1993 CB750 Engine cuts out instantly when releasing clutch in gear, side stand deleted, clutch switch tests good

[Cilich]

Bike: 1993 Honda CB750 Nighthawk

Problem:
Runs perfectly in neutral and idles fine in gear with the clutch lever fully pulled in. The moment I release the clutch even a fraction of a millimeter in any gear, the engine cuts out instantly. Not a stall, a clean ignition kill, like flipping a switch. It restarts immediately and runs fine the second I pull the clutch back in or drop it to neutral. It originally died on its own while riding, then started doing this. I was in gear, revving and the engine just killed, all lights were still on.

The side stand switch was deleted by the previous owner. The delete was done the normal way: the Green/White side stand switch line is jumpered straight to Green ground so it sits permanently grounded, which makes the ICM read the side stand as always up. The clutch switch was not rewired or touched at the ICM. Stock otherwise.

What I’ve tested:
• Clutch switch: tests good, continuity when lever pulled in, open when released
• Clutch cable: loosened and retightened, no change to the symptom
• Followed clutch switch cables to 9P mini but after that it’s bundled into a main look running towards the ICM. Side stand switch splice test good for continuity.
• Connections look good other than that.

What I’ve ruled out:

• Clutch dragging (switch is functional, drag I saw earlier was just an over-loose cable)

Where I’m stuck:
Per the manual, the ICM fires spark when either the neutral switch line OR the side stand line is grounded. Since the side stand delete keeps Green/White permanently grounded, the side stand permit should always be satisfied, so the bike should run in any gear regardless of the side stand. But it doesn’t. It only stays alive in gear while the clutch is held in. I even try to rev it to try and make it catch but it just dies.

That points to the permanent Green/White to Green ground having failed or gone intermittent somewhere in the main harness loom, with the clutch lever position somehow making or breaking the permit path through that same buried section.

Questions:

• Anyone seen a deleted side stand ground go intermittent and cause an in-gear cutout like this?
• Would this be a mechanical fail in the transmission? To be it does not sound like it since it just cuts off
• At first glance it sounds like the side stand switch but again, there’s non. I’m running out of ideas, does anyone have any other diagnosis?

 

[Cilich]

Oh also, hello everyone, thank you for your help in advance I really appreciate it

 

[Raggaren]

I’m not familiar with 84 and up.
But earlier years the clutch and neutral switch only effected the starter circuit…
I’m thinking maybe neutral switch/circuit could be the problem…
When running the neutral light is on , when in neutral also as no effect on clutch handle ..
I wonder if any wires/connector have been switched accidentally switched around on headlight bucket.
Or at neutral safety switch…???
Strange issue for sure.

 

[Cilich]

It’s gotta be something electrical. I tested the bundle, the G/W and G cables, that the previous owner connected permanently to see if I get continuity. I get continuity on the G cables at the ICM and the bundle but not with the G/W at the ICM and the bundle. Which mean I have a break somewhere in the main loom correct? Right?
Idk ¯\_(ツ)_/¯ im running out of ideas here.

I was thinking maybe the neutral switch being the problem but it runs fine in neutral and runs fine if I pull in the clutch all the way in, as soon as the neutral light turns off and the engine cuts off. I’m super stumped

 

[Cilich]

The bundle is the snipped side stand switch delete btw. I’m thinking that when the previous owner retired it, he just connected the G and G/W so that there will be a ground always. But I’m not entirely sure if that’s how that works

 

[Raggaren]

It’s gotta be something electrical.

I was thinking maybe the neutral switch being the problem but it runs fine in neutral and runs fine if I pull in the clutch all the way in, as soon as the neutral light turns off and the engine cuts off. I’m super stumped

That’s why I’m thinking some wires might been switched accidentally switched..
When in neutral there’s power to neutral light.
Which in turn may also back feed ign system
And clutch switch have no effect..
But in gear no power to neutral light
Release clutch power cuts off like safety switch would do. ???
Any relays in those circuits???

 

[Cilich]

Well the clutch switch wires go to the G/W and G/R cables at the 9P mini. Then the G/R wire goes to the clutch diode and the G/W goes all the way back to the ICM. It splices only at the side stand switch. Which relays are you referring to?

 

[Raggaren]

I was just wondering if there was any in circuit??..
Newer the bike more electronic’s.

 

[Raggaren]

Look/print out this wiring diagram…
It may help figure out issue.

 

[Raggaren]

Have you tried cranking over engine in gear
Engaging playing with clutch in/out to see if switch working engine turning over.
If releasing more than a couple mm does it still turn over.
Just thinking that the switch might be messing you up…
Maybe try jumper clutch switch ????

 

[slmjim n Z1BEBE]

Two thoughts:

1) Clutch lever sw. continuity, but at what resistance? Some devices (ICU?) don't like say, 300 ohms (as an example) in the circuit that drops voltage when the ICU expects to see near zero resistance / full voltage when the sw is closed.

2) I've seen intermittent opens develop in a harness at the steering head where it flexes every time the front end is turned. Have you tried testing at different front end positions of rotation?

Good Ridin' (eventually...)
slmjim & Z1BEBE

 

[VFFreedom]

Looking at the wiring diagram provided by Raggaren, I would be checking the circuit at the clutch diode connector. Its probably easily accessible. I doubt the actual diodes are bad, though never say never again ( side note one of my favorite Bond Movies). But since the bike runs in neutral, the neutral switch connects right to the ICM bypassing the clutch diodes and the safetys connected to it. Think of the diodes like electical check valves. Electric can only flow one way. You should be able to test the circuit, switchs and wire, integrity and closure there. The center G/R on the clutch diode plug goes to the ICM. We know that is good as the bike will run in neutral. Testing here cuts the circuit in half, test, then narrow down some more. You'll find it.

 

[Cilich]

Thank you very much to everyone who’s responded. I really appreciate it. I had some company this weekend so I haven’t been able to work on the bike just yet, I’ll be there tomorrow pretty much all day so I will update you all.

Thank you raggaren for providing that diagram. I have been studying that and also my service manual like a bible and I thing I found something of note.
I think that you’re right Z1, the clutch sw is definitely providing a ground and letting the bike run while in gear and with the clutch fully pulled in. I have tried at different positions before and I have had the same result so I think the sw might be good.
Also Raggaren, I have tried starting it in gear and it turns over fine but as soon as the neutral light turns off the bike just switches off.
VFF, yes the diode seems to be fine and the wiring that runs to the ICM is intact.
I probed both the harness sides and check for continuity. I also checked to see if there’s continuity given that the clutch sw goes to the diode and then to the ICM.

My next test will be to ground the G/W wire since i am not getting continuity from the side stand switch. The G wire seems to ground just fine but since the G/W is the only other wire that provides a ground while the bike is not in neutral I think that might be the culprit. Imma pierce it with a needle and clamp it to the battery negative, hopefully that will maintain the ground regardless if I am in neutral or in gear. So fingers crossed that solves the issue.

Next thing if that does fix it is imma rewire the whole bike bc *beep* trying to find wiring I have not done myself.

Again thank you everyone for giving me ideas and suggestions. It’s really awesome having this forum. I’ll update you tmr. I’ll also add some pictures of the wiring so you guys get an idea

 

[Cilich]

SORRY for the radio silence!!
I FIXED IT!!! It was that bad ground on the green and white wire! Easy fix and now I get my bike back . Thank you so much to everyone for helping.

 

[Dewe]

SORRY for the radio silence!!
I FIXED IT!!! It was that bad ground on the green and white wire! Easy fix and now I get my bike back . Thank you so much to everyone for helping.

That is AWESOME that You found the cause and fixed it Cilich! I ride a 2001 Honda CB750 Nighthawk (exact same engine,ICU,and wiring as yours!) and though my Side-stand wiring is stock and has never been touched or played with............Your situation DOES INDEED help me if I ever have a problem with my electronics! (as I'm strong in Mechanical-things,but have always fell short with any type of "electrical" thing,whether it be wiring,diodes,resistance,etc.) As my comment would've been something like "make sure your grounds are Clean and solid and make sure your getting full contact within your G/GW wiring running all the way to your ICU!" So again,it's so awesome to read You figured it all out! Congrats man.!