Fuel Gauge not working at all

[Deesta]

When I restored my BMW 3.0CS (1974) I removed the wiring loom and replaced some faulty wires and reinstalled it. I have everything working except the fuel gauge (I might add it took about 40 hours). In summary:

1. Temp gauge works fine;
2. Fuel sender unit is working and is correctly earthed however I am not sure how to test the other two connections;
3. I installed an 'O' ring on the fuel sender (there wasn't one previously and boy did the car smell of fuel).

Any advice on how to troubleshoot this?

 

[lloyd]

I would check the easiest things first. Since it is (probably) easiest to access the tank sender rather than the gauge and the under dash wiring - here is what the manual says:

Check fuel gauge.
Detach brown-yellow wire from fuel tank float and connect
to earth (ground). Switch on the ignition momentarily.
The fuel gauge needle should be deflected right over to
the full position.

With the sender out of the tank, I recall it being fairly easy to test the resistance with an ohmmeter.


Make certain that the in tank float moves freely. In my limited experience the nut holding the fine wire attached to the tank float sender has a habit of loosening. I've found this situation where the wire became slack or totally disengaged on at least two different cars - where the gauges seemed stuck.

If the tank unit checks out, then the next step would be to determine if the signal is received at the gauge. Without consulting a wiring diagram, there is always the old fashion manual method. I suppose it is possible for the wiring at the gauge to be reversed, but this would likely peg the gauge opposite to the amount of fuel in the tank. I only mention this as something to consider, but it has been so long since I personally looked at the wiring connections, I can't recall if there is anything that might prevent this.) Finally, although rare, it is certainly possible for a 40+ year old gauge to fall prey to rust, corrosion, dirt and/or constant vibration. The last time I recall dealing directly with a stubborn gauge was on an E10 and a couple blasts with compressed air had the gauge registering correctly and moving freely. Sadly, accessing the gauge on an E9 is not as easy as some of it's contemporaries.

 

[sfdon]

Connect brown/black wire in trunk to ground with key on
Light on yes or no?
Connect brown/yellow wire in trunk to ground with key on
Gauge on full yes or no?
Connect brown wire in trunk to ohmmeter
Less than . 5 ohms?
The wires and bulb are pretty easy for access.
Green/white is battery positive

 

[Deesta]

Update - looks like brown/yellow wire issue

Connect brown/black wire in trunk to ground with key on
Light on yes or no? Yes.
Connect brown/yellow wire in trunk to ground with key on
Gauge on full yes or no? No
Connect brown wire in trunk to ohmmeter
Less than . 5 ohms? All good.
The wires and bulb are pretty easy for access.
Green/white is battery positive

Responses above. Can I assume it's an issue with the brown/yellow wire

 

[Deesta]

Great advice. As per my response above. Looks like an issue with the brown yellow wire. I think I might have to check the connection to the gauge. As you stated its very difficult to get too.

 

[sfdon]

Test for continuity on brown/yellow wire from trunk to dash