1974 911 - Green mystery wires behind temp/pressure gauge?

[maninblack]

On my 74 911 there are some green wires exiting the harness behind the temp/pressure combo gauge. The first is a set of 2 green/white wires that terminate together and the second is a single solid green wire.

Anyone smarter than me know what these may be?

[gomezoneill]

Instrument / gauge lighting?

[maninblack]

No. Instrument lighting is black/blue. And while I appreciate the attempt, I'm already REALLY good at guessing. I'm looking for an authority on the subject.

Looks like the solid green wire is unused based on other threads. But can't find the anything on the two green/white wires. I'm guessing it has to do with the oil pressure gauge but mine is already working.

[maninblack]

Think I got it. Looks like oil level.

[gomezoneill]

Here's a schematic for a 74. It shows the g/w wire is for the alt charging light.

https://uc3acbc47c1c28b045bb6be53653.previews.dropboxuserc ontent.com/p/thumb/AAiwtAC-Viiezz4B-RKWObmpZ6PA4HRgqVHNYPaozvZ8iuyQsqYRiQkvBnM30t9Shk5 ZqYYkD60umySwnHpusHEemp2_QkNkiEX_eWVVYyCJKLKDIu3YD iyJvFToIAHd38yq99LoTLBmXKfjMKKy9jVlqnRHyc4F78of-WP1hKGg3t-2iLTWnKA8k8lpEJ9JC1KqxqB7m9B5w3GHRw8CcYQgu1dwQoD8y HvuZM3te_3yjWzyWZXjOLL4bsRkkY4c_pv-4LHzzCYwLf6bVKhO-VjdMQGCfyNNMmxipsaTPoZX-jHc_0WyC2-ik8Em5Kbr_TpTCIeZhOv3gGmhI3YLEf49JnMTiPNXFiJn9JHn7 wSWEy4K3hGLXBVbAw4WCvIJfgrvkXhM2OGn_an968YemPNMnRL p84RumPVaNfRxoigQhQ/p.jpeg?fv_content=true&size_mode=5

[timmy2]

Green is a spare it runs all the way to the 14 pin connector for Sportomatic transmission.
Green/White is used for oil idiot light and oil level gauge. One to the 14 pin connector and one to the driver side tail light harness area. Either in the 14 and 6 pin connectors or external depending on the year.
White/Green is speedo lamp and also used for fog lamps.

[timmy2]

Use the ‘74 factory wiring diagram....
https://www.dropbox.com/sh/s292jtsyvl6llw2/AABF-ofgMfoMltGa99Asphr5a?dl=0

[maninblack]

Originally Posted by timmy2

Green is a spare it runs all the way to the 14 pin connector for Sportomatic transmission.
Green/White is used for oil idiot light and oil level gauge. One to the 14 pin connector and one to the driver side tail light harness area. Either in the 14 and 6 pin connectors or external depending on the year.
White/Green is speedo lamp and also used for fog lamps.

We have a winner! Thanks much.

[mikedsilva]

Originally Posted by timmy2

Green is a spare it runs all the way to the 14 pin connector for Sportomatic transmission.
Green/White is used for oil idiot light and oil level gauge. One to the 14 pin connector and one to the driver side tail light harness area. Either in the 14 and 6 pin connectors or external depending on the year.
White/Green is speedo lamp and also used for fog lamps.

I know this is an old post but the info is good so no need to start a new one.
I'm an electrical dunce.

A friends car 79 SC, idiot light does not light up.
Pulling the gauge out, I removed and tested the bulb and it works fine.

Clarification: Does the idiot light switch connect ground when no pressure and allows the bulb to light, but when pressure is built, the ground is broken causing the light to go out?

Does that mean the green/white wire supplies 12vpower to the bulb?

I tried putting my meter on that wire, and we are only seeing 0.2volts with ignition on.
Tried to follow the wire, but it appears to run into a larger sub harness that runs to the left side of the car...

Not sure where to go from here... and didn't really understand that current flow diagram either.. (sadly)

[wazzz]

Originally Posted by mikedsilva

Clarification: Does the idiot light switch connect ground when no pressure and allows the bulb to light, but when pressure is built, the ground is broken causing the light to go out?

Mike, yes, you are correct. Other side of bulb (red/black wire) is the power (+12V) supplied when ignition is on.

Originally Posted by mikedsilva

Does that mean the green/white wire supplies 12vpower to the bulb?

No, the green/white wire brings a ground to the ground side of the bulb through the oil pressure switch. This switch is closed when oil pressure is zero.
So when ignition is on and engine is not running, power (+12V) is fed to the bulb through the red/black wire and ground is fed to the bulb through the oil switch and green/white wire.
When engine is running, oil pressure makes the oil switch open and therefore ground is no longer provided to the bulb, which goes off.

If for some reason oil pressure is dropped, oil switch will close and ground will again be provided to bulb, which will go on to signal failure.

[mikedsilva]

Originally Posted by wazzz

Mike, yes, you are correct. Other side of bulb (red/black wire) is the power (+12V) supplied when ignition is on.



No, the green/white wire brings a ground to the ground side of the bulb through the oil pressure switch. This switch is closed when oil pressure is zero.
So when ignition is on and engine is not running, power (+12V) is fed to the bulb through the red/black wire and ground is fed to the bulb through the oil switch and green/white wire.
When engine is running, oil pressure makes the oil switch open and therefore ground is no longer provided to the bulb, which goes off.

If for some reason oil pressure is dropped, oil switch will close and ground will again be provided to bulb, which will go on to signal failure.

ah ok. I dont really understand why I see a small voltage at that greeen/white wire...
And I don't think we saw any voltage at the other wire going to the bulb. In fact, using continuity, it showed it was connected to ground (ignition off) which is why I assumed that was the ground wire..

See what I mean when I say I'm a dunce with electrics...

[wazzz]

The small voltage of 0.2V could be due to the high sensitivity of your voltmeter, don't know what you use for testing. The wire from engine to dash is lengthy and could grab some interference on the way.
Now to make sure, remove the bulb and test continuity between the green/white wire at the bulb socket and a known good ground, like dashboard chassis nearby or an accessible ground point. No need to turn ignition on for this. If it checks OK (near zero ohms), keep your ohmmeter set up the same way and start the engine. Continuity should go off (ohmmeter showing infinity), meaning the oil pressure switch and its ground circuit are OK.

As for the power wire (red/black), check again if you see 12V at this wire when ignition is on. What about other instruments and lights, like the alternator light? They all get power from the same red/black wires that are all connected together. If they are functional you should see power at the bulb too. Otherwise check where this wire goes and see if it is properly connected with other red/black wires nearby.