Author Topic: Persistent CB750 starter button issues (Read 4538 times)

cshanek · « **on:** November 24, 2018, 04:20:46 pm »

This probably breaks some forum bylaw, but $50 to whoever gets me past this issue (provided the solution isn't "you have a short in your loom dummy").

I have a 76 CB750 that I have rewired, installed a new loom, new rectifier, battery, switch box, ignition switch, and handle bar switches a WHILE ago, and I have not been able to get past a persistent issue with the handlebar starter switch assembly. At first I thought the aftermarket switch I purchased (Vintagecb750 I think) was to blame so I emailed them and received a replacement, however the problem persisted. Now I am leaning toward a short or an issue with the starter motor safety unit connections (I have tried in gear, in neutral, clutch engaged).

Here is a link to a short video I shot showing the symptoms and open connections (I felt it was just easier this way).

https://photos.app.goo.gl/xW4kkA2X5b8ASvih6

Any help/thoughts/pointers would be greatly appreciated.

Mantree · « **Reply #1 on:** November 24, 2018, 07:46:02 pm »

Have you tried jumping the solenoid with a big screwdriver? If that works the next move is to grab some alligator clips and test to see if the solenoid is working. If that tests out grab a multi meter and work your way up the harness to the switch. Dont forget to check for ground. Tou might have a bad diode.

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golfman · « **Reply #2 on:** November 25, 2018, 06:33:19 am »

You need to look into how that new aftermarket switch is wired. My understanding is that the 1976s had 3 wires to the starter button and all other years have 2 wires .

Mantree · « **Reply #3 on:** November 25, 2018, 07:24:14 am »

Quote from: golfman on November 25, 2018, 06:33:19 am

You need to look into how that new aftermarket switch is wired. My understanding is that the 1976s had 3 wires to the starter button and all other years have 2 wires .

You are correct it looks like the headlight is run through the starter switch to kill the light on start up you can acheave the same thing with a relay in the headlight bucket if you want although I just have a switch to shut off my headlight.

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bryanj · « **Reply #4 on:** November 25, 2018, 07:27:18 am »

Early bikes had 1 wire to the starter button that grounded to the bars whilst later ones had 3 wires that turned power from lights to starter.
This happened when the US safety nerds insisted on lights being "always on" unlike us Brits who are alowed to turn them off!!

Jerry Rxman Griffin aka MuthaF'er · « **Reply #5 on:** November 25, 2018, 08:04:15 am »

I see wiring for a starter safety switch in the left control. Perhaps you addressed this in your video. I stopped watching when you pressed the starter button. Did you pull in the clutch lever to disengage the safety switch?

cshanek · « **Reply #6 on:** November 25, 2018, 09:25:16 am »

All, thanks for the questions/pointers.

Mantree, yes I short the solenoid in the video.

Jerry Rxman Griffin aka MuthaF'er, yes I have tried it with the clutch engaged, and in and out of neutral.

You all are correct about the switch ... from VintageCB750.com (https://www.vintagecb750.com/products/5/electrical/90/handlebar-switches) ...

Quote

Fits: Honda CB750K (1973-75)

PLUS- Also, fits the models below but the original switches for these bikes below did not include the OFF/ON switch on the side like this switch does.

Fits: Honda CB750K (1976),

According to their instructions (https://www.vintagecb750.com/parts/instructions/12-0301.pdf) the brown and white and black and red aren't necessary. I am going to take a look at the headlamp switch instructions too. Perhaps something is going on there?

bryanj · « **Reply #7 on:** November 25, 2018, 10:36:39 am »

That wiring description sounds wrong to me. You need the black connected to a black in the headlamp then verify that you get 12v on the yellow/red with ignition on and button pushed.
If that happens report back and i will give you the next test

Mantree · « **Reply #8 on:** November 25, 2018, 01:30:36 pm »

If you are up for a field trip I am in fort Collins. If you trailer it up I could have a look. I have rewired a few bikes from the ground up.

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Mantree · « **Reply #9 on:** November 25, 2018, 01:48:29 pm »

Is it a 76 harness or a universal k3-k7 harness

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05c50 · « **Reply #10 on:** November 25, 2018, 02:05:31 pm »

I think that you're on the right path, if you fix your headlight problem, you'll fix the start problem. Do you have power going to the contacts inside the switch that you showed in the video? There should be power there with the key on to power the headlight and then pass through the switch when depressed to activate the solenoid. If you get power there, both problems will be fixed.

......Paul

05c50 · « **Reply #11 on:** November 25, 2018, 02:11:08 pm »

Another question: What is the yellow and red plugged into inside the headlight? Your video moved too fast to see where it goes.

......Paul

Jerry Rxman Griffin aka MuthaF'er · « **Reply #12 on:** November 25, 2018, 03:10:31 pm »

Well #$%* cshanek, you didn't mention the Springs. I'm in Briargate. Where are you? Be home Saturday. On the road. Maybe I can find time to come over. I have to head out next Wednesday.

cshanek · « **Reply #13 on:** November 25, 2018, 04:00:47 pm »

Another round of updates ...

I tested and just eventually replaced all 3 fuses just for the heck of it. I also unplugged the brown/white and red/black connectors from the handlebar ignition switch and clamped them off to the side. I plugged the red/black from the handlebar light switch assembly into the open red/black port. Now all I have open are the two clamped connectors mentioned above, a brown/black male from the handlebar light switch, and two females from the loom (brown/black (hot) and brown/red). I noticed that when I plug the brown/black connector from the handlebar light switch, the turn signals are much quicker. Could this be the "speed lights" described here (https://www.vintagecb750.com/parts/instructions/12-0300.pdf)? I also have an open connector to the beeper/squawker. Pictures of these connectors are attached.

Mantree ... the harness I purchased was the 1976 harness (vintage CB750 part # Part #: 24-4004)

05c50 ... The red/yellow is plugged into a female red/yellow from the loom, which runs to the solenoid (I think).

BrianJ ... I have green/blue/white lines on my headlamp connector. The wiring diagram I have shows green/white/black. I tried connecting the blue to a blue receptor (that had the ignition button black connected to it) and no nothing changed. I connected the positive lead of the multimeter to the yellow/red set (one from ignition handlebar switch, another from loom) and the negative lead to ground connectors. Oddly enough I had about 9-10 v when I didn't press the ignition button or engage the clutch, but 0 when I did either of those. 30 and 60 second videos illustrating this are below:

https://photos.app.goo.gl/31mT3nFnyZETYoPp8
https://photos.app.goo.gl/LuY3fYcNA2f4oWH29

To the Colorado riders, I will happily take you up on your offers if I cannot hash this out by the weekend.

-Shane

bryanj · « **Reply #14 on:** November 25, 2018, 04:10:13 pm »

Black wire from ignition should ONLY go to black definately not blue. Blue on headlamp is main beam, white is dip beam and green ground. The test i wanted you to do is tell me if there is 12v between the yellow/red that comes from the switch, ignore the loom, and ground when the button is pushed and the black from the switch is connected to a black and the ignition is on. You could also check that there is 12v between the black and ground when the ignition is on.
Dont forget i am in th UK so on a different time scale to you!

Mantree · « **Reply #15 on:** November 25, 2018, 05:19:38 pm »

Going off of the instructions it looks like you need to bundle the red and black wires from the switch and conect them to harness black to get power to the starter swith, headlight and kill switch

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smacd11 · « **Reply #16 on:** November 26, 2018, 08:38:12 pm »

In your video, you turned on the ignition, then the off/on switch to "on"(isn't that the headlight on/off switch). Website says that the '76 didn't have this switch. The switch you show in the video doesn't appear to have the 3 wires to the start button like the '76 should which is probably why it has the on/off switch. Also, when you jumped the solenoid with the screwdriver, that didn't confirm the magnetic switch is good, it bypassed it and confirmed the starter is good.

Here's what I see looking at the wiring diagram (assuming I am looking at the right one as there seem to be conflicting diagrams. I am using the Honda service manual):
1. The start button supplies ground to the solenoid (green/red wire goes through the neutral switch to ground, yellow/red goes to the solenoid.
2. The black wire at the solenoid should have 12v from ignition switch. I would check with your volt meter to confirm voltage there. If there is no power at the black wire to the solenoid I would check wiring from ignition switch to confirm all connections are correct. I assume some of them must be correct for the signals to work. My best guess is that the black wire is not connected to the solenoid as well as one of the connections for the headlight as mentioned.
3. With the ignition on, and not pressing the start button, there should also be 12v at the yellow/red wire. If you jumper the yellow/red to ground (bypassing the start button and neutral switch) it should energize the solenoid and, if the solenoid is good, crank the starter.

Hope this helps.
Shawn

bryanj · « **Reply #17 on:** November 27, 2018, 06:27:39 am »

Shawn, you are looking at the earlier wiring diagtam. 76 should have the solenoid wired the other way round as i described before.

Don R · « **Reply #18 on:** November 27, 2018, 01:28:16 pm »

Am I correct in thinking he has the 76 harness for no headlight switch but the pic shows the k3 or so right control with a switch? The bikes with no headlight switch use a male end to male end jumper in the harness to bypass where the headlight switch would be on a non us bike.
I installed a Euro right control with a switch and park lamp on my 75 400F, once I removed the jumper all colors plugged in correctly and worked.

cshanek · « **Reply #19 on:** November 27, 2018, 08:44:41 pm »

According to vintage cb750 that switch "fits the models below but the original switches for these bikes below did not include the OFF/ON switch on the side like this switch does." For installation they simply instruct you to not use the brown/white and black/red wire pair that runs to the on/off feature.

cshanek · « **Reply #20 on:** November 27, 2018, 08:48:05 pm »

All, I really appreciate all of the great feedback. I have not had time/energy to head out to garage and test/play over the last few days as I have come down with the flu (or something like it). I am hoping to get some time out there tomorrow or Thursday and I should have some results.

my name is nobody · « **Reply #21 on:** November 28, 2018, 06:11:03 am »

Just throwin this out there, Is the new MAIN loom you replaced for a '76 as well?

cshanek · « **Reply #22 on:** December 02, 2018, 05:44:58 pm »

BryanJ ... I ran those two tests ... the results are below:

Black & Ground = 11.7 volts with ignition on
Yellow/Red & Ground = +- 20mv without clutch engaged and without pressing ignition. 0V with either clutch engaged or ignition button pressed.

My name is nobody ... The loom I purchased was for 1976 CB750Ks only. (Top item (24-2004) here: https://www.vintagecb750.com/search/?q=24-4004

-Shane

bryanj · « **Reply #23 on:** December 03, 2018, 12:20:19 am »

Was the 0 volt at the yellow/red with the black connected to black and the ignition on

pjlogue · « **Reply #24 on:** December 03, 2018, 01:24:39 am »

Cshanak, I too have a '76K and put the same right control switch you have on my bike. I ran into the same problem as you. What I did was trace out the wires with a multi-meter then drew a diagram of what should go where. The wires need to be "crossed over" in the headlamp bucket to the left control wiring so your high/low beam works. When I was done, my headlight needs to be on in order to start the bike. After it's running I can turn it off if needed. I can't remember if I did it this way on purpose or by accident but it keeps me from riding off with the headlight off. The headlamp still goes off when starting to prevent excess battery drain.

Sorry I can't tell you what wires got to what as I did this ~4 years ago and it was confusing when I did it, let alone 4 years later. Memory isn't that great these days. Doing a trace out and then putting it on paper so I had a diagram to go off of is what got me solving the problem. The wires on the aftermarket switches and harness are not color correct. Hence the wire trace out with the multi-meter.

-P.

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Author Topic: Persistent CB750 starter button issues (Read 4538 times)