Topic: CB750K6 No Electrical Power

[M 750K6]

Hi,

I recently joined and posted a brief intro in the new members section. I have run across a problem:

I am reassembling my 750K6 from a bare frame. The engine is not yet back in, but the front and rear end are back on, along with the wiring loom, including in the headlamp bowl and all the electrics connected to a new 12v battery (tested at 13.2v). So, the only things not connected are the wiring with the engine. Prior to strip down, all the electrics worked fine. I've cleaned them up and replaced one bullet connector, otherwise, as it was before.

I thought I would check everything works before putting the engine back in. I turned on the ignition and nothing, no tell-tales and nothing is working (lights, horn, indicator, brake lights). The fuses are good and I have 12v in various parts of the loom, including to the coils. But these were random stab in the dark tests, not very systematic.

I started looking at the wiring diagram, but I find it a bit unreadable! Electrics are a bit of a black art, to me.

-Should I expect lights and telltales to work before I install and connect up the wiring to the engine?
- If so, is there a good way to start checking different parts of the loom, to try to isolate where the problem starts?

Thank you,

Martin

[rocket johnny]

check and clean all the grounds to start

[M 750K6]

So far I have only come across 2 earth or ground points. One is on the horn bracket, the other is the main lead from the battery -(ve) to the engine mount. I've ground off the powdercoat for a clean connection for both.

[calj737]

Yes, you should have lights and a horn without the engine in. You may not get the indicators to flash unless the motor is running depending upon the type of signal relay you use.

Start by backtracking from the headlight bucket to the main loom. With the key ON, check for 12v to the colored wires that supply the power to the headlight and Horn. You should also check with a meter that the fuses are truly passing voltage from one side to the other. If youre using the stock glass fuses, the clips (especially on the backside of the block) can be problematic if you don't get the fuse seated properly.

While the attached diagram may not be exact for your bike, it should be more readable and allow to descipher the wires involved for testing.

[newday777]

I've not tried to test without the motor in the bike, the only thing that comes to mind is that the starter connector needs to be plugged in to complete the circuits for power to energize the system.

[BenelliSEI]

So far I have only come across 2 earth or ground points. One is on the horn bracket, the other is the main lead from the battery -(ve) to the engine mount. I've ground off the powdercoat for a clean connection for both.

There is another ground wire that ties all the grounds in the loom to the frame. From memory, it’s a green wire ending in an eyelet. It bolts under one of the two screws that hold down the seat latch/lock. Without it attached, you’ll get nothing. I’ll take a picture of mine, later today.

[M 750K6]

@BenelliSEI: You're right. I forgot that one. I will take it off and check it is a good surface contact.

That said, I am getting 12v at the coils. But, I may have cooked that reading up, because, thinking about it, I had the kill switch off, so surprised I'm getting power to the coils.

[M 750K6]

Yes, you should have lights and a horn without the engine in. You may not get the indicators to flash unless the motor is running depending upon the type of signal relay you use.

Start by backtracking from the headlight bucket to the main loom. With the key ON, check for 12v to the colored wires that supply the power to the headlight and Horn. You should also check with a meter that the fuses are truly passing voltage from one side to the other. If youre using the stock glass fuses, the clips (especially on the backside of the block) can be problematic if you don't get the fuse seated properly.

While the attached diagram may not be exact for your bike, it should be more readable and allow to descipher the wires involved for testing.

Thank you for this. I will print the diagram. It is clearer. Are the black oblongs in the diagram the connectors?

It looks like the only positive connection is via the starter solenoid? I do have this connected into my loom, except the lead to the starter motor, which is with the motor, to be bolted onto the terminal on top of the solenoid when I install the motor. However, that missing part of the circuit is only live when the starter switch is depressed, so I think I should have a positive feed through the solenoid into the rest of the loom, without that feed to the starter jnstalled.

I've given up for the day as it's dark and I'm cold! Will crack on tomorrow. Thanks for input so far.

[BenelliSEI]

Here’s the ground wire (bright green). Bolt it down under your seat latch to nice clean metal. It delivers a ground to ALL lights, brake switches, horn, etc. via the loom, Also, even without the engine in place, you must bolt the battery ground lead to the frame, otherwise this green wire shown gets no ground (-) from the battery.

[BenelliSEI]

That diagram is for a K1, single 15 amp fuse. Not for you. Find a K6 diagram. Power is fed through the solenoid. A lead off your solenoid should have a large, single (“shielded”) red that plugs into the loom and feeds your fuse panel. See photo.

[M 750K6]

Thank you guys. I have cleaned off the frame where the ground is on the main engine mounting. For now, I tightly clamped the black negative cable direct from the battery to that point of the frame. The contact should be clean and good. However, i didn't appreciate the importance of the green eyelet point behind the seat lock and i will check that first thing tomorrow..I think that's most likely my problem. I hope so.
👍
Martin

[BenelliSEI]

Martin…… I’ll bet money you got it solved! John D.

[HondaMan]

Another tip or two:
Some of the 750 wire looms after the K2 have that green ground eyelet up near the spark coils, and it is supposed to then mount between the left coil's front metal bracket and the frame: the frame will be unpainted there if yours is that way.

Another tip: the old-style glass fuses (type SFE) are 1" long and have solid-web elements inside. These have not been made since 1996, and the world ran out of stock in about 2006 or earlier. If the fuse has been replaced since then, it is likely and AGC type, which will self-destruct when used on the road while carrying current: they are not shock-rated and they fall apart from being soft (hot) while passing power thru them. And, the little clips that hold the fuses were also intended for a 10-year life, circa 1976: that coating is now a nice resistor that drops considerable power while also heating the fuse caps, often melting the fuses in the process.

So, if you find yours in that shape, consider replacing it with one of my ATC-style fuseholders, built to expressly solve this problem. If you buy a new OEM-style fuseholder (as are being sold today in many places) they do NOT contain the proper fuse, and the 15A main fuse WILL melt and fail when riding on a bumpy road. I bring this up to try to help detour the situation so many have found themselves in after buying these look-alike fuseholders, only to not solve the actual problem.

[M 750K6]

Few hours in. Some progress, some problems. This is what I've done and found:
The earth behind the seat latch was poor, so I've addressed it.

I now have good, low resistance continuity across the fuse block.
From the right side of the fuse to the +(ve) battery terminal is less than 0.2 Ohms
From the battery +(ve) to the red connector in the ignition switch is also below 0.2 Ohms.

On 1st position of the ignition key, I now have a headlight and rear light, indicators, horn, starter solenoid clicks, tacho and speedo lights and brake lights. So I think that's everything.

Other than the indicators and blue main beam, I have no telltale lights on the headstock dashboard. I'm hoping that is correct?
Am I right to assume no neutral light, as the motor neutral switch is not yet connected?, ditto for the oil pressure light?

When I go to the next position round on the ignition key, I only get lights. Is that correct?

I have friends coming over tomorrow to provide muscle for the motor install (excited about that).

Martin

[newday777]

Neutral should come on if you ground the wire that hooks to the Neutral switch. Same with the oil pressure connector.
Second key position is for park mode, taillight only should be on.

[calj737]

Other than the indicators and blue main beam, I have no telltale lights on the headstock dashboard. I'm hoping that is correct?
Am I right to assume no neutral light, as the motor neutral switch is not yet connected?, ditto for the oil pressure light?

Correct.

[newday777]

Other than the indicators and blue main beam, I have no telltale lights on the headstock dashboard. I'm hoping that is correct?
Am I right to assume no neutral light, as the motor neutral switch is not yet connected?, ditto for the oil pressure light?

Correct.

They can be tested though by grounding the wire to each to see if the dash light comes on as if it were in neutral and has no oil pressure.

[M 750K6]

All good. Thank you for your help. 👍

[PeWe]

There is a jumper wire between black and brown or brown/white at front. Connectors in headlight bucket.

Visible in Honda diagram.
That will be missing if replacing the entire loom.
Gauges etc will not lit without it.

[rotortiller]

the old-style glass fuses (type SFE) are 1" long and have solid-web elements inside. These have not been made since 1996, and the world ran out of stock in about 2006 or earlier.

 Bought these kits last summer at Canadian Tire. They measure 1 inch in length and fit my CB750, spec is AGX whatever that means so we have a replacement that works good, lasts a long time. They also sell SFE sizes too. https://m.littelfuse.com/~/media/commercial-vehicle/catalogs/littelfuse-aftermarket-catalog-cvp150.pdf

[BenelliSEI]

the old-style glass fuses (type SFE) are 1" long and have solid-web elements inside. These have not been made since 1996, and the world ran out of stock in about 2006 or earlier.

 Bought these kits last summer at Canadian Tire. They measure 1 inch in length and fit my CB750, spec is AGX whatever that means so we have a replacement that works good, lasts a long time. They also sell SFE sizes too. https://m.littelfuse.com/~/media/commercial-vehicle/catalogs/littelfuse-aftermarket-catalog-cvp150.pdf

I get those at CTC a too. Maybe we can sell them on here? 🤣

[rotortiller]

I get those at CTC a too. Maybe we can sell them on here? 🤣

The way I figure it we should double our money but only after working the exchange rate and taxes. We could also open the package and sell singles for 5 times the cost just like some did with smokes and firecrackers back in the school yard! lol. OEM fuse box works fine if cleaned once every decade or two with scotchbrite to remove oxides, so we should have a stable market  .

[Deltarider]

Maybe my eyes are not good enough, but... where does the package say what amperage they are?

[Nurse Julie]

Hi Martin
Don't forget yours is a UK model, therefore some of the wiring will be different than the USA models, like we don't have running lights on ours where the USA models do.

[M 750K6]

Hi Julie,

Yes, that's the case, a UK bike. I'm pretty confident it's now all sorted. The cause was the bad earth at the seat latch area. My poor workmanship. The neutral and oil tell tales will be due to the switches being with the engine. I was getting power to the coils, too. So I think it's ready.

Finding a UK K6 wiring diagram that I could read on one screen has proved impossible, so far. The black and white one I have in the downloaded manual was poor resolution, so I couldn't read the labels. But I worked back through the connections on the earlier model diagram and it seems very similar, it was certainly enough to get a good idea. Getting that K1 version in colour helped me.

Excited about getting the engine back in tomorrow, then on the home straight. That's tempting fate!