65 Mustang Fuel level Problems

[mygt350]

Have a 65 fastback A code in shop with a fuel gauge issue. When it came in, fuel gauge always registered less than empty so a loose wire, faulty gauge or faulty sender was suspect.

I had the cluster out and assumed gauge was functional because when I grounded the fuel gauge signal wire with key on, gauge went from empty to full.

Checked for open/short from fuel gauge to sending unit and it was fine. When sending unit signal wire connected to ground with key on, gauge properly reads full tank. So assume signal wire is intact.

When the fuel level sensor in tank was removed, there were several cracks in the brass float allowing the float to sink causing the empty reading. Plus, the filter/pickup sock was missing.

New sending unit ordered and installed. Gauge still does not accurately read fuel level. Needle moves just a tiny bit.

Took full/empty ohms readings  (10/80) and seems old sender was ok, except for float and sock.

Connected sending unit signal wire to old sender with good ground. With key on and float in up position, gauge only reads 3/4 tank.

Meter sweeps from empty to full when signal wire is grounded and key on. But when sender is injected, gauge only reads 3/4 tank.
Is there a problem with the sender?
Any idea where to really start looking?

[mustmore]

Most of the fuel sending units sold today do not read accurately.  There are a few manufacturers,
but I have yet to find one brand that is consistently good.

My first question is, was the unit being removed an original Ford or a repro?

If a repro, installing a new repro will either have the same problem or possibly a different
problem. 

I think a lot of the sender units are being tested by their manufacturers with repro gauges,
thus not reading properly with original gauges.  I have not thoroughly tested this yet, but
I am suspect.

Many repro fuel senders read 3/4 when full.  Others read full, but you are out of gas at 1/4.
Hell on cars (owners) that are driven often.

[Don Johnston]

Also, the brass floats are subject to leaks.  I changed to plastic folats on the sender to correct the problem.  The senders are rarely perfectly accurate and can be off by a quarter of a tank.  Sometimes, removing and slightly bending the float arm corrects the problem, but it is a hassle.

[mygt350]

Unit I pulled out with the cracks on the brass float and missing sock was a Scott Drake. But it does measure 10/80 ohms full/empty so its relatively close. When I ground the fuel sender wire with key on, gauge pegs full. When I insert the Scott Drake sender (via jumpers), gauge reads 3/4 full when I know float is being held all way against the stops. Don't see how bending float lever at this point would fix 3/4 full reading as rod is against the stops already.
Martin

[sg66]

The gauges can be adjusted http://www.mustangandfords.com/how-to/interior-electrical/mump-1005-ford-mustang-instrument-panel-troubleshooting/photo-07.html


Also a long thread covering other things that can cause the problem https://www.vintage-mustang.com/threads/fuel-gauge-adjustment.535407/

[mygt350]

Update.
Removed the recently replaced sender (Scott Drake) in tank and bench tested empty/full ohms and measure 10.4 ohms full and 76.4 ohms empty. Sender originally in car was also a Scott Drake and measured 9.5 ohms full and 80.5 ohms empty so both senders are really close to each other.

When I ground sender signal wire with key on, fuel gauge sweeps past full.

Connected jumper from fuel sender to both sender units and grounded each.
Both senders read empty on gauge when float is bottomed out. One actually goes a bit past empty.

Both senders read 3/4 full on gauge when float is all way up.

Is this indicative of a gauge issue?
Thanks for your thoughts before I rip cluster out, again...Frustration level getting too high.

[mygt350]

Found a youtube video about how to adjust a 65 temp gauge. Adjustment was with gauge out of cluster. On back side, two holes. One was for "cold" temp adjustment, assume other was for "hot". Guy inserted small screwdriver and adjusted it until needle was on "cold". Video cut off without addressing adjusting "hot" reading. Fuel gauge has same 2 adjustment holes. Assume one is to adjust until needle is on "Empty" and other is on "Full". Thought when key was off, fuel gauge needle would go to empty side. So how does a fuel gauge get adjusted to match sender it will be used with?