Topic: Weird power thing going on. Bad ignition switch?

[Patrick]

I am trying to trace an electrical loss in my '75 CB750K. I think it may be the ignition switch, but I am looking for confirmation before I buy one.

I have power over the red wire from the battery. I have no power over the main fuse. I have power at the red wire before the fuse, however, and I have power at the ignition switch after the fuse, so I ruled out the fuse. At the ignition (square connection aftermarket type), with the switch on, I have power at the red wire and the black wire, but no power at the brown and white or brown wires. Should I have juice on all four after I turn the switch on? Doesn't the switch distribute power from the red wire to all three of the other wires?

Help, please.

Patrick

[Patrick]

OK, maybe a few more details will entice a reply from someone....

I have power on the red wire at the ignition switch. I have power on the black wire on the ignition switch when the switch is in the running position. I have no power at the brown wire of the ignition switch, but the taillight works on the parking setting of the ignition so I think it must be getting power. I have power on the black wire in the headlight bucket, but I have no power on the B/W wire on the headlight bucket or anywhere else, so I have no headlight, speedo light, tach light etc. The turn signals work and the brake lights work. The starter does not respond to the button. The horn works.

I read no power on the headlight fuse, nor on the main fuse. This I cannot explain, as if I had no power on the main switch how do I have power on the red wire at the ignition switch, which is downstream from the fuse? I also have power on the red wire at the harness into the fuse box and I have power on the red wire when it leaves the fuse box.

The wiring inside the bucket got disconnected and that's when the power issues first arose. I have reconnected everything, however, and I cannot find any other place where the B/W wire would get juice except through the ignition switch.

This is the second ignition switch I have tried. The first gave me no power on anything. The second - which I found in a drawer and whose pedigree is long lost in my misty past - gave me the limited power I have now.

If I am missing something I am open to suggestions. I don't want to buy another ignition switch if that won't solve the problem.

All I really need to know is when I turn on the ignition switch should all four of the wires that connect to the ignition switch be hot?

TIA.

Patrick

[HondaMan]

This problem is not in the Ignition switch: it is in the right handlebar START button switch.

On the K4/K5/K6 bikes, the Start switch supplies Ignition power to the BLK/WHT wire when the button is relaxed. It switches this same power to the YEL/RED when you push the button for Start, as it opens the headlight (BLK/WHT) side of this switch. If the headlight is halogen, it may have burned the switch contact out (or melted it out) on the headlight side: very common.

There's a couple of fixes for this, should yours be found to be a burned-out contact (like so many I have seen). One is, of course, a new switch, but be aware that a halogen headlight will take it out, too. Another fix is the Switch Saver relay kit I make for these, but if the contacts are already gone then it must be wired in a little differently from "Plan A" (I can explain, if necessary...).

The other fix is to wire the headlight's BLK/WHT wire inside the headlight directly to the BLACK inside the headlight bucket. This leaves the headlight on all the time, even when starting, so make sure your battery is a good one. 

[Spanner 1]

With respect, the K1 thru' K5 has a separate headlight on/off switch and the 'start' button is a 2-wire deal.
I think the poster has a K5.... maybe the h/l on/off is bad or the 'stop' switch which passes power from the Black to the Black/White ?
                                                 OR,
The fuses need replacing and the fuse clips need to be cleaned and polished ( I'd do that for sure, anyway  . More likely IMO.

[TwoTired]

All I really need to know is when I turn on the ignition switch should all four of the wires that connect to the ignition switch be hot?

This is a trick question.  All four will be hot only if the headlight switch is also on.  If the lighting switch is off, only red and black will be hot with the key switch on.

See pg 224 of the Cb750 Honda shop manual.  The wire diagram there also shows what the key switch connects in each available position.

Cheers,

[Patrick]

Thanks, guys. Those are the bits of information I was looking for. I can fix this now.

Patrick

[Patrick]

OK, an update, but not a total solution yet.

I jumped the headlight on/off switch in the right controller - black wire to Br/W. I got power on the Br/W wire and the gauge lights came on. So that is good. However, the little jumper wire I used quickly began heating up, like there is a short somewhere or like I was connecting hot to hot. Tracing wires appears to be in my immediate future. A question before I begin.

Can this condition be caused by a bad ground, or lack of adequate grounding? Or would a bad ground only cause the wire to be dead?

PatricK

[TwoTired]

Connecting hot to hot is like dunking a pipe full of water into a pond.  There is no reason for water (or current) to flow through it.

Your switch bypass (jumper) connects power to load and enables current flow at whatever rate the lighting circuit draws.  What do you have on that circuit?   Is it still fused in series with the Main?  The head and tail fuses are normally 7A and 5A, which should limit excessive current draw.

"Grounds" are simply the return path for current to flow in the circuit back to the battery NEG terminal.  Current flows in a loop between battery post POS and battery post NEG.  A connection break interrupts current flow.  A Short (short circuit) bridges the battery power source before the load can receive the power.  The load normally limits the current flow and wires are sized to handle the expected flow.  A short removes the current limiter and allows full current available to flow through the circuit.  If not fused, then the circuits heats until something melts.

Cheers,

[Patrick]

The wire in the jumper is from another MC wiring harness, so it should handle the load even of the 15 amp main circuit. Especially since the headlight is not connected, so the actual draw should be low.

Sounds like I get to trace some wiring and look for a short.

Thanks, TT.

Patrick

[HondaMan]

With respect, the K1 thru' K5 has a separate headlight on/off switch and the 'start' button is a 2-wire deal.
I think the poster has a K5.... maybe the h/l on/off is bad or the 'stop' switch which passes power from the Black to the Black/White ?
                                                 OR,
The fuses need replacing and the fuse clips need to be cleaned and polished ( I'd do that for sure, anyway  . More likely IMO.

Sadly, it's more complicated that that for the K5 owners, it goes like this:

K0-K2: the right-hand handlebar switch has an OFF/LO/HI switch for the headlight. Nice and simple! 

K3-K5 up to 3/75 build (or so) have an OFF/ON on the right side, with a HI/LO on the left side. Not terribly complex (except they came in several wiring colors schemes...). 

K5 after that, and those retitled as K6, may have either the K3-K5 arrangement or the F0-F1 arrangement with no OFF switch on the right side, having a little plastic cover on the switch slot instead. Even some K4 bikes were set up this way: not a lot of them, though.   

To confuse things a little more:
On the K3-K5 bikes with the ON/OFF switch on the right, some have a 2-wire START and some have a 3-wire START button. Of the ones that have a 3-wire START, there may be EITHER: 2 BLK/WHT wires coming from the right hand switches to the headlight bucket, OR, one BLK/WHT wire and one BLK/YEL wire instead. Both of these are fed from the BLK in the headlight, and some bikes have 1 BLK wire feed going up into the handlebars (2-wire START) while others have 2 (3-wire START).

The difference is:

If 2 BLK/WHT wires appear, one is coming from the RUN/OFF switch, and one is from the relaxed side of the START button. The former is for the coils, the latter for the middle fuse of the fusebox, which comes back to the headlight bucket and goes up to the left HI/LO switch, where it returns as BRN/WHT, or sometimes with a RED stripe instead. (This caused much consternation when we were assembling these bikes new, as Honda did not document this setup, and if connected wrongly, caused the RUN/OFF switch to operate the headlight...). In a few cases, a BLK/WHT might also have a little YEL collar around the wire near the connector, which was Honda's "signal" to us as to which BLK/WHT went to the fuseblock.
(A side note: in these bikes, the color of the BLK/YEL in the main harness becomes BRN before it reaches the middle fuse clips, returning as BRN to the headlight, enroute to the HI/LO switch).

If one is BLK/WHT and one is BLK/YEL, the arrangement is simpler to figure out: the BLK/WHT goes to the coils and the other to the middle fuse of the fuseblock, for the headlight circuit. In the K6 and early F0 bikes, this BLK/YEL wire might also be found to be BLK/RED, so watch for that, too.
 

Pretty simple...

[Spanner 1]

 The panel lights come on when the headlight on/off switch is bypassed ( black to brown/white ) but has an obvious short/ overload which has burnt-out this switch.... need to look closely at the brown/white wires for any wrong connections inc. tach. and speedo lights and h/light... also look at the orange/white and blue/white running light ( to front turnsignals ) wires..... Good luck 

[Patrick]

OK, found the short. Now I have power on the Br/W wire and nothing is melting or burning. I bypassed the headlight on/off switch and the kill switch (damage in the switch housing). The starter turns, I have juice at the coils, the blinkers work, the running lights work, the horn works. I still have no headlight and I still have no juice registering at the main fuse. I have juice on the headlight and tail light fuses, so the headlight outta work. This is bizarre. How can ANYTHING work if I have no juice at the main fuse? I removed the main fuse and nothing works. I put it back in and I get everything but headlight and fuse.

I may just get it working and pretend the main fuse will work a test light....

Suggestions? How can this be?

Patrick

[TwoTired]

Is the bike stock?
Does the wire diagram on page 224 of the Honda Shop manual apply to your bike?

[Patrick]

The bike is stock and the wiring diagram is accurate. Everything is hooked up the way it is supposed to be. I am starting to believe that it is the fuse box itself. A few minutes ago everything went suddenly dark on the bike. I have nothing, no matter how many new fuses I plug in. I am starting to think the main fuse is shorted at the fuse box and the short carried enough juice around the fuse to get the lights to work. I also think that short just burned itself out.

I have thew fuse box off the bike and it is dirty, but I cannot see an obvious short. I may just replace the main with an inline blade fuse and see what happens.

I thought, with all the help I've gotten from y'all over the years, that I was getting pretty good at electrical work on these old bikes. But CB750s are wonderful at renewing a man's humility.

Patrick

[TwoTired]

Did you look at the back inside of the fuse box?

[Patrick]

Yeah, it's pretty corroded, but it doesn't look burned. I haven't tried making a jumper and seeing if I can jump around the fuse just to see if the lights come back. The more I think about it the less likely my scenario seems, but I can find no other way to explain why my test light would not light up on either end of the main fuse and yet I had juice on all the right connections (except the headlight). Something has now failed catastrophically and killed the electrical system and I can find no other common link. If it was the ignition switch I should still be able to juice on both sides of the fuse. Before, I had juice on the red wire as it went into the fuse and juice on the red wire that came out, even if the fuse didn't show any juice. Now I have juice just on the red wire on the battery side of the fuse.

If I could have fixed this with a wrench it would have taken about 10 minutes.....

Patrick

[bollingball]

Just get a inline fuse holder and bypass the old fuse to see what happens.

[TwoTired]

Electrical problems like this are always mechanical in nature.  A voltmeter allows you to "chase" the voltage from source to destination by probing along the mechanical path.  This allows you to "see" what the electrons do on their journey.

FYI, corrosion *can* take out the mechanical bridge between connections, including intermittent performance.

Also, check the fuse for integrity.  The end caps sometimes come loose from the inner conductor, and motion of the end caps, as might happen during probing can make the fuse either bridge or come undun.

There IS a science to this.  Your test technique may be part of the difficulty you are experiencing.  To sort that, we need to know exactly where you are probing, and under what conditions the circuit is in, (switch positions, etc.).

Cheers,

[Patrick]

My testing method is pretty basic, TT. When I started, two days ago, the tail light, the dash lights and the turn signals worked, but that was all. I measured the voltage on the red wire on both sides of the fuse. It was 10.6 volts on both ends, so I figured there was some resistance that was keeping voltage low. Touching the probe to either end of the main fuse, or to either end of the fuse holder, showed nothing. I planned to get back to that. The rest of the testing was done with a continuity checker (just a light on a probe with a ground wire). The first day I traced power on all black wires, except through the right switch to the coils. That was a broken solder on the kill switch, so I bypassed it. Then I had juice on all black wires, but connecting black to Br/W (as the headlight on/off switch does) caused the jumper wire between B and Br/W to melt. Found a spot on Br/W where the insulation had cracked and the wire was grounding. Cut and spliced it and then I had juice, as seen on my probe/bulb, on all Br/W wires. Everything worked but the headlight. I pulled the left hand switch and took it apart to make sure there was not also a broken solder in the left switch. It looked OK. Put it back together and hooked everything back up. It all worked but the headlight. While I was tracing the headlight wires I noticed I no longer had juice. Checked the red wire again and I had nothing. Tried a new fuse. Nothing. Tried another new fuse (new, not different) and still nothing. Now I had juice from the battery to the red wire into the fuse but nothing at or after that.

I did not check the voltage at the various points along the way while I was getting juice back to the various parts of the harness. It is entirely possible that it was just a trickle, enough to light up a test light but not enough to do anything else. I was able to turn the starter with the starter buttion, though, so it must have been allowing substantial power at least through the starter button/solenoid.

I just cannot how it could be anything except for the main fuse. It just makes no sense otherwise.

[Patrick]

A postscript, for those interested or for archival purposes....

When I was studying my fuse block it looked OK. It was somewhat corroded, but seemed intact. After I installed an inline fuse to replace the main fuse, however, I went to pull the old tube main fuse from the fuse block to save it for another day. But when I attempted to pull the fuse from the holders, the entire plastic backing came with it, leaving a rectangular hole in the fuse block. It physically LOOKED okay, but I suspect has been heated enough over the last 35 years that the plastic deteriorated. With the inline fuse now everything works.

That led me to wonder. Since the fuse block is really nothing but a mount for the fuses, I don't understand why the fuse block had anything to do with the flow of electricity. Does the electrical system ground to the fuse box? If so, how does an inline fuse work? If not, why would the plastic backing - no matter its condition - having anything to do with the flow of juice? It should have worked as well with the fuse hanging in air with plastic shards hanging from it. Unless, of course, the two separate fuse holders came into intermittent contact because of the deteriorated plastic.

I now test hot on both ends of the new fuse, as well as everywhere else in the system.

Just a lesson. When the electrical system starts doing really weird stuff, check the fuse block.

Patrick

[TwoTired]

That is exactly why I told you to look at the back of the fuse block in post #14.  Did you take off the back cover?
The wires are soldered to the metal clips that hold the fuses.  If the solder fails, or corrosion separates the connections, then the circuit is opened.

The plastic is just an insulated holder to keep the bits in proper orientation.  It has nothing else to do with electrical flow.

I think you may still have a lesson to learn. 

Cheers,