider trying the following,
When you are checking power to the light sw and blowing the horn, make sire your ground for your test equip is right from the ground post of the battery.
That will either verify or eliminate any possibility of a grounding problem if you get the same dropping out problem, then you know it is in the positive side of the system and I still suspect the positive feed or the fuse panel.
We do appreciate the diagrams and info and thank you.
Instead of going through a lot of work that may not be necessary consider trying the following,
When you are checking power to the light sw and blowing the horn, make sire your ground for your test equip is right from the ground post of the battery.
That will either verify or eliminate any possibility of a grounding problem if you get the same dropping out problem, then you know it is in the positive side of the system and I still suspect the positive feed or the fuse panel.
So an update for this as follows. The problem is still persisting, but with a few developments.
I put a constant live from the battery to the headlight switch in place of the constant live (BLK/orange wire) in the headlight switch plug. I kept all other pins the same, and the headlights worked on the switch and on the dimmer.
I checked for constant live from the BLK/O wire which there was. I plugged the BLK/O back into the switch plug and tried again. No lights. What's more, the voltage across the BLK/O wire drops from 12v (engine off) or 14.5v (engine running) to 0v when the switch is either on side light or main beam setting. The voltage also drops if I press the horn button, although the horn doesn't sound.
This is still confusing me a lot as to why this happens.
Can I ask where are you connecting your test instrument ground connection ?
If you by passed everything with a DIRECT, SEPARATE wire from battery positive to the switch , headlights on, and blew the horn and , no change , you just eliminated the positive feed in the vehicle.
The key to all this may lie in the ground system after all.
Since we are pretty sure the positive feed is good, can you do the same thing with that wire jumper you used and by pass the grounds ?
