Stupid question - brake light on cluster

Hi All

A friend of mine gave me his 79 xs650. It’s been sitting for decades. Trying to get her back on the road.

Electrical loom cleanup should have been the first thing to do. You were fixated on the carbs, and often carb issues that look like carbs is an electrical issue.

I am on my phone and don't have access to my bookmarks, on my laptop.

Post #5 in this link has a list of links for electrical, (loom), cleanup and a great description from XSLeo on how everything works when turning the key on
Ignition Switch. Check power voltage before then after switch.

I did a quick test. I took off headlight and checked connector going to on/off switch. Battery voltage on red wire with key off. Then checked brown wire voltage with key on, dropped to less than 11v. Then I did another test. I disconnected the connector, jumper wired red to red, turned key on and checked brown wire voltage. I got battery voltage. I think this test proves the on/off switch is ok and the issue is somewhere further down the line. Am I correct on this?
Update. I’ve been chasing the 0.8v drop when I turn the on/off switch on. When I disconnect the voltage regulator connector the issue goes away. I then checked the regulator with a meter. I get .074 ohms when testing brown (in) to ground. I get the same reading for ground to green (out).

Do I have a bad voltage regulator? If yes, any recommendation for a new one (1979)?

Thanks again for your help.
On off switch???..........i that the kill switch or the Ignition switch.???

Test the red wire form the battery, +, to the battery side of the Fuse box.
Test the red wire from the battery, +, to the far side, Ignition side of the Fuse box. through the main, (20amp), fuse.

If no voltage drop, test the red wire from the fuse box, (main fuse), to the ignition switch.
Now test the red wire from the fuse box to the brown wire after the ignition switch......This will test for a drop in voltage through the ignition switch.

I didn't understand your explanation.

The kill switch can also b a cause of a potential voltage drop. May need to be dismantled and cleaned.

The bike has been sitting for decades. The whole loom connectors, both plastic and bullet, need to be dismantled cleaned, checked for corrosion and any damage repaired replaced and reassembled, using a conductive grease. All earth/grounds removed cleaned and resembled.......All switches, Ignition, Kill, left and right bar switches need to be taken apart checked for loose or broken wires, corrosion on contacts, cleaned and reassembled.

I done my loom and switches doing a continuity test on every wire between connectors then after a connector then between that connector and the next on every circuit. Tedious but after going through the whole loom without any continuity/resistance i picked up any voltage drop, when i did run live power there was next to no loss, any more than would normally b expected after multiple connections.

I put a second hand loom on my bike, some before and after shots of the bullet connectors.
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The mechanical regulators can b adjusted......Fiddly and often need to b regularly checked...........for a few $'s an automotive regulator and a bridge rectifier and be put together. This has been tried and tested and is a regular way to make your charging system reliable without costing much. The aftermarket, SS combined R/R, are plagued with Chinese products that are cheap and have had a lot if reliability problems.

Yours is a points model, B type regulator. Important to make sure you buy the regulator for the point model B type Automotive regulator.
Go to Post #2 by 5Twins

Both poits models and Factory TCI ignition type are covered in the link
Hi All

A friend of mine gave me his 79 xs650. It’s been sitting for decades. Trying to get her back on the road.

So far I rebuilt carbs, changed oil, adjusted valves and worked on brakes.

Brakes are where my question comes in.

When I turn the key to the on position the brake/tail light comes on. Is this normal? There’s another switch over on the right handlebar for engine run\stop. If I switch that to stop the brake/tail light goes off.

Is this normal or do I have an electrical issue to chase down?

I did replace the front brake cylinder and rebuilt the rear. Maybe I did something wrong?

Thanks for your help!
I don't know where you are located, but in the USA the headlight, and tail light are on all the time that the bike is running. It's the law.
I found a couple of issues. I’m not sure what it’s called but this tube was broken off going to carb.

I put it back on.

Also, voltage regulator was bad, so replaced that.

I am still checking electrical. So far no major issues. Some corrosion on ground wire to battery so got that cleaned up good. Every other connection so far looks clean.

I did start bike and let it idle for a while. Seemed good. Also rev’d it up, good as well.

I still want to do some more testing when I have time, but definitely some progress today.
That's the vaccum tube for the vaccume operated petcocks. When the bike is running it draws petrol to the carb. If the tube lets in air the bike runs lean and very rough.