headlight beam not working

[wannabay]

Hi
I'm new on here but have had my 400/4 for last 6 years
it's a blue 1977 and has done just over 50k miles
I use it most days for commuting and getting around
As the subject says, my headlight beam has just stopped working. It's not the bulb nor the fuse; no loose wires in headlight case and all earths seem OK
Multimeter on blue and white wires into bulb holder reads only 5v whereas on brown to parking light it reads 10v
switching between hi and lo beam has no effect
Is it likely to be a deterioration/breakage in wires coming from headlight switch to bulb holder?
If so, is easiest option to swap in a new switch/loom?
thanks in advance
gerry

[PatM]

Hi Gerry
Firstly, I dodnt know the answer to your problem- beasue you have almost soved it yourself- I hate problems like that- I replaced my headlamp and had such difficulties- I got ther in the end and the found the popular wiring diagram was incorrect. Id use a buzzer and buz out the healight switch to see if you have lost continuity- it means disconecting the wires inside the headlight- but noting in writing which ones go where- which is what I didnt do..

[Bryanj]

Bad connection probably at the fuse box,and just cos the fuse looks good don't mean its electricaly good

[wannabay]

PatM and Bryanj

thanks and will check the fuse box connection - be easier than swapping switch/loom - we swapped fuse out so that's eliminated

and how do I get an avatar up on this site - it accepted the photobucket URL address for a resized pic of my 400/4 but then said 'image removed'

gerry

[SteveD CB500K0]

Never heard of it doing that before.

Can't you upload a resized picture from your PC? That's what most do.

[wannabay]

OK, had a quick look today
Whipped out fuse box but all connections at back look like new
Also, as the parking lightbulb runs from headlight feed, and is working, this rules out the fusebox/connections
I've got a Haynes manual with a b/w wiring diagram but what I'd really like is a colour one - I've found a link on this site to a US one but it's not the same as the UK loom - any ideas? it would be a whole lot easier following the different wires - i get quite scrambled looking at the b/w diagram!
for the moment, I've swapped the parking light feed wire (brown) into the dipped beam in the headlight shell and can turn this on/off using the P on the lighting switch
oh, and I've sorted my avatar pic
cheers

[SteveD CB500K0]

The Haynes manuals only come with b/w wiring diagrams (at least, mine do) - I think those colour ones may the result of someone with a computer and too much time on their hands!

[Bryanj]

OK its either the lights on/off switch fubared OR the connection accross to the dip switch unpluged ( i believe that is a rarity on Honda's in that it does not plug colour to colour in the headlamp bucket)

[wannabay]

I acquired a colour wiring diagram from kojaycat.com and have just spent an hour or so going over this again.
There is voltage all the way thru to headlight bulb on both dip and main, so fuse, loom and switches would appear to be OK. Voltage measures 8.70 on dip and main feed wires in headlight shell and 10 on side light wire.
Connecting side light wire feed to dip wire lead into headlight bulb holder works dip filament in bulb; same with main wire/bulb filament.
Is the difference in voltage critical? And/or is this pointing to a difference in eg amps or resistance? It's a long time since I did physics!
And not sure I understand your comment below, Bryanj, about connection across to dip switch coming unplugged
Keep those thoughts, comments and ideas coming.
Cheers, gerry

[Bryanj]

Isn't the lights on off on the RH switch and the high dip on the left hand switch?

[wannabay]

Morning.
Bryanj, Yes you are correct re handlebar switch configuration.
Am thinking that voltage drop to 8.7 at the bulb itself means that insufficient power can be carried to the bulb and so it won't light up.
As voltage on side light is higher at 10v and this comes from lights switch, which also controls main light, it's looking like the dip high switch is the culprit
I'll check voltage drop around lighting circuit later, shopping trip intervenes, and report back
Gerry

[Bryanj]

Even at 6v it would glow, there is a wire from the RH switch that goes via the headlamp bucket to the dip switch. check that it is connected OK

[wannabay]

After all that it was a rogue headlight fuse!
There was a 1.5 voltage drop across the fuse holder. I swapped in a new fuse and hey presto no voltage drop and everything works!
No idea what/why but I do have working dip/main beam now.
Also found a dud earth connection to backlight in speedo so need to sort that too - I can see a similar repair has been done to tachometer as the wrong colour wire has been used.
At least I now know my way around the wiring loom
Cheers all
Gerry