RD400 Headlight Fuse

[2steve]

Went to ride today. The RZ has to stay in the neighborhood because it's not tagged, so got a short putt around the block. The RD400 is ready for anything - except today the headlight didn't come on and that's not lawful. Only 7,700 original miles with decades of unknown history. 10A fuse was blown. Turning the switch to "ON" grounds the headlight system through the fuse box - high or low beam.

Inside the bucket looks okay and the H4 bulb is intact. The problem might be in that big umbilical that goes from the headlight bucket and runs under the tank.

Anyone know of a common problem/rub area to look for in the RD400 wiring for the headlight? Switches are in pretty good shape. They are still pretty fresh for 1977 vintage.

[RDnuTZ]

if it was me, I start looking for disconnected/loose bullet connectors and grounds. making sure all connections are clean and tight (especially grounds). easy to check continuity on cluster switches too. eliminate all the basic stuff 1st before diving in looking for serious problems.

[gargantua350]

I remember back in 03 when I had my RD400 I was riding home and stopped for gas and noticed my headlight was off.  I thought it was the wiring in the bucket so I removed that to look...nothing.  I narrowed it down to the fuse block.  There was bad contact between the fuse and the holder spring clips.  sprayed contact cleaner and put in a new fuse and problem solved.

[SoCal250]

There was bad contact between the fuse and the holder spring clips.

I also had this happen on my 400 many years ago

[2steve]

The fuse blew hard. Black inside the glass and even cracked the glass. Flipping the headlight switch to "ON" grounds the lower fuse box spring clip, so use in blowing another fuse. Gotta be something simple. Maybe I can dig into it on Saturday.

Thanks for the inputs!

[m in sc]

look in the HL switch as well. good luck w it.

[2steve]

"Narrowed it down" tonight, but not really. Both yellow-to-the-bulb and blue wire from the switch are grounded when checking inside the headlight bucket.
This seems to point to a tach bulb circuit, dimmer switch, speedo bulb and/or somewhere under the seat going to the tail light.
Need a new itty-bitty JIS screw for inside the right switch assembly wiring clamp which apparently isn't the problem. JIS screwdriver wouldn't bite because it was already abused, but got it out anyway with a mini reverse drill easy-out tool.
Wiring colors don't quite agree with the factory service manual. This might take awhile.

[2steve]

Further poking around shows that the blue wire out of the main switch is grounded. The blue wire from the speedo is grounded (goes to a bulb - why grounded in those two branches but the other blue ones are fine?), and the yellow wire from a bulb in the tach is grounded. That yellow one connects to the headlight and dimmer switch but no problem with those branches.
I'm baffled, but next thing to do is to try and pick apart the main switch and look for a smoking gun. Then, the speedo and tachometer.

The bike really only has 7k miles and change on it. Not corroded, all switches pretty fresh. Something stupid happened and I can't find it.

[m in sc]

age is age. have you removed all the bulbs and checked then?

[1976RD400C]

Maybe this can help.

Headlight on/off switch - red/yellow is hot from fuse, blue/black goes to hi/low switch, blue goes to dash lights.
 
Try disconnecting blue to see if the problem is in the dash light circuit (if fuse still blows, problem is not that circuit)

Try disconnecting blue/black to see if the problem is in the hi/low or beyond.

Yellow is hi beam, so if it blowing the fuse when using low beam that's not the problem.

How about the taillight and the wires going to that?

If it is blowing the 10 amp headlight fuse, the main switch wouldn't cause that.

[2steve]

Excellent guidance there, THANKS!

Fooling around tonight, the blue wire to the taillight was grounded. Tracked it down all the way to the light and - of course - the blues and yellows ground thru the bulbs to the black. I'm just listening for the meter to beep, so a bulb makes that happen when it connects to ground on the other side.

Main switch is clear. Hi/lo is good. Headlight switch is good too.

I'm ripping thru all this and missing something obvious. Almost need to put it all together and start all over with a new fuse - while carefully following your lead, 1976RD400C.

[2steve]

Just got it all put back together and hooked up the wires. Stuck the new 10A fuse in, hooked up a charger for battery support without it running and everything works like it should.
No smoking gun to point a finger at what happened.

Maybe these fuses just go after a hard life of running that high amp draw, old headlight. Should have replaced it in the beginning.

1976RD400C's procedure looks like a keeper. I printed it off and am going to keep in with the service manual.