Curious issue with 77 GS750 Neutral and Oil Pressure Light

[Holdawayt]

Hello and thank you for taking the time to read my post.
I recently picked up a non-running 1977 GS750 and one of the first issues I came across was the dodgy wiring loom.

After lots of repairs to the wiring, I have reached a point where I have the bike turning over, I have a spark, and the headlight / brake lighting circuit etc is all ok.
The dash will light up when I turn the headlight on (the lights that back-light the speedo and rev counter) but I don't have the neutral, gear position indicator or oil pressure light illuminating.

Now here's the curious bit, I understand that the gear position lights and neutral lights work by grounding the wires, however if I put 12v to the blue neutral wire coming from the instrument cluster (the one that's separate to the gear indicator plug) the neutral and oil pressure lights will illuminate (Still no gear position indicator). If I ground the blue wire (or any of the gear position wires) nothing happens.

I don't understand how this would work but I don't have the greatest electrical knowledge. I know I could probably put 12v to the blue wire and it'll work but I don't think that's right and I'm trying to make everything correct.
I have three plugs coming from my instrument cluster as follows:
1st plug - Orange - Ignition Power, Green with yellow trace - Oil pressure, Yellow with blue trace - no idea, Light green
2nd plug - Black - Left turn signal, Black with white trace - Ground, Grey - Dash back lights.
3rd plug - Gear position indicator with separate blue wire - Neutral.

I understand the previous owner did these multiplugs but as far as I can tell it doesn't matter as long as the colours match.
I have a wiring diagram from the Haynes manual and I have a multimeter but I'm still learning how to use this.

Any help would be greatly appreciated. Tonight was the first night I had the bike turning over with a spark so I'm pleased but this has caused some confusion.

Thanks, Tom.

[Burque73]

Hi Tom. Welcome to the forum.

It's good you have a wiring diagram and multimeter. If the Haynes manual has black and white you should be able to find a color diagram on bikecliff's site to download. I don't have a link handy, sorry.

Silly question, is the kill switch in the run position? I've been perplexed why mine was suddenly dead and realized I accidentally hit that switch. I'm not the best with electrical issues either.
Good luck.

[Redman]

Kill switch cuts out the ignition (and starter solenoid) circuit, so should have no effect on the instruments.

.

[Redman]

Yes, netural light and gear indication light (and oil pressure light) should have power directly to the light, from a fuse.. byway of some jumper in a certain connector somewhere....

And yes, the specific circuiit for each seperate light is to be connected to ground (earth) to operate the specific light.

I am not that familiar with pre1980 bikes to quote wire colors, nor to state what specific fuse for what, so get a schematic . . .

CAnt explain the two lights lighting when you put power to that wire, but strange things happen when back feed things like that.

[Redman]

PS: does a 77 have sidestand light?

[Burque73]

Kill switch cuts out the ignition (and starter solenoid) circuit, so should have no effect on the instruments.

.

Yes, of course. Was thinking starter would spin but it wouldn't run. My bad.

[Holdawayt]

Hi and thanks for the replies.
No side stand switch on this bike and it only has one fuse on the main 12v power cable off the battery.

[Redman]

.......
No side stand switch on this bike ..................

Okay Thanks. On bikes that do have sidestand switch&light, there are two lights whos circuits are somewhat tied together . . . . . . . nevermind.

.. . . . .
. . . . . . . . . . . . . on this bike only has one fuse on the main 12v power cable off the battery.

Okay Thanks, that is the situation (on some pre 80 bikes) that I am not familiar with, so I not say much specific about powerfeed to the instrument lights.

On a schematic, find the wire color that feeds power to all the instrument lights (oil, netural, gear, etcetera). ANd then find that wire in connector that goes to the instrument cluster. See if you have power there (both sides of conector). If do, then problem is in the instrument cluster. If dont, then look on schematic back into the bike wiring to see where it is suppose to get power from, probably(?) some jumper in come connector somewhere that connects that circuit over to the main fused circuit.

.

[Redman]

https://gsarchive.bwringer.com/mtsac/~cliff/storage/gs/GS750E_1978_Owners_Manual.pdf

>> later note:
Schematic (page 52) doesnt print clear enough to be of much use of specific wire colors.
But can see that is only one fuse (not a 4 or 5 fuse fuseblock).


.

[Holdawayt]

Thanks Redman, that appears to match how I've wired mine up so still scratching my head. I have 12v power going to both sides of the connector going into the instrument cluster. This then splits off to the individual lamps.
I'll take the clocks apart tomorrow and see if I can see a dodgy ground anywhere.

[Redman]

You say you ground out the netural wire and netural light doesnt light.
You say you ground out the oil pressure wire and the oil pressure light doesnt light.
(Just confirming.)

. . . . . . . . . . I have 12v power going to both sides of the connector going into the instrument cluster. This then splits off to the individual lamps.
I'll take the clocks apart tomorrow and see if I can see a dodgy ground anywhere.

Good that you are able to check that. now know the problem is not in the bike main harness, but is more in the instrument panel.

Relative to the oil light and the netural light: the ground in the instrument panel is of no concern.
THe individaul wire for the oil light is grounded out by the oil pressure swtich to light the light.
THe individaul wire for the netural light is grounded out by the netural swtich to light the light.

Yet, THe ground in the instrument panel will be of signifiacance for the instrument panel backlights though.

You say you checked and found power at the connector going to the instrument panel. Next check if power inside the instrument panel.

Other comment: If you are looking for dodgy connection, your eyeballs are not very effective at that (unless wire is complelty disconnected). Is best to devise a way to test with a meter, either by voltage testing somehow, or by continuity (ohms) testing somehow.

[jborum]

Long story short, your orange wire is probably not bringing battery voltage as it should.

Page 177 of this PDF is probably the circuit diagram you want. It's spread over 2 letter-sized pages and seems quite clear for something of this vintage, so you're in luck. If that's the right schematic, then it confirms what you wrote in your original post.

https://gsarchive.bwringer.com/mtsac/~cliff/storage/gs/GS750_Service_Manual_early8valve.pdf

It's not clear to me either what wire brings power TO your gear position lights. It's most likely O because that brings power to the neutral light, but it could be G/R also. In the diagram O and G/R both have dead-ends right by the gear position lights, and separately O seems to dead-end into the middle of the connector.
This is my read, mostly confirming what you wrote originally:

1st plug:
O is power to the neutral and oil pressure lights (also splits off to ignition, brake lights, and other things)
G/Y is the ground side of the oil pressure light, heading to that switch and ground.
Y/Bu isn't on the diagram, but Y is the high beam
Lg is power to the right turn signal

O becomes Gr in the lighting switch

2nd plug:
B is left turn signal
B/W is ground for lots of things
Gr is tach and speedo

3rd plug:
All are heading away from the gear position lights and towards the position switch and ground.
Bl is neutral light
W/Y R/B Y/Bl G/Bl Br/R are gears 1 2 3 4 5

I hope that helps.

[Holdawayt]

I found the issue for anybody having something similar happen.

The orange Ignition 12v cable going to my clocks splits off into 3 to provide power to the various lamps in the instrument cluster.
Where the cable split into 3, the wires had frayed and after a simple repair they were working properly again.
I don't have anything happening at the gear indicator but that can wait.

Thanks for the advice, what a great forum!