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Author Topic: Headlamp fuse blowing  (Read 377 times)

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Offline pmiller

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Headlamp fuse blowing
« on: May 14, 2016, 08:15:31 PM »
Hello all. I have a 78 cb750k. Recently I have been experiencing the headlamp 7.5A fuse blowing. I noticed that it only happens when the headlamp dimmer switch gets accidentally set in between the high and low settings. Took me a minute to figure that out after banging my head against a wall. So I figured I would just be careful to not let that switch get set to that position. Since I told myself that, it has happened again twice so now I'm trying to sort it out for good.

 My thought is that it is trying to fill both circuits of the high and low and it's too much for the fuse. So I took apart the switch and looked at the wiring diagram as well and now I'm not sure what to do since the diagram shows a middle "N" setting where the hot would be connected to both the high and low at the same time. Which is how the inside of the switch looks, with the pass-through connection being able to touch them both at the same time when set in the middle. Has anyone else experienced this?

Also, the whole bike shuts down and won't restart with the headlamp fuse blown. Is that normal?
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Offline Airborne 82nd

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Re: Headlamp fuse blowing
« Reply #1 on: May 14, 2016, 08:56:36 PM »
Will you post a link of the wiring diagram you are using.
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Offline calj737

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Re: Headlamp fuse blowing
« Reply #2 on: May 15, 2016, 07:03:19 AM »
It sounds as though your headlight wiring to the switch from the harness is improperly fed. I woner if someone prior to you used a different KILL switch wire feed thwt is somehow linked to the headlight fuse? The headlight should have absolutely nothing to do with the BLK, BLK/WHT to the coils.
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Offline pmiller

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Re: Headlamp fuse blowing
« Reply #3 on: May 15, 2016, 09:30:55 AM »
Here's the wiring diagram I'm referencing. It's from the Clymer manual.
I'm sure the PO messed with some things for sure. I mean he had been riding with the front brake off because he couldn't get it to go back on without it gripping the rotor under no brake pressure. I'm new to working on bikes but I eventually figured out that he had the caliper piston in backwards. 

Not sure all what he did to the wiring. Seems to be fine except when the headlight switch is in that "N" position.
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Offline Bodi

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Re: Headlamp fuse blowing
« Reply #4 on: May 15, 2016, 11:44:29 AM »
The HEAD fuse might pop with both filaments powered, it's probably more than 7.5A - that's around 90W. A fuse will be OK for more current than it's rated for for a while.
However... the ignition should NOT be running through the HEAD fuse. With its load plus both beams on... a 7.5A fuse is almost certainly going to blow in short order.
Figure out what's wrong. It would take some doing to power ignition from the HEAD switch though. Ignition power goes to the right switchpod and from there to ignition coils via the kill switch, and also to the HEAD fuse power feed via the starter button. From the HEAD fuse, headlight power goes to the left switchpod where hi/lo is selected. One would have to do some fancy rearrangement to make it do what you report.
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