94 Brake light troubleshooting

I'm terrible with electrics, so would appreciate any pointers.

Had the rear tail section off of the bike as part of sorting out the carbs for the last few months. Carbs are back on but once I put the tail section back on the brake light was intermittent, with a slight delay, and then stopped working altogether. Tail light has worked consistently throughout.

The brake light fails with both the front and rear, so my conjecture is this is not the switches (although the rear needs adjustment).

• Checked the bulbs, OK
• Fuses OK
• For good measure tested the reg/rectifier, OK. Battery is holding up fine after recharge.
• [I am not mistaking the tail light for the brake light, nor are brake lights constantly on! Check]
• Test lamp on brake wire going into unit (yellow/green) responds to front brake being applied, so not switch.
• Continuity on all wires from the tail section through the sub-harness.
• Continuity (? negative probe of multi-meter on negative battery terminal, set to continuity test) through the rest of the harness on all wires, including the brake wire when front brake switch applied. Although the continuity reading isn't as low as the other wires i.e. it only goes down to about 0.21 (others go down to 0.01).

I have no experience of splitting open these wiring harnesses, and from the point that it connects to the sub-harness (just to the left of where the battery sits, it snakes behind the battery then connects to the rear switch and snakes up under the frame heading forward). I'm not sure where to look next. In terms of voltage drop tests I can't see how i'm supposed to get the probes into the back of these massive connectors while the system is live.

I'm missing something obvious surely. Everything else seems to be working fine.

 

Dirty connector would my my first guess. I've certainly had that on other bikes. Check the connectors in that circuit, including the fuse and fuse holder.

 

Also give the lamp holders a good clean

Sent from my TECNO KG5j using Tapatalk

 

"In terms of voltage drop tests I can't see how i'm supposed to get the probes into the back of these massive connectors while the system is live." T pins? back probing

 

Thanks all, I went back through the sub-harness and fuse/holder and gave them as good a scrub as I could with some wd40. It didn't seem to have any effect.

Now I had an idiot idea at the start of this from looking at the wiring diagram (perhaps mistakenly) so this could all be a placebo - but the tank was barely full, maybe 2 or 3 litres while i've been working on it. I think the ground is shared with the fuel level indicator. I wonder if the tank being empty for so long might have screwed around with that somehow. So I filled it up with another c.5litres today, and the brake light started working. Initially with a bit of a delay but now it seems spot on from the second try.

What I really should have done before refilling it, was disconnect the fuel level connector and try the brake with an empty but disconnected tank. Let me know if this is all in my head and the problem will recur later!

 

I suspect that your WD-40 did some cleaning, as did disconnecting and reconnecting the...connectors.

 

According to the drawing I have for a 93 onwards 3rd gen. The Fuel Level Sender and Brake Globe Grounds are two separate wires, a Green/Black for the Fuel Sender and a Green for the Brake Light Ground. There should be no effect of the Fuel Sender with respect to the Brake Light. The only thing common with these two Ground wires is they both end up at the same Frame Ground point. Refer to attached wiring diagram. So it might be worth while making sure the Frame Ground is clean and good, and that it has zero ohms resistance/continuity back to the Battery Negative.

 

Thanks Grum, I should really have not skipped that video on understanding wiring diagrams! Continuing the theme of ignorance - roughly whereabouts on the frame is that grounding point do you know?

 

Here you go, hopefully it's applicable to your bike.

 

Ah thanks so much, can't believe I missed that from the manual!