Fuel gauge ground?

[RyanH]

1970 911T.
Hi. Trying to fix my fuel gauge as the needle sits about 1/4 full and lower as the fuel empties regardless of fuel amount and sinks to the bottom periodically when driving. From research I think it's the ground, the sender looks to be in good shape so I just cleaned it. The ground wire (green?) is in place, when I remove this and use an alternative ground the needle shoots straight to the top? Not sure what to do, the fuel tank is not that full. Any auto electrical guys out there that could explain? Thanks...

[porwolf]

The green wire is not ground. All Porsches use brown wires for ground connections. The green wire from the sending unit reads the resistance of the wires that the float slides up and down on to ground. The higher the float goes the less the resistance if the slide wires. With the float at the top, tank full, the green wire is practically at ground level. So if you connect ground to where the green wire is supposed to go on the instrument it reads the tank is full.

I suspect the sending unit is not working correctly. Somehow the slider does not go higher than 1/4 tank full. You can test the sending unit function by taking it out off the tank connect all the wires and move it from slowly vertical to sideways and upside down while watching the tank level instrument. If the sending unit works right it should display a fuel level corresponding to the position of the float position. Be careful with ventilation, should be done outdoors.

A safer way to test the fuel sending unit is taking it completely out of the car and test the resistance between the ground connection on the unit, G, and terminal 31. That resistance should vary between about 2.7 Ohm with the float at the top and about 72 Ohm with the float at the bottom.

The third connection from the sending unit (black wire) shorts to ground when the float is a the reserve level or lower.

I did repair the sending unit of my '79SC a couple of years ago and that is the information I looked up in my Bentley Manual. But I also checked with the wiring diagrams for 1965+ years and the 1971 models here at the Pelican site:

Pelican Parts - Porsche 911 Parts Listings & Diagrams

and the tank level metering circuits seem to work very similarly.

[RyanH]

Thanks Porwolf. I disconnected the brown ground and the gauge now works! Why? This brown is also looped to the next gauge and does not seem to be affecting it. Not sure what to do, would leaving it ungrounded be unsafe? Cheers...

[RyanH]

could anyone please explain?

[T77911S]

if you grounded the green wire and the gauge indicated max delfection or full, and the other wire should turn on the light, i would say the sending unit is bad.
me, i would pull the sending unit and see if the float is moving up and down as it should and i would check the gauge to see if it follows.

how do you know the gauge is reading the correct fuel level?
where did you disconnect the ground? at the sending unit or the gauge?

[mrdieseldude]

Originally Posted by RyanH

Thanks Porwolf. I disconnected the brown ground and the gauge now works! Why? This brown is also looped to the next gauge and does not seem to be affecting it. Not sure what to do, would leaving it ungrounded be unsafe? Cheers...

That doesn't sound right. How do you know the gauge is working? Have you cycled it through a tank of fuel? The circuit may also be finding ground elsewhere, but in order for it to work correctly, the sender needs to be isolated. If you put one meter lead from the sender terminal (where the brown wire should connect) and the other lead to a good clean ground (might as well use the battery's ground since it is close), you should have a very high resistance value (in the million ohm range). Of course, you need to disconnect the brown wire from the sender for this test. The brown wire is not for bonding, so it should not be a safty concern.

[E Sully]

Porwolf is on the right track.
The fuel gage works when a float in the sending unit moves along a looped wire varying the resistance to ground, by way of the green wire. There is also a contact that turns on the warning light by way of the black wire when the float bottoms out. Main power is from the Rd/Bk wire connected to the gage.
If the float sticks and does not move smoothly your readings will also be off. If the wire and contacts are corroded you will also get incorrect readings.


Here is a thread showing the internal wires and parts. It shows some readings you can do to check.
Broken wire inside fuel tank sending unit
And this is a Pelican Tech article on it.
Repairing the Porsche 911 Fuel Tank Sending Unit - Pelican Technical Article
The ground is from the Brown wires terminated behind the blower in the trunk.

This picture shows the wiring on the back of the gage, with the wires on the bottom for the fuel gage, and the upper half for the oil level.

[RyanH]

Great feed back, thanks for all your time.

The plot thickens! My sender and gauge work in unison, I removed it again and tipped it up and down and the needle responds. Also took the gauge apart and looked for any faults.

I left the ground off the gauge and re fuelled and the gauge needle responded and has dropped as the fuel empties.

My oil volume needle on the same gauge has never been consistent but with the ground off now reads correctly but drops as I press on the accelerator pedal, then rises again as it lifts.

The temp needle in the next gauge has also never been consistent so I tried removing the ground on this gauge too, this also now appears to be working.

What has stopped working is the oil pressure needle, I can live with that.

From this description, what is happening?

Cheers....

[timmy2]

It sounds like the brown wire you removed may have an impressed voltage on it from another device in the ground loop or it is chafed and making contact with a powered wire.
The oil level gauge sounds normal now as the level in the tank drops when above idle speed.
Was it the ground to the lamp circuits you removed? Do all the gauge lamps work?
As Ed pointed out, the ground for the fuel level gauge sender is not run to the gauge.

[RyanH]

Hi again! Thought I solved this but was driving yesterday and all my needles flew to the top of the gauges?

E Sully, you posted some great info on diagrams and photos. Quick question. I'm trying to find the ground location in the trunk, do I have to remove the blower housing to get to this?

I want to track these ground wires back for any faults.

Thanks...

[DRACO A5OG]

It should be near the gauge holes on the driver side chassis wall.

Not for nuttin, but if you carefully re-flow the solder joint in the gauge, it may solve the problem before tearing into the sending unit. does your reserve light come on properly?

It pegged because ground was lost.

[RyanH]

Ha ha this gets more confusing...

Thanks Draco, I have just found a random green wire at the back of the fuel and oil gauges that does not attach to anything. There doesn't seem to be anymore places for it to go on the back of the gauges, but it must go somewhere?

I must be getting closer?? Lol

[timmy2]

There is a spare green wire run to the back of the gauges that is not used on a number of different years. Porsche ran spare wires in their harnesses so they could be used over multiple years.
You can see it in the drawing that was posted straight up from the letter v.


Sent from my iPhone using Tapatalk

[RyanH]

Bugger...

Back to tracing the ground

[sugarwood]

A safer way to test the fuel sending unit is taking it completely out of the car and test the resistance between the ground connection on the unit, G, and terminal 31. That resistance should vary between about 2.7 Ohm with the float at the top and about 72 Ohm with the float at the bottom.

Por,

I want to do this test but need clarification.
I know how to remove the sender unit.

Sender has 3 prongs on the top.
How do I know which is the ground connection?
What is "G"?
What is "terminal 31" ?

Can you tell me what setting to put the multi-meter on
and where to place the red & black test leads?

[DRACO A5OG]

G is ground and on my old sender it is marked G

The two terminals he is referring to is the two that are closest together G and the copper strip connected one.

[sugarwood]

Ok, so put red on G and black on the copper one and then set to measure resistance.....I'll do it this week..

[DRACO A5OG]

Black on G Red on the one with copper strip

[Gretz]

Apologies for resurrecting an ancient thread. Can someone confirm the location of the ground for the instruments in my 1982 SC? I always thought it was the one behind/underneath the dash near the back of the radio in the passenger compartment. Based on the above, it looks like there is a location behind the blower in the frunk. If so, I'll need to dig in there and clean it up.

Or is this only the location on older cars? Here's the pic originally shared by Sully above:

[timmy2]

Same location on your car.
It is a real bear to reach, and is likely under a few other harnesses that use the same ground stud depending on the options on your car.