Neutral & Oil Light issues! Please Help

[Sellner91]

Hey guys,

Looking for some help with a neutral light and oil pressure light issue on my 1982 CB750K.

The bike came to me with an aftermarket mini speedometer/tach setup, and since the day I bought it neither the neutral light nor the oil pressure light has ever worked. Everything else on the gauge works fine.

Over the years I've tried quite a few things to fix it:

• Replaced the complete wiring harness.
• Replaced the neutral switch.
• Replaced the oil pressure switch.
• Checked and reworked wiring connections several times.
• Verified that the indicator LEDs in the aftermarket gauge actually work by powering them directly.
• Using the OEM gauge cluster connector for most of the wiring.

The aftermarket gauge has paired wires for each indicator (green/green-black for neutral, red/red-black for oil, etc.). Through testing I found that if I feed 12V directly to the neutral indicator wire and ground the corresponding green/black wire, the neutral light comes on, so the gauge itself appears to be working.

Following the wiring diagram, I connected the neutral light positive to the brown/white gauge illumination wire and connected the neutral light ground side to what I believe is the light green/red neutral switch wire from the Honda harness. The neutral light then comes on, but it stays on all the time, even with the engine running and the bike shifted into gear.

At this point I'm trying to figure out whether:

1. I'm somehow on the wrong wire in the OEM connector.
2. The neutral circuit is grounded all the time somewhere in the harness.
3. I'm misunderstanding how these aftermarket gauge indicators are supposed to be wired.

The oil pressure light has a very similar issue and has never functioned either.

idk what i am doing wrong but there is obviously some thing out of wack.

Has anyone run into this on an 81-82 CB750K with aftermarket gauges? If anyone has a photo of how they wired their neutral and oil pressure indicators or can point out what I'm missing, I'd really appreciate it. ( will include photo of the gauges i have tried to wire but with no success for the neutral or oil light)

cheers,

 

[mrtwowheel]

I've been through the same thing. This is what I remember about those gauges. The wiring diagram that they give you is NOT how the gauge is internally wired. The neutral light is triggered by the ground wire, not the positive wire. There is a way to do it using a relay.

 

[SenseAmidMadness]

I've been through the same thing. This is what I remember about those gauges. The wiring diagram that they give you is NOT how the gauge is internally wired. The neutral light is triggered by the ground wire, not the positive wire. There is a way to do it using a relay.

Yes. The way I'd phrase it is that the circuits for both of these lights are interrupted by their individual switches, and both switches are on the "ground" leg of their circuits. Both the oil pressure switch and the neutral switch are screwed into the aluminum engine case and serve as the "ground" and complete the circuit when the switch is closed: the neutral plunger switch by the shift drum poking it and the oil pressure switch by seeing no oil pressure.

If those indicator lights are LED's in that little aftermarket gauge, which they look to be, try this:
Whichever way you have the wires for the neutral switch hooked up to the connector for the gauge, switch them around to the inverse of what you currently think is "positive" and "negative". Do the same for the oil pressure switch. That might just solve it.

I had a similar problem on my own 1980 when I installed a Koso gauge. Also when I installed smaller aftermarket turn signals. Flipping the wires around fixed it.

The original light setup, because everything was incandescent, didn't care which direction current flowed through the lights because incandescent bulbs are electrically the same as resistors and resistors will do their job in either direction. LED's, however, are Light Emitting Diodes and Diodes are a special resistor that only allows current to flow in one direction. Polarity suddenly matters when you swap to them.

Edit: My fault for not reading your post thoroughly. The fact that you wired the Neutral through the gauge illumination circuit seems off to me; the other end should not always be grounded unless your neutral switch is constantly engaged. Check the switch and make sure it's not stuck closed. You should have no continuity to ground at the center pole of the neutral switch if you're not in Neutral. Inversely, the oil pressure switch should always have continuity to ground from its exterior terminal unless the engine is spinning and has oil pressure.

 

[mrtwowheel]

You need to use a relay. The relay needs to be triggered by the switch/sensor wire (ground wire). Relay must be wired to connect neutral or oil + wire on gauge to positive power.

 

[SenseAmidMadness]

Why does the new gauge need a relay for a pair of simple switched circuits going through tiny LED bulbs? The original doesn't have a relay. See: 1980 CB750K FSM wiring diagram.

Edit: I just checked the 1981 Addendum's wiring diagram, which I don't have in as good of resolution, and it also does not have a relay for either circuit. I see no reason why you'd need a relay; it's not one of those reverse-circuits where another circuit being opened triggers the light circuit closed, and the amperage running through the circuit is less than .5 amps (unlike the horn or starter). From the factory it's just two mechanical switches and two small bulbs. I didn't need a relay when installing my Koso gauge.

 

[Raggaren]

Like Mr2wheel said it could be done with relay.
But no need to over complicate a simple circuit
Just need 1 wire with power from ignition switch to lights and other wire to Switches..they provide the necessary ground for lights to function just like original gauges did.

 

[mrtwowheel]

This is how the gauge that I had was built. There were two ground wires from the gauge, one ground wire was supposed to be for the neutral light, the other was supposed to be for the back lighting, per their wiring diagram. That is how it was designed, but that is not how it was built. both of those ground wires were connected somehow inside the gauge.

So, there was no way to wire the neutral without using a relay or pulling the gauge apart to correct the connections inside.

In other words, someone designed it right, then someone built/assembled it wrong.

Seen this on other forums too, still building them wrong I think.

 

[mrtwowheel]

unhook the gauge, check for continuity between the ground wires. there should be none