NGC Honda Cb 750 chopper headlight/taillight problem

[bobrebos1]

I consider myself a decent mechanic/troubleshooter. BUT!!!

I give up on my headlight/taillight voltage!  I brought my bike to a shop to try and find the problem.

1972 Cb 750 chopper.  When turn key to on position voltage at headlight is 9.6volts

Battery is 14.4 about when engine running.  13 volts about when engine turned off.

When I turn key to start position ignition voltage drops from about 12.8volts at the ignition to like 10volts if I remember correctly.

I thought it might be a bad ignition switch so I replaced it.  Same numbers.  Ground wires seem fine at least the one main ground wire.  Anyways I threw in the towel and brought it to a shop.  Anybody want to guess what the cause ends up being?  I should know in about two weeks as shop is pretty busy.  Note the taillight is also very dim.  I forgot to check voltage there but it is very dim like the headlight.  So dim headlight, dim taillight, good ground and switch seems ok and I don’t think taillight is wired through headlight switch????  Let’s see what the shop says.

[slopokes]

Shop just called me to tell you to buy a v85………

[twowheeladdict]

I consider myself a decent mechanic/troubleshooter. BUT!!!

I give up on my headlight/taillight voltage!  I brought my bike to a shop to try and find the problem.

1972 Cb 750 chopper.  When turn key to on position voltage at headlight is 9.6volts

Battery is 14.4 about when engine running.  13 volts about when engine turned off.

When I turn key to start position ignition voltage drops from about 12.8volts at the ignition to like 10volts if I remember correctly.

I thought it might be a bad ignition switch so I replaced it.  Same numbers.  Ground wires seem fine at least the one main ground wire.  Anyways I threw in the towel and brought it to a shop.  Anybody want to guess what the cause ends up being?  I should know in about two weeks as shop is pretty busy.  Note the taillight is also very dim.  I forgot to check voltage there but it is very dim like the headlight.  So dim headlight, dim taillight, good ground and switch seems ok and I don’t think taillight is wired through headlight switch????  Let’s see what the shop says.

Something is providing too much resistance and drawing down the voltage.  I would disconnect the light and measure the voltage at the connectors to see if it is somewhere in that circuit.  If not then it is in the light itself. 

[bobrebos1]

Something is providing too much resistance and drawing down the voltage.  I would disconnect the light and measure the voltage at the connectors to see if it is somewhere in that circuit.  If not then it is in the light itself.

It’s in the circuit and not the light as voltage at light plug/wiring with light disconnected is 9.8 volts.  I just don’t know where in circuit but I’m thinking bad ignition switch!  Il see if I’m right. 

[twowheeladdict]

It’s in the circuit and not the light as voltage at light plug/wiring with light disconnected is 9.8 volts.  I just don’t know where in circuit but I’m thinking bad ignition switch!  Il see if I’m right.

I'm thinking bad relay. 

[Vagrant]

I would have done a load test on the battery before anything else. Relay replacement next.

[bobrebos1]

I'm thinking bad relay.

Bare bones wired chopper.  There are no relays in the wiring system

[bobrebos1]

I would have done a load test on the battery before anything else. Relay replacement next.

Load test on battery was fine.  Good battery, and it’s basic bare bones wiring with no relays installed!

[twowheeladdict]

Bare bones wired chopper.  There are no relays in the wiring system

I’d be interested to see a wiring diagram of the bare bones chopper. 

[Scout63]

Maybe in the wiring from the battery to the switch and back.  I’ll bet there’s a lot of vibration under the tank.  Also check the ground points for the forward wiring, coil etc on the forward frame area. Also maybe in the headlight wiring harness. I keep hearing about this chopper but need some pictures.

[normzone]

In an effort to participate in the conversation, I will mention that the last time I found mysterious rear light issues on my bike, eventually I found wires inside the light housings that the insulation had melted on, and they were making tenuous contact. I addressed that issue and the problem went away. Never did figure out why they melted.

We now return you to your pertinent conversation, which is already in progress. 

[John A]

In an effort to participate in the conversation, I will mention that the last time I found mysterious rear light issues on my bike, eventually I found wires inside the light housings that the insulation had melted on, and they were making tenuous contact. I addressed that issue and the problem went away. Never did figure out why they melted.

We now return you to your pertinent conversation, which is already in progress. 

I’ve had this with the plastic sockets. I attributed it to me using halogen bulbs and a relay on the brake lights. My fix was to modify the reflector and epoxy in a metal socket. On the headlight I found a high temp connector as used on BMW’s. This was before LED’s came along of course. My apologies for contributing to one of those pesky thread drifts

[bobrebos1]

I’d be interested to see a wiring diagram of the bare bones chopper.

Like this except no horn!  https://www.google.com/search?q=honda+cb+750+chopper+basic+wiring+diagram&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari#vhid=WADETvpkGdd-_M&vssid=mosaic

[bobrebos1]

Maybe in the wiring from the battery to the switch and back.  I’ll bet there’s a lot of vibration under the tank.  Also check the ground points for the forward wiring, coil etc on the forward frame area. Also maybe in the headlight wiring harness. I keep hearing about this chopper but need some pictures.

Here’s the basic wiring diagram and pics of the chopper;  https://www.google.com/search?q=honda+cb+750+chopper+basic+wiring+diagram&ie=UTF-8&oe=UTF-8&hl=en-us&client=safari#vhid=WADETvpkGdd-_M&vssid=mosaic















image sharing

[John A]

https://www.cb750.com/threads/honda-cb750-wiring-diagrams.9/
Is this it?

[Paul_Tim]

Thin wires, corrosion in connectors/switches, poor earth return. Had a similar problem on a Yamaha TDM last year which showed itself when changing to LED bulbs and the low voltage caused them to flash      Voltage losses all over the place, rather than chase 40 year old wiring issues I fitted a couple of relays to switch a direct battery feed to the lights, problem solved.

[Stretch]

Thin wires, corrosion in connectors/switches, poor earth return.

If you have any "Scotch Block" connectors I'd look there first.

Yer gonna have to go wire by wire and connection by connection in an
old wiring harness like that.   
I'd start with the alternator connections first, as a number of things
appear to be affected. I'd also suspect the ignition switch. Check
ALL of your ground connections. Every last one.......

Good Luck!

                                            -Stretch

[Scout63]

Beautiful chopper.  Thanks for the pictures.  That’s as authentic as it gets.

[bobrebos1]

Beautiful chopper.  Thanks for the pictures.  That’s as authentic as it gets.

Thanks,.  I’m going over everything front to back on it as it’s due.  Synching carbs, checking wiring, replacing fork seals etc etc etc

[Grabcon]

Okay I am going out on a limb here it has been sometime since I worked on a CB750, but I have owned many. First since it is a chopper the same goes for the wiring, it has been chopped. Sliced and diced so a factory schematic may not be of use.

Check the main ground, it should go from the battery to one of the top rear engine mounts. Typically the grounding point between the frame and engine has either corrosion or paint that interfere with the ground. The grounding lug slips between the engine lug and frame bolster. If there is paint on either the ground will be bad. The unfortunate part of this is the motor needs to be pulled to make those areas bare metal. Keep in mind that there are three different metals at that connection. Aluminum for the motor, steel for the frame and copper for the lug. There maybe some electrolysis going on there as well.

Voltage drop when bike is turned on. This is normal, a load has been added to the circuit, headlight and taillight. These old CB's did not have a relay to turn the light off when the electric start is used. I used to install them but that memory is long gone as to the wiring.

If I recall the original headlight assembly had a ground wire in the headlight case that was connected to the bolt that held the headlight assembly to the blinker ears. I will assume that that ground is long gone so make sure the headlight has a good ground.

That is it for now. Let us know what happens.

[John Croucher]

I have made this comment several times.  After chasing electric problems around on a couple of bikes, I have found that the ignition switch was the problem.  Poor contact inside the switch would allow voltage through.  But when a load is applied, not enough amps are passing through due to very little surface area of the ignition switch contacts being dirty or corroded.

This is a chopper I bought.  The first mistake I made was destroying the original paint job.  It was filled with bondo and smoothed at all the weld joints.  And then painted.  I sand blasted all the bondo out and then repainted. 

The original springer front end was deadly. 

[Tom H]

That was a girder front end, not a springer. The girder front ends had a rougher ride if memory serves. I had a CB500 with extended tube forks, I don't think the front end did anything unless I hit a curb. Tubes looked more like they flexed.

Good luck chasing down the issue, you have good advise already.

Tom

[bobrebos1]

Thanks everyone for your ideas.  I for one in looking at the chopper schematic believe it’s a bad ignition switch but I will post on here when the cause is discovered and fixed.  Thanks again!