Fuel gauge pegged full

[NYNick]

I switched to an in tank 328is pump on my tii. Per Blunt, I tried hooking it up properly for the fuel gauge to read but it keeps pegging "Full" no matter how much fuel I have in the tank. My odometer doesn't work either so this is a problem.

 

To check it, I dug out my old sending unit, hooked it up and placed it in a half pail of water to see what it might read. I should've read 1/2 or less but it pegged "Full" as well. I tested the wires in the trunk and I'm getting a solid 12 volts.

 

What gives? Do I have to remove the gauge? I was sure I hooked up the gauge cluster correctly but maybe not?

(Electrics are not my strong suit!)

 

Thx

 

Nick

[69Bimmer02]

The sender has a float inside that slides up and down based on fuel level in the tank, in turn changing resistance across the leads.  Sounds like your float might be stuck near the top.  You can take the sender apart to come confirm that.  Just be careful of the very small wires inside.

 

Jeff

[NYNick]

9 minutes ago, 69Bimmer02 said:

The sender has a float inside that slides up and down based on fuel level in the tank, in turn changing resistance across the leads.  Sounds like your float might be stuck near the top.  You can take the sender apart to come confirm that.  Just be careful of the very small wires inside.

 

Jeff

Thx. doubtful this is the case as both the new one and the old one are reading FULL when hooked up to power.

[69Bimmer02]

With them out can you measure ohm resistance across the leads?  If you rotate the sender the resistance should technically change if the float is in fact sliding.

[69Bimmer02]

All the gauge is doing is converting that resistance to an indicator on the dial.  But you able to test the senders with nothing connected to them.  Once you confirm they actually change resistance then you can work forward to the gauge.

[69Bimmer02]

One last note there is a ground wire location right next to the tank.  That one is notorious for being a bad ground from the sender.

[Simeon]

If the gauge is pegged with both senders then you have a dead short in your wiring somewhere. 

 

Take a good detailed look at the terminals at the top of the tank. Any chance of the wires or crimp terminals touching the top of the tank? Having disturbed the wiring at the tank, this is the most likely site but the short could be anywhere along the wire from the tank to the gauge. Probably a good idea to eyeball the wiring loom from rear to front to look for any obvious issues. 

[2002Scoob]

New sender?

 

If so, they’ve got a wire clip-thing that holds the float in position during shipping. You’ve gotta remove it.

 

 

And, just a theory, but I’m pretty sure gasoline doesn’t conduct electricity like water does. By putting your original in a pail of water to test could likely be giving you a false reading.

 

-been wrong before. And has also installed a sender without removing said clip.

 

Sent from my iPhone using Tapatalk

 

[NYNick]

New sender/fuel pump combo pegging at full when hooked up to power.

Old sender pegs Full when hooked up to power.

 

Simeon's theory seems logical. Difficult to trace, but logical.?

[BarrettN]

Full on a working sender typically is zero ohms across the terminals, empty about 75 ohms. Maybe somewhere the lead is shorted out. Given both senders behave the same I wouldn't expect it to be the sender itself. If you want to understand how the sensor works look for the thread I wrote on rebuilding senders. 

[jimk]

Lost ground on gauge, gauge goes to top, just like a temp gauge goes to top on lost ground.

An electrical engineer explained it to me in 1973 when my gauges went to the top.

[PaulTWinterton]

7 hours ago, jimk said:

Lost ground on gauge, gauge goes to top, just like a temp gauge goes to top on lost ground.

 

Sounds right.  I just had a failure (pegging intermittently) and traced it to the gauge.  

 

Not the sender.

 

Swapped it out and not only did it fix the issue, but now my fuel readings are more accurate.  So it was getting worse over time.

 

FWIW I tested my sender and confirmed that it was operating properly.

 

Edited May 20, 2019 by PaulTWinterton

[NYNick]

7 hours ago, PaulTWinterton said:

 

Sounds right.  I just had a failure (pegging intermittently) and traced it to the gauge.  

 

Not the sender.

 

Swapped it out and not only did it fix the issue, but now my fuel readings are more accurate.  So it was getting worse over time.

 

FWIW I tested my sender and confirmed that it was operating properly.

 

Paul,

You swapped out the fuel gauge? Where would I get one?

[PaulTWinterton]

I have a spare gauge pod.  I scuttled it and replaced the whole left side instrument cluster, which includes the temp gauge, oil pressure light, alt light.  I could have removed the fuel gauge specifically, as it's only attached with 2 tiny screws, but I got lazy.

 

I would recommend a posting in the Parts Wanted forum.  I've seen pictures of stacks of hoarded clusters so I'm sure someone on the FAQ will sell either a complete pod or the cluster you need at a reasonable price.

 

Good luck.

[NYNick]

Is there a way to test the gauge before I replace it? Should I test for a dead short first and try to find it?