1972 - CB750 Taillight Issue - Request guidance

[tyler]

Hello guys,

First post from a new member. However I've been a fan of the site for a while now and have found it to be very helpful. With that being said here is the situation:

I own an amber yellow 1972 CB750 that I bought back in mid October 2011. The bike was restored after sitting in a garage in southern California for 10+ years and until now has given me no problems.  As far as I am aware the all the lights and electrical worked correctly expect the high beam; more on that later.

Two weekends ago I was riding up and down the coast when I was pulled over by a pair of what turn out to be very nice motorcycle officers. They told me my rear taillight was not working.  Since both officers owned or own (can't remember) vintage bikes they could sympathize and decided not to give me a fix it ticket. Turns out officers can be nice, who knew?

Nevertheless I immediately headed home, opened up the rear taillight assembly and sure enough found a dead bulb.  A few days later I replaced the rear light bulb according to the Clymer's manuel with a new SAE 1157 compliant bulb and did some checks.  Unfortunately only part of my problem has been solved because it seems that the rear tail light only works uncertain conditions.

If I apply the front, rear or both brakes with the bike running or the key in the on position the rear taillight lights up and operates correctly.  However if I have the bike started or the key in the on position with the headlight on the low setting, the the rear taillight does not come on nor get brighter when either or both brakes are applied.  

I took a reading with my voltmeter from the two contacts in the rear taillight assembly where the bulb screws in and with the headlights off the bike registers 14.2 volts. If the bike is started the reading remains the same at 14.2 volts. However if I switched the headlight on the voltmeter registers 0.29 volts with & without the brakes applied regardless if the bike is started or not.  I can confirm all readings were taken three times to eliminate the possibility of human error and with the help of an assistant.

I can also confirm both turn signals in the front and back operate and flash correctly with the headlight on and off. As I eluded to earlier the front headlight bulb is not stock. I was told that when the bike was restored a new brighter headlight bulb was installed. The one draw back being the fuse would blow if the high beam was used. And so the high beam was disconnected in the front headlight assembly and and electrical wires tapped off. I wrote down the details of the headlight bulb but I cant seem to locate those notes due to a recent move.

To try and fix the problem I've performed these two scenarios:
1) I installed a tested a new & different SAE 1157 rear taillight bulb. My local auto parts store only offered the bulb in packs of two so I conveniently had a spare.  Unfortunately no change to report.
2) Thinking the headlight bulb could be the issue, I disconnected the front headlight cables, switched on the headlight, and checked the rear taillights with & without the brakes applied.  Still the same issue  with the same electrical reading of 0.29 volts.

With that being said, I'm hoping someone has had similar experience and can point me in the right direction.  I'm unable to figure out the problem after searching online, reading through the Clymer's manuel and trouble shooting on my own.

Lastly, thanks for reading my post as I realize its fairly long and probably contains a few grammatical mistakes.

-Tyler

[sgtslag]

Brain fart...  Removed non-sensical crap post.  Cheers!

[TOOLS1]

Welcome to the forum. You either have a ground problem (the green wires), or your ignition switch is bad. The light circuits all go through the ignition switch, so I would start there.
TOOLS

[tyler]

sgtslag wrote

Tyler, welcome to the forum!  Now, for the nitty-gritty...

Are you certain it takes an #1157, dual filament bulb, in the rear?  A lot of bikes only have signal lights (#1156, single filament bulbs) in the rear, with running lights (dual filament, #1157 bulbs) up front.  Please go back to Clymer, and confirm, or, better yet, pull the bulb out of the rear turn signal assembly, and look inside at the contacts:  are there two contacts in the bottom, or just one?  If there are two contacts, it takes an #1157 bulb (7W running light/23W turn signal filament -- same bulb used in the brake light assembly).

Now, about the voltage reading of 0.29 Volts.  That ain't right!  You have some sort of short circuit going on somewhere.  Trouble shooting electrical is very difficult to explain verbally.  If you have no experience reading a schematic, it is best to either find a YouTube Video to explain it, or find a friend who is knowledgeable in electrical/electronic circuits to assist you.  Most guys can be bought with liquid refreshments.


Now, about that uber-headlight...  The standard automotive headlight is a 55W/60W headlight.  Anything brighter, is illegal in most every state.  If you install a standard, 55W/60W Halogen bulb, you won't have any issues seeing at night, and you will have a high-beam!  "And there was much rejoicing!..."

If necessary, you can use an automotive relay (~$5) to control that uber-headlight, and power it directly from the battery, with an inline fuse to prevent a fire, but this is overkill, IMO.  It can be done, but I would recommend more liquid refreshments offered to your electrically experienced friend, or more YouTube Videos (and more liquid refreshments...).

If you still need more lights, there are LED's that put out 900-1,800 Lumens (standard 55W/60W headlights put out 700 Lumens), which can be purchased, and added to your bike's engine guard bar (assuming you have one installed already), and wired into the battery with a relay (see notes above about liquid refreshments).  These LED's consume around 6W-15W only, per LED, but they cost a bit of change.

Check the light sockets for how many connectors are in the bottom, and get back to us.  Cheers!

Are you certain it takes an #1157...?
Yes, I'm certain.  The old taillight bulb was 1157 and the Clymer book says the same (M341, page 146, table 1). I just took the rear taillight lens apart and confirmed two connectors.  Here is a photo:


As for the headlight the book says stock is 12v 40/50w but does not supply a part or SAE number. I just took apart the headlight assembly and saw "AutoPal 12V 60/55W" written on the bulb. As for staying legal... I'll take it under advisement.

I appreciate the suggestion of adding LED lights or hard-wiring the headlight but I do not want to take the bike that direction.

I'm a novice when it comes to wiring / electrical for bikes so hopefully I can get the job done.

Thanks sgtslag.

[tyler]

TOOLS1 wrote

Welcome to the forum. You either have a ground problem (the green wires), or your ignition switch is bad. The light circuits all go through the ignition switch, so I would start there.
TOOLS

I'll check both of those out tomorrow.  Thanks Tools1.

I forgot to mention the tach and speedo lights work fine.  If that means anything.

[sgtslag]

Brain fart...  Removed non-sensical crap post.  Cheers!

[TOOLS1]

Hey Sarg. It's the taillight, not the turn signals.
TOOLS

[tyler]

A quick update for those who are following...

Yesterday I identified a ground wire that i thought was the culprit. So I replaced it but alas no success. Unfortunately I had a late started and ran out of useable light so I didn't get to pull the tank and look at the ignition switch.

I did spend quit a bit of time just trying to make sense of the wiring and contrasting it with the documentation.  One of the major problems I have comes from working in a shared underground garage at my condo complex.  Lighting is not ideal, lack of resources, no workbench and small space all makes troubleshooting more of a challenge.  With that being said I'm going to reach out to my father and ask if I can take the bike to his house. It's about half an hour away and I can probably rope him into the project which is a good thing.

[rrgunslinger]

Don't forget fathers day is the 14th!

On Mon, Jun 11, 2012 at 1:25 PM, tyler [via Honda CB750'S] <[hidden email]> wrote:

A quick update for those who are following...

Yesterday I identified a ground wire that i thought was the culprit. So I replaced it but alas no success. Unfortunately I had a late started and ran out of useable light so I didn't get to pull the tank and look at the ignition.

I did spend quit a bit of time just trying to make sense of the wiring and contrasting it with the documentation.  One of the major problems I have is comes from working in an shared underground garage at my condo complex.  Lighting is not ideal, lack of resources, no workbench and small space all makes troubleshooting more of a challenge.  With that being said I'm going to reach out to my father and ask if I can take the bike to his house. It's about half an hour away and I can probably rope him into the project which is a good thing.

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/1972-CB750-Taillight-Issue-Request-guidance-tp4022629p4022769.html

To start a new topic under Honda CB750'S, email [hidden email]
To unsubscribe from Honda CB750'S, click here.
NAML

[sgtslag]

Brain fart...  Removed non-sensical crap post.  Cheers!

[Jeremy]

I second checking the ignition switch.

[Trailblazer1969]

Hi Tyler and hello to all. This is my first post in the forum. Tyler I'm having a similar problem with my bike. The other nite my headlight went out and to make a long story short I went through a process of finding fuses blown until I discovered the blown fuse located under the left side panel. I also have a cb 750 1977 Hondamatic. I replaced the fuse and now the tail light is not working but when I apply the brakes that light works and the the number one fuse keeps blowing and my left gauge light on my speedometer is not working. I'm not very smart at all with wiring problems and have no clue what to do to resolve these issues. Any help would be very appreciated. Again it's great to be a part of this forum. Bill