Fuel gauge responded slowly, then not at all

[Dug Nichols]

On my drive out to the vintage, at one refueling the fuel gauge took about 30 minutes to climb from 1/4 tank to full after a refuel. Then it was fine every refill for a while, popping right back to full. Then suddenly on the drive back after a stop to refuel the gauge just sat at the very bottom, not getting any level at all. 

 

The gauge cluster has been grounded previously and for quite some time. I've had the car for 5 years and never an issue until this past week.

 

I tested at the sender by grounding the non-ground wire, expecting to see the gauge go to full when I turn on the ignition. BUT, when turning on the ignition the gauge spiked to top and then fell back to lifeless. On subsequent ignition-on, it would give a tiny little movement.

 

I swapped in an alt cluster I have in my parts stash, and the fuel gauge did a similar tiny blip of movement. (note, I'm not 100% certain of the gauge in the parts cluster's condition)

 

If I'm reading previous posts right, and what my limited electrical logic tells me, the gauge should pin full and stay there with the ignition on when I'm bypassing the sender, right??

 

Also, any other tests I should run given the diagnosis at this point, before resorting to opening up the fuel sending unit?

 

TIA

 

 

[John76]

The float in your sender is sticking in the tube and on the slide wire,

Need to remove the sender and tilt it back and forth several times to clean the varnish off the wire.

No need to disassemble...just remove and "exercise".

[Mike Self]

42 minutes ago, John76 said:

The float in your sender is sticking in the tube and on the slide wire,

Need to remove the sender and tilt it back and forth several times to clean the varnish off the wire.

No need to disassemble...just remove and "exercise".

Another thing to check is the screen at the bottom of the sender tube.  If it's all gummed up, it takes awhile for fresh fuel to work its way into the tube, especially if the tanks' nearly empty.  So when you have the sender out, I'd clean the screen just on general principles.  It pops out and is easy to clean. 

 

And if "exercising" doesn't work, it's not terribly difficult to remove the aluminum outer housing to expose the float and resistance wires.  Just be very careful cleaning the wires; they're delicate, easily broken and the very devil to replace properly.  Clean with a cotton ball soaked in lacquer thinner, wrapped around the wire and gently slid up and down.  And make sure the float itself hasn't developed a leak.  Like carb floats, they can spring a leak and go down with all hands.

 

mike

[jimk]

Doing a dead short to ground using the gauge wire doesn't do the gauge any favors.  When the needle hit the top, no telling what happened in the gauge.

[Dug Nichols]

19 hours ago, jimk said:

Doing a dead short to ground using the gauge wire doesn't do the gauge any favors.  When the needle hit the top, no telling what happened in the gauge.

Bummer! I saw multiple posters posts recommending this test so I did it. I hope I didn't mess it up.

I opened up the sending unit and the wire broke on one side, so it's completely disconnected. Also the inner part pulled out separately from the outer housing ring, which I'm guessing isn't supposed to happen. Smells like a new part for this job.

[jimk]

I believe there should be a small nut on the bottom of the center rod that passes thru the bottom inlet screen holder to keep it all together.  The nub might be loose in the tank

[Mike Self]

I have an old (1976) BMW no-space CCA Tech Tips book article on repairing those senders when the wire breaks. 

 

You need to find resistance wire that measures out to 6 ohms to the inch.  IIRC a source for this wire has been written up in the FAQ, along with some details on how to install it.

 

mike

[BarrettN]

I have a big spool of the wire needed to rebuild standard length senders. PM me for my address and then use it to send me a self addressed stamped envelope and I'll return it with a chunk of wire that you can use to rebuild your sender. I have a how-to post here with pictures to walk you through it. 

 

I've been incredibly lucky in my adventures with 02s, especially with finding hard to find parts and whole cars even, this is one way I try to pay it forward. 

 

Barrett