Fuel gauge / level warning lamp woes, duff sender?

[dj_efk]

Guys now that I've finally got my coupe (March 69 2800cs) back on the road doing shakedown runs, I hope you don't mind me posting each of the inevitable gremlins that are inevitably cropping up

So my fuel gauge is playing up: after brimming the tank and doing 180 miles it still reads more than 3/4 full, it also dances around fairly violently when the car is in motion, plus the low fuel lamp is permanently on!

Is the fuel lamp powered from the same sender as the gauge itself or is there a separate sensor?

I haven't checked the gauge yet but in my experience fuel gauges either work or they don't at all, so I'm assuming the problem(s) to be either with the voltage stabiliser (if there is one, where is it and how would you check it with a multimeter?), or with the sender unit itself (ditto?)

All opinions greatfully received!

 

[MMercury]

http://www.e9coupe.com/forum/showthread.php?t=1390

 

[dj_efk]

Thanks MMercury. However I'm still not clear on two things that thread does not specifically address:

1) Is the low fuel level light switch built into the sender unit itself then? having read that I'm now assuming that you could have a perfectly functioning gauge but an iffy light, so therefore they are on seperate circuits?

2) Can anyone shed some light on possible faults with the actual wiring of the fuel guage E.g. where the voltage stabiliser is located etc. ?

Thanks.

 

[Mike Goble]

Thanks MMercury. However I'm still not clear on two things that thread does not specifically address:

1) Is the low fuel level light switch built into the sender unit itself then? having read that I'm now assuming that you could have a perfectly functioning gauge but an iffy light, so therefore they are on seperate circuits?

2) Can anyone shed some light on possible faults with the actual wiring of the fuel guage E.g. where the voltage stabiliser is located etc. ?

Thanks.

My Bav sending unit has three wires, a hot wire for the gauge (brown with a light tracer), a hot wire for the lamp (brown with a dark tracer, and the solid brown Elvis wire (Return to Sender). The gauge and lamp use the common return wire, which is connected directly to the case of the sending unit. The connections on the sending unit are riveted and may degrade with age, so you may find that wiggling them will change the gauge reading.
When the float is at the bottom of the unit, the gauge connection should read about 90 ohms relative to the ground connection, while the lamp connection should read close to 0 ohms. As the float rises in the sending unit, the gauge connection should read a lower resistance, and at with the float at the top it will read about 5 ohms. At any point above the bottom the lamp connection should read open.
If the reading jumps around a lot, you may have a bad connection from the slide wire inside the sender to the float. Remember, it's the moving parts that fail most often.