Fuel Level Senders - Which one does what?

[5Toes]

Okay so my gas gauge has never worked, its not the biggest deal, but it is annoying.

I have a late model so I understand there are two senders.

Anybody want to send a link for testing these?

For now I want to get my reserve light working, so I atleast know when to gas up (keeping track on odometer isnt most accurate). Does one sender tell when to put reserve light on, and other one is for fuel level itself?

[bmwguy325is]

So the way they work is the vary the load/resistance. Full is either open circut or fully closed/continuity.
An easy wau to check would to be to pull them out and plug them in an tilt them up and down and watch the guage in the car.
If they just stopped working check for power and ground at the plugs

[5Toes]

My gauge shoots from completely empty to full constantly while driving.

[TobyB]

That's a loose contact... somewhere.

The late car has 2 senders in series, and as the tank empties, the resistance
of the 2 increases.

From the ETM, looks like current flows from the gauge, through the left sender, then
through the right one to ground. My bet would be one of them has worn out,
and is only making occasional contact, but you'd have to meter them both to find
out. You could get lucky and find bad connections at the tank...

The wire's brown with a green stripe, and then the ground for the tank sender's brown.

Time to pull the back seat and have a look.

hth

t

[5Toes]

I had the back seat over the weekend, and if I pressed very hard on the left tank's sender then the gauge stayed put.

But I have no clue if the reading was accurate, I was parked on the steepest hill around.. (my driveway)

[jlevie]

As TobyB said, the sensors are wired in series on a 63l tank. When the tank is full the resistance of each sensor is zero and rises to ~60 ohms when the tank is empty (~120 ohms total). So a short in the wiring or a sensor will result in the gauge indicating full all the time and an open will result in the gauge reading empty all the time. If one of the sensors is bad the gauge will only have about half it's travel. The warning light is driven by a switch in the right side sensor.

The other common problem with the fuel (or temp) gauge is a bad connection where the gauge is fastened to the PCB in the cluster. Simply tightening the nut can help, but the best fix is to disassemble the cluster and reflow solder on the pads of the PCB.

[5Toes]

As I said before the gauge flops around or just reads nothing

When I pulled my rear seat, I held the two prong connector on tightly by hand and the gauge stayed put.. I had no clue if the reading was accurate or not though

The rear nut on the cluster is on tight

[nando]

there is also a 68 ohm resistor on the PCB of the cluster, the solder joints can crack rendering the gauge useless.

[bmwguy325is]

As I said before the gauge flops around or just reads nothing

When I pulled my rear seat, I held the two prong connector on tightly by hand and the gauge stayed put.. I had no clue if the reading was accurate or not though

The rear nut on the cluster is on tight

SO go fill the car up or drive around until it quits. Then you will know what the reading should be. I may suggest the first one.

[5Toes]

there is also a 68 ohm resistor on the PCB of the cluster, the solder joints can crack rendering the gauge useless.

any pics of this?

[nando]

really?

http://lmgtfy.com/?q=resistor

[5Toes]

Yes really.

[nando]

it's a resistor by the gauge. It's obvious if you open it up. It even says "68" on it..

[5Toes]

Okay. Cool