Fuel gauge reads half full after fill up

[Mucci]

My fuel gauge seems to only read half of what it should. After a fill up it shows a half tank. Never goes above the half mark but will slowly fall to E at a normal rate as gas is used.

[John76]

Half full or half empty??

[Mucci]

3 minutes ago, John76 said:

Half full or half empty??

I'll check later. I've reserved the third half of the day for car work. 

[PaulTWinterton]

Can't give you a reason, but I can add some knowledge.

 

If you had a bad (no) ground, the needle would read above full, pinned at the top of the gauge.  So, this doesn't sound like a grounding issue.

 

The float in the tank creates resistance at different levels depending on its position (up and down).  Approx. 0 ohms resistance at full and 70ish ohms at empty.

 

 

Question: does the needle drop proportionally as you use up the tank of gas?  If so, it sounds to me like a faulty component within the gauge that is interpreting less resistance than required. IMHO.

 

This is a bit boggling as most problems are with grounding issues or contamination causing poor grounding. Both would give you higher readings , not lower at a full tank, I believe.  

 

I look forward to your discoveries.

 

 

Edited March 18, 2020 by PaulTWinterton

[John76]

Nice pictures Paul.  And I concur...bad/weak ground would cause gauge to read high.

 

Mucci, try removing the sender from the tank and tilting it back and forth, making the float travel from empty to full a few times.

Could be just some crud on the rod, causing the float to stick.

 

John

 

PS; My sender reads 3.4 Ohms when full, and 79.5 Ohms when empty....but I'm much closer to the equator than Paul.

[PaulTWinterton]

Just now, John76 said:

Could be just some crud on the rod, causing the float to stick.

 

John

 

Yeah.

 

1 minute ago, John76 said:

PS; My sender reads 3.4 Ohms when full, and 79.5 Ohms when empty....but I'm much closer to the equator than Paul.

 

Funny.

[Mucci]

That's what the sender unit looks like?! God I love these cars. So simple. 
 

Will check and report back...

[jimk]

1 hour ago, Mucci said:

After a fill up it shows a half tank.

Pull the wire off the sender that goes to the gauge and ground it.  The gauge needle should go to the full/full.  Leave it ungrounded and off the sender and the gauge should read empty/empty.

If gauge is ok, check the ground screw at the rear in the spare tire section of the trunk.  if clean and good, the sender needs attention as has been said.

[halboyles]

5 minutes ago, jimk said:

check the ground screw at the rear in the spare tire section of the trunk

 

This is a real common source of grounding problems in the electrical components especially in the rear of the car.  The common ground wire is only held on with a sheet metal screw and the connection is usually screwed in over the paint.  Also, there is a crimped connection common to the passenger side components that is usually corroded (see circled wires in pic). That can be cut and re-terminated.  Clean everything down to bare metal, coat with a di-electric grease and use a bolt and nut to hold the ground wire eyelet to the sheet metal.

[Mike Self]

While you have the sender out of the tank (after you loosen it, pull it part-way out and let it drain--if the tank is nearly full it'll have a lot of gas inside, and it takes 15-20 seconds to drain), carefully run your fingers up and down both wires that the float rides on.  A small kink in the wire can cause the float to hang up. 

 

While you're at it, take a clean rag and soak with some carburetor cleaner, and VERY gently wipe up and down the two resistance wires.  They're fragile, so be careful.  You can also use a q-tip but don't press hard.  Then use the Q-tip to clean the contacts on the float.  Over the years crud can build up on the wires and serve as insulation, causing a false reading.

 

Just don't break the wires!

 

Mike