Topic: tail light driving me crazy

[nate33]

i have a '76 550 and can't get the tail light to work.

originally i had a light with two wires (with no ground) and the brake light worked but no running/parking light.

i tried to hook up a three wire light like the one pictured below and got a running light but no brake light.

then i pulled another two wire light off a 750 (which worked) and tried that by copying the way it was wired on the 750. that included a ground wire coming out of the harness (where the turn signals connect) and grounding to the light housing. no luck.

all other lights work and i am not running turn signals if that makes a difference. i have the wiring diagram and a 750 for comparison but electricity is not my strong point. this is my first bike and i am slowly cleaning it up but am getting very discouraged. any help would be great.

[tlbranth]

It'd help if you had a multimeter. You should be able to get one for $10 or less at radio shack or schuck's. A wild guess is that there's no good ground connection between your light fixture and the frame. Try clipping a wire between the light fixture and the battery ground. If it works, you may need to run a ground wire from the fixture to the frame.
If not......
With light activated (light on or brake pressed, depending on which one you're testing), connect voltmeter between wire and ground. If you have 12v then the problem is in the lamp. If not, it's in the supply.
How about trying that much and report back.
Good luck

[JS550]

Is it an L.E.D your trying to run? Dont know if this makes since but, my bro picked up a nice LED for his 550, wired with 3 wires, at idle he has brake light & a blue dot in the middle working. The tail light will come on at cruising speed or when the rpms are up. Strange stuff?

[nate33]

will grab a multimeter this afternoon and report back.
one more question ... on my 750 the ground wire is coming from the opposite end of where the signals plug in and connects to the light housing. it is next to a green wire. that light works perfectly. tried this on the 550 with no luck. so do i need to ground another way?

thanks.

[tlbranth]

Well, you need to find a good ground. You can test with the ohm setting on the multimeter looking for 0 ohms between the battery ground (-) and whatever your ground candidate is.

[nate33]

Is it an L.E.D your trying to run? Dont know if this makes since but, my bro picked up a nice LED for his 550, wired with 3 wires, at idle he has brake light & a blue dot in the middle working. The tail light will come on at cruising speed or when the rpms are up. Strange stuff?

not an LED. thanks for the response.

[jaknight]

Hi nate,

I had tail light and turn signal problems that plagued me for a long time.  I am not an electrical know how guy either.

I traced and cleaned all connections everywhere.  Still didn't solve the tail light not working or turn signal oddities.  But, the clean connections helped the bike's overall electrical efficiency immensely.

I finally ran a new ground wire connection from each turn signal and the tail light to ground connections up under the seat and lo and behold everything worked properly immediately.

There are guys here who can step you through doing new ground connections if you need help for that......... Since I fixed my bike, I have come to believe that the vast majority of electrical problems on my bike have always been ground connection related.

Ground is what I would be looking for on your bike.

~ ~ ~ jaknight ~ ~ ~

[nate33]

Hi nate,

I had tail light and turn signal problems that plagued me for a long time.  I am not an electrical know how guy either.

I traced and cleaned all connections everywhere.  Still didn't solve the tail light not working or turn signal oddities.  But, the clean connections helped the bike's overall electrical efficiency immensely.

I finally ran a new ground wire connection from each turn signal and the tail light to ground connections up under the seat and lo and behold everything worked properly immediately.

There are guys here who can step you through doing new ground connections if you need help for that......... Since I fixed my bike, I have come to believe that the vast majority of electrical problems on my bike have always been ground connection related.

Ground is what I would be looking for on your bike.

~ ~ ~ jaknight ~ ~ ~

thanks jaknight.
i am going over the wiring diagram now to see if i can figure out the ground.
since i am not running signals it is a little confusing.
gonna take tlbranth's advice and get a multimeter and see if i can't figure it out.

[nate33]

It'd help if you had a multimeter. You should be able to get one for $10 or less at radio shack or schuck's. A wild guess is that there's no good ground connection between your light fixture and the frame. Try clipping a wire between the light fixture and the battery ground. If it works, you may need to run a ground wire from the fixture to the frame.
If not......
With light activated (light on or brake pressed, depending on which one you're testing), connect voltmeter between wire and ground. If you have 12v then the problem is in the lamp. If not, it's in the supply.
How about trying that much and report back.
Good luck

ok. got a multimeter this afternoon and watched a few tutorials on youtube because i have never worked with one before. after testing a few batteries and fuses i went out to the garage. activated the tail light. connected the positive end of the multimeter to the light lead and the negative end to the main ground. got a reading of around 6v. did the exact same thing with the brake light, pressed brake, and got the same reading.
should i have 12v each?

[tlbranth]

Yes, shou should see 12v but I'm guessing you're looking at the wrong scale. Put the leads across the battery. That's what you should see at the leads. Is the bulb good? You can check it by putting the solder tip on one battery terminal and with your other 3 hands, run one of your meter leads from the side of the bulb to the other battery terminal. If the bulb is good, test the fixture by hooking your signal wires back up to the fixture and with the bulb still out, use your voltmeter to measure between the contact point down in the bulb holder and battery ground. If no voltage, clean contacts, look for broken wire or corroded connector. If you get a reading, try the same thing between the contact in the bulb holder and the side of the bulb holder. If that fails, you have a bad ground connection from your light fixture to your frame. This could be because there's paint or rust in the threads where the fixture bolts to the fender or the fender to the frame. I'd be inclined, in that case, to run a green wire from the fixture body to the frame.
Good luck, keep us posted.

[nate33]

Yes, shou should see 12v but I'm guessing you're looking at the wrong scale. Put the leads across the battery. That's what you should see at the leads. Is the bulb good? You can check it by putting the solder tip on one battery terminal and with your other 3 hands, run one of your meter leads from the side of the bulb to the other battery terminal. If the bulb is good, test the fixture by hooking your signal wires back up to the fixture and with the bulb still out, use your voltmeter to measure between the contact point down in the bulb holder and battery ground. If no voltage, clean contacts, look for broken wire or corroded connector. If you get a reading, try the same thing between the contact in the bulb holder and the side of the bulb holder. If that fails, you have a bad ground connection from your light fixture to your frame. This could be because there's paint or rust in the threads where the fixture bolts to the fender or the fender to the frame. I'd be inclined, in that case, to run a green wire from the fixture body to the frame.
Good luck, keep us posted.

below is almost exactly the same meter i have and how it was set (see green line) when i got a reading of 6v. if this is wrong how should it be set? i also checked the battery but can't remember exactly what i got. all other lights are nice and bright and the bike starts easily so i am assuming the battery is ok. i bought a few new bulbs and new contact points yesterday so i will test those too. i also tested the ground i was using and it looks like it wasn't a very good ground. noticed a little rust on the main ground bolt so i will clean that up too. i will check everything you suggested and try a ground from the light fixture to the frame.

[tlbranth]

Oh, I was thinking analog (i'm stuck in the 60's). Couldn't tell from the pic but you'd probably get a 6v reading on 12v if you were set to read ac instead of dc but. Try reading directly across your battery. If it's 12v there and 6v at your lights, you have a resistance somewhere causing a voltage drop - bad connection probably.

[nate33]

Oh, I was thinking analog (i'm stuck in the 60's). Couldn't tell from the pic but you'd probably get a 6v reading on 12v if you were set to read ac instead of dc but. Try reading directly across your battery. If it's 12v there and 6v at your lights, you have a resistance somewhere causing a voltage drop - bad connection probably.

got an 11.5 reading across the battery and around 8 at the lights. getting a reading of slightly less at the contact points in the bulb holder. also checked the bulb the way you suggested and it WAS good, until i dropped it. i have a few new bulbs that i will check when i get home. shouldn't this be enough power to at least turn the light on?

[tlbranth]

You'd think. But 11.5 is low for the battery and a drop to 8 at the light shouldn't be. Are other lights going on when you test? I think you ought to trace your wiring back from the light to the battery taking voltage readings at each connector - and clean each connector - until you get back up to 12v. That should show you where the problem is. Don't like the low battery voltage though. That could drop to nothing with a load applied if there's something wrong with it.

[nate33]

You'd think. But 11.5 is low for the battery and a drop to 8 at the light shouldn't be. Are other lights going on when you test? I think you ought to trace your wiring back from the light to the battery taking voltage readings at each connector - and clean each connector - until you get back up to 12v. That should show you where the problem is. Don't like the low battery voltage though. That could drop to nothing with a load applied if there's something wrong with it.

all other lights are nice and bright and the bike starts right up. the battery just came off the charger too. it is possible that i am not reading the meter correctly but it is set on the only setting that will give me any reading at all and this particular meter does not have an ac/dc switch. any recommendations for connector cleaning solutions? i feel like i am complicating something fairly simple.

[tlbranth]

I use a brass-bristle brush for stuff like that - the males anyway. The females, tube-up some fine wet & dry silicon carbide paper and work it around. I've used vinegar before and it's worked but there may be a reason not to that I'm not aware of. Mechanical cleaning, though, should be sufficient.

[nate33]

IT WORKS!!!
turns out it was a silly ground issue that i thought i had checked at least ten times.
thanks for everyones input (especially tlbranth).

[tlbranth]

Glad you got it fixed.

[jaknight]

Congrats Nate,

Glad to hear you got it straightened out!

A good preventive maintenance would be to buy a product called Deoxit.  You should be able to find it at Radio Shack or some similar electronics outlet type of place.  It comes in a spray can and will remove corrosion, dirt, grease, etc.

You might see it in a "kit", the Deoxit cleaner and a so called Deoxit Gold, which is a "preservative" to keep it clean and help prevent corrosion after the cleaning you give it.  I would go for the kit.  Keep you eyes open in whatever store you do go to......... the spray on cans are small.......... they are not in very large cans at all.

I think it is one of the best on the market.

~ ~ ~ jaknight ~ ~ ~