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TOPIC: No taillight, headlight still works

No taillight, headlight still works 09 Aug 2018 18:04 #1

  • Mike MPD
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Taillight went out while on a ride, hadn't been working on the bike, had not changed anything. Headlight still works fine. I do not have a horn or any other lights connected.

I have a good battery and tested the light directly to a battery and it works fine. Since the taillight is on a separate dc circuit with the battery I was thinking rectifier. I replaced it and it didn't fix anything. I checked both brake switches for loose connections. I had a bad light switch previously but I replaced and have had no issues since, so I do not think that is the issue.

Where to look next?

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No taillight, headlight still works 09 Aug 2018 20:20 #2

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75 DT250?

Bad ground. Broken wire.

Headlight switch has three sets of contacts when "on". Brown connects to Blue with switch on. Brown is battery power from key switch and blue goes back to taillight.

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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

No taillight, headlight still works 09 Aug 2018 23:10 #3

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If it's the type of tail bulb that runs off the mag along with headlight & if the dc stop light still works then possibly a broken wire in the loom or disconnected in the headlamp. Wires in the loom 'do' break inside their covering where they flex at the steering head. Even if the tail light 'is' DC it should still work via the battery even if the battery is flat but just pumped along via ac rectified charging. Bulbs don't care if it's ac or dc. Make sure you haven't got the blinkers & stop light operating at the same time dragging any chance a flat battery [via charging circuit] might have of running tail light [if it's dc]. We have mostly ac tail lights here on all magneto ignition bikes--but dc stop light of course. Would think if tail light 'is' dc it'd be a brown wire powering it but blue if mag powered-- but 'might be wrong' on 'that'!!.

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No taillight, headlight still works 09 Aug 2018 23:24 #4

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yes that's a point. If the rectifier had a broken wire would that stop a dc tail light or would it just get ac? unrectified. if that does happen then nothing dc would work when the battery goes flat.

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No taillight, headlight still works 10 Aug 2018 20:38 #5

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Might be a dumb questions but would a bad turn signal flasher affect that circuit at all?
It has black, brown, and brown/white wires connected to it.

I cannot find a loose connection anywhere and all the grounds have been cleaned up and are good. I cannot find anywhere where the harness is obviously worn or anything either.

I should also probably mention that I am running and LED taillight. I know sometimes those dont play nice with ac. If I cover the light with my hands I can just barely see the faintest amount of light so it seems to be getting at least some power from somewhere. I am wondering if maybe the taillight just is getting pure AC and is super dim? I guess I should find an regular filament bulb and see if that behaves any differently.

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Last Edit: by Mike MPD.

No taillight, headlight still works 10 Aug 2018 21:38 #6

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USA laws required the tail light to stay on with the engine off starting in about 1970.

Tail light is just a simple 6V DC circuit. Switch on bars on your bike connects the brown to the tail light. If engine is not running, tail light should be on when switches are on and there is no chance that AC is interfering.

Do you have a specific 6V LED? Or one of those LED's with a wide range of running voltages like 6 to 30 volts? I've seen those be sensitive to lower voltage. I have two brand new LED 1157 bulbs that came in the same package... one works fine on a 7.2v RC car battery pack even when the battery is almost dead... the other bulb works if the pack is freshly charged and voltage is around 8 volts... then it goes out when the voltage drops a little. That one works great on 12v, not 6v. The other bulb works on either voltage but is significantly dimmer on 7.2 v (or 6v)

It's easy to over complicate things. The tail light circuit is not complicated. Do you have a good battery? Is the key switch getting solid voltage from the battery through the red wire? (There should be a fuse... and since you don't have anything turn signals or horn you wouldn't know if the tail light is the only battery powered item not working because it's the only battery powered item you're using.)

With the key on, is the brown wire out of the key switch getting solid battery voltage? Is the brown wire into the handlebar switch getting solid battery voltage when the light switch is on? Is the tail light wire coming out of the switch getting solid battery voltage? Does the tail light have solid battery voltage?

When I say "solid" voltage I mean with a load like a light bulb. Voltmeters can show voltage that quits under load if you have a bad connection somewhere.

I'd start with a good battery, key and light switch on, and then start at the tail light and work your way toward the battery until you find where the voltage starts... there are connectors along the way that make it easy and the Yamaha wiring diagram often shows generally where the connectors are... by the tail light... headlight bucket, near battery (fuse). Most are in the headlight bucket.

Never overlook a bad ground, defective bulb, or bulb socket. Those should be first checks.

Good luck!

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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

No taillight, headlight still works 08 Sep 2018 11:05 #7

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Ok so after some testing...

Battery has good voltage and ground. I am getting voltage on the red wire where the ignition switch plugs in, but this may be irrelevant because I am not running one. Bike was running and all lights worked fine for about a year with no ignition switch.

I get no voltage on any other brown wire in the system . I believe this means that there is an issue where the red wire from battery is connected with all the brown wires inside the harness?

Anywhere else I should check before I start unwrapping this harness and taking a closer look?

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No taillight, headlight still works 08 Sep 2018 11:10 #8

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The red and brown connect at the switch, not in the harness.

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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

No taillight, headlight still works 08 Sep 2018 11:44 #9

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They do connect at the switch but the wiring diagram shows them connecting within the harness as well.

If they only connected at the ignition switch, lights would not have been working with no ignition switch as they had been all this time.

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No taillight, headlight still works 08 Sep 2018 12:18 #10

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I don't think you've even confirmed what bike this is but if it's a 75 DT250 as a guessed (probably because of another post you made someplace) here is the lighting diagram from the service manual:



I circled the Red wire. It only goes to the rectifier and battery. Then there is a set of switch contacts in the ignition switch that connect red to brown with key on.

If you had battery power to brake light and tail light without a key switch, some "non-standard" wiring was done to connect the battery to brown. The battery powered accessories (horn, brake lights, turn signals, and taillight on later US models) simply do not work on a stock bike unless key is on.
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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