Calibrating 1955 fuel gauge sending unit

[5390john]

The latest item to fine tune on my '55 6237DX is the fuel gauge. Everything works, but the getting indicated level to be correct is proving a bit challenging.
I completely drained the tank, then put in a measured 5 gallons. I removed the sending unit and adjusted it per the manual's diagram, on page 15-8. I then installed it and the gauge barely registered empty. I then removed the sending unit again and repeatedly adjusted the float arm until the gauge read 1/4 full. I figured a 20 gallon tank with 5 gallons = a 1/4 gauge reading.
I then added another measured 5 gallons, now the gauge reads 3/4 full.
BTW, also checked and confirmed that the car is level.
Comments appreciated!
John Adams

[Caddy Wizard]

I would adjust the float now until it reads 1/2 on the gauge.  It will then be right at least once during a tank of fuel!  From now on, remember that when it reads 1/2 it really is half.  Then take out 5 gallons and note the error (slightly above or below the 1/4 mark).  Make a mental note of that so that you know how to interpret the gauge in the bottom register.

Then live with it...

[5390john]

I was thinking along the same lines, but just leaving it accurate at 1/4 full. That way, I'll know its accurate at that point and time to fill up.
John Adams

[J. Gomez]

John,

You may need to take in consideration the float will not be resting on the gas tank when empty; you would have a few extra gallons to clear the pickup line before the gauge register “E”.

The fuel pickup line seats on the bottom of the tank (I know the ’56 does I think the same is for the ’55), so the engine will died before the float hits “E” if it seats at the bottom of the tank. You also need to consider these tanks have no baffles so fuel will be sloshing all over one would need the extra gallons to clear the pickup line at or above it.

[Jeff Rosansky CLC #28373]

Be sure to check the tank's ground. I put in a little ground line when I put the tank back in.
Jeff

[Caddy Wizard]

I was thinking along the same lines, but just leaving it accurate at 1/4 full. That way, I'll know its accurate at that point and time to fill up.
John Adams

Yeah, pick one or the other.  Either 1/4 or 1/2 and calibrate the float to that.  Obviously, there is no need for the gauge to be accurate when completely full.  So pick either 1/4 or 1/2.

[bcroe]

Never worked on a 55, but I suppose it is like mine a couple dozen
years later.  The first issue is the float needs to move from just
touching the bottom, to just touching the top.  In order to make
that happen, I have had to lengthen the float arm, then bend it to
just at both extremes. 

The other test is to connect the pickup properly drives the gauge. 
I connect the bare pickup to the car and move the arm.  The gauge
needs to move between empty to full.  Once assembled, this test
can be done by turning the (empty) tank upside down. 

I added this pic of a common sender unit on the left, which cannot
reach the top of the tank. So it reads FULL for about the first 1/3
tank, then drops like a rock after that.  The one on the right has a
loger float arm allowing the float to just touch the top and bottom. 
It will give accurate readout always if not defective.  Bruce Roe

Old pickups can have all sorts of poor connections, for example
a rivit connection has developed some resistance.  If the gauge
is not responding, check voltage across the wiring of the pickup,
or check  the resistance against specs.  A lot of extra resistance
means there is cleanup to be done.  Bruce Roe