Fuel sender reserve light FYI

[Shern]

Been trying to track down the source of a flaky fuel gauge for a week now... removed my sender today to clean and check resistance values (all but one were fine) and realized I was getting nothing on the reserve circuit. Couldn't find much in the archive so FYI to anyone without a reserve light:



That little copper tab had a hairline fracture - easily remedied with a dab of solder. Anyway, very easy to miss and worth a look if you're at wits end.

Now for the gauge. I'm trying to rule out a faulty one but everything seems to point to that. My grounds are good, both on the cluster and on the brass rivet behind the gauge itself (ran new ground and resoldered connection which ended flickering issues).

The needle experiences the full range of movement but seems to be off by just over a quarter of a tank. When I refill it goes to F, and after 100 miles it's down to half and around 300miles it's down to E.

Do these things lose calibration? Does this sound like something else I've missed?

I'd happily buy another but my understanding is that the 300d gauges are not compatible with my 240. 14pins vs 15 and different position I'm told.

[Diseasel300]

I'd be more inclined to suspect the sender than the gauge. The gauge is nothing more than a coil of wire. Not really much to lose calibration. If the fuel sender has a layer of varnish on the wires or otherwise contaminated, you'll get really weird readings. Try (GENTLY) cleaning the wires with steel wool, then hosing down the wires and the wipers that ride on them with carburetor cleaner. Worked for my SDL and you couldn't believe the varnish layer that was on there.

[Shern]

This is my second time pulling the sender... have sanded, have carb cleaned, it's pretty spotless at this point.
I even followed the float up and down with my multimeter, it appears to be working fine.
The coil of wire on my gauge however, has become dislodged (that little plastic spool) and I imagine the tension isn't what it used to be.

[Diseasel300]

Sometimes the sender is just junk. The one in my SL had a bad habit of just quitting below ~1/2 tank and being flicky at higher levels. It would rapidly drop to 1/2 tank (even though it wasn't down 1/2 tank), then finally drop out. Sanding and cleaning got it to work mostly, but the levels were still way off (saying 1/2 tank after ~40 miles). New sender stopped the madness. Just my own experience with a similar problem.

[Shern]

Hmm... maybe it is worth a punt after all. No experience with a faulty gauge?
They must go bad at some point no?

[Diseasel300]

The gauge is a coil of wire, they either work or they don't. What controls their position is the fuel sender, it acts as a big resistor. Low resistance at high fuel levels makes the gauge high, resistance increases as the fuel level drops. If you have a crappy fuel sender or the wipers are contaminated, the resistance will be increased over normal and you'll read lower than you are in reality. That's also why flaky senders cause the tank to read empty, infinite resistance is "high".

[Shern]

When I reinstalled the sender last night, it read a resistance value that was precisely the amount of fuel I had in the tank (about 30ohms/half tank). The gauge however read just under a quarter.

[Shern]

Anyone ever had a sender float that perhaps developed a leak?

My gauge is typically 15-25% off. When I refuel, It never quite hits F. By the time I've done 75miles, it's either at or just below 3/4. I've cleaned and repaired the sender twice, I have no shorts in my cluster grounds... boy would I love to figure this one out.

[Mxfrank]

This sounds like high resistance somewhere in the circuit. If you've already cleaned the resistance wires, try cleaning the connectors at the top of the sender. Do the same for the dash side if that doesn't work.

[vwnate1]

? Have you tested the system by connecting the gauge when it's out of the car, holding it above the rear seat and slowly tipping it one way or t' other to see if the gauge makes its full range ? .

If you've sanded the sender's wires you've likely killed it ~ they should only be chemically cleaned with a very soft acid brush .

[funola]

Originally Posted by vwnate1

? Have you tested the system by connecting the gauge when it's out of the car, holding it above the rear seat and slowly tipping it one way or t' other to see if the gauge makes its full range ? .

If you've sanded the sender's wires you've likely killed it ~ they should only be chemically cleaned with a very soft acid brush .

I used 1200 grit wet/dry paper to clean the resistance wires (Nichrome) and the wipers on the floats. Worked great, didn't kill it.

[vwnate1]

Let's see how long that lasts .

Electronic contacts are delicate, you're never supposed to sand/abrade them .

[Diseasel300]

Sanding the wires won't damage anything. There's virtually no current being carried, so the risks of sanding are nil. The wires and contacts build a varnish layer over time that makes an insulating layer. You have to remove that and the safest way is mechanically (meaning sanding). Be gentle and you can easily and safely restore good connection.

If any varnish is left, or if the contacts (meaning the slides on the float too) are tarnished, you'll have high resistance and the level reported can be off.

[Shern]

I have in fact removed the sender and tested resistance with a multimeter -slid the float up and down the shaft, observed the change in current as consistent with what I’ve read to be “full” ”halfway” and “empty.” There was no sludge or crud on the wires either. I’ve never had any issues with fungus in my tank. I was almost a little disappointed after pulling the tank screen only to find it perfectly clean!

[funola]

Originally Posted by Shern

I have in fact removed the sender and tested resistance with a multimeter -slid the float up and down the shaft, observed the change in current as consistent with what I’ve read to be “full” ”halfway” and “empty.” There was no sludge or crud on the wires either. I’ve never had any issues with fungus in my tank. I was almost a little disappointed after pulling the tank screen only to find it perfectly clean!

If the sender measures fine resistance wise, then you have to look elsewhere in the system for unwanted resistance. Have you tried a new fuel gauge in the cluster? Sometimes there is binding in the meter movement bearings that can give erroneous readings.