Replacement fuel tank sending unit won't read full

[scotth3886]

In my 66 Fleetwood, I just replaced the tank sending unit.  Original had a bad gasket and stunk the garage up when full with the vapor leak.  Also gauge read correctly, but bounced back and forth when it was around half tank or so.

Bought a replacement sending unit from Summit, which came from Restoration Parts.  Would only read to about 7/8ths and went bad a few hours later (made in China).  It didn't look like the original.  We mistakenly threw away the original sending unit.

Got a rebuilt original from Tom at Cadillac Kingdom.  Just picked up the car.  This one is a match for the original, but only reads 2/3rds when full.

Does anyone have any idea why it's reading like this?

Thanks in advance.

[klinebau]

The float might be bent.  The factory service manual might contain a section on correct float measurements (it does for 1970).

When I did replaced mine, I had several problems

• Float height was off by a half inch or more
• There was a problem with the shunt resistor in the fuel gauge.  I forget if it was pegged full or empty, but I think it showed full.  I replaced the shunt resistor, but it might have just been a dirty contact.
• Due to the angle (or lack of) of the filler neck, the tank is 3-4 gallons shy after the pump shuts off automatically.  I have learned to wait
  a few seconds and then slowly add the remaining gas to get a full tank.

[scotth3886]

The float might be bent.  The factory service manual might contain a section on correct float measurements (it does for 1970).

When I did replaced mine, I had several problems

• Float height was off by a half inch or more
• There was a problem with the shunt resistor in the fuel gauge.  I forget if it was pegged full or empty, but I think it showed full.  I replaced the shunt resistor, but it might have just been a dirty contact.
• Due to the angle (or lack of) of the filler neck, the tank is 3-4 gallons shy after the pump shuts off automatically.  I have learned to wait
  a few seconds and then slowly add the remaining gas to get a full tank.

Thanks. 

Mine shuts off right at the top.  I can get only a few ounces in after auto shut off.  Today the pump didn't shut off with predictable results.

[scotth3886]

I'm going to have to pull it out again.  It's reading about half of what it should be.  This is a refurbished original unit from Cadillac King in SoCal. 

It reads under a 1/4 tank when it takes 12 to 13 gallon to fill and once completely full, it reads just over a half tank.  This will fill all the way to the top of the neck before the pump shuts off.

Is the sending unit adjustable / fixable on our end somehow? 

[Kurt Weess]

Scott, I just replaced the fuel sending unit on my 70 Fleetwood on Monday.  Here are a few things you may want to consider when checking the unit out when you pull the tank.  Once you pull the sending unit out of the tank hook it up to an ohm meter and confirm that the resistance sweep empty to full is 0-90 ohms.  Then connect the vehicle wires to it and check to make sure when the float is at its highest point that the vehicle fuel gage reads just above full.  Do the same when the float is at its lowest point and gage should be just below E.  Then reference your shop manual and see if there is a picture similar to the one I’ve attached to set up the float.  You can bend the rod that the float is on to make the adjustment.  I also noticed that the new sending unit’s pick up tube didn’t appear to be as deep as the tank was.  The tank measured 5-1/2” deep where the sending unit was mounted.  The pick-up tube without the sock on it measured 4-1/2” from the mounting flange.  I used a tube bender and bent the pickup tube so that it would extend further down into the tank.  I was able to bend it to 5-1/4”.  I put the sock on it and installed it in the empty tank.  I took a resistance reading at that point to ensure the float was down all the way.  While the tank was sitting level on the short edge of 2 concrete blocks I put 2 gallons of gas in the tank.  I then measured what I could siphon out through the sending unit fuel line to ensure the pickup was low enough.  I got most of the 2 gallons out.  I added 1 gallon at a time to the tank and then measure the resistance of the sending unit while it was still connected to the vehicle wires and observed the gas gage reading.  I stopped at 8 gallons when the gas gage was reading just above ¼.  Your tank is a 26 gallon tank and designed to hold 2 gallons of gas reserve when the gas gage reads empty.  I siphoned the 8 gallons back out of the tank and reinstalled the tank.  Once installed I added back the 8 gallons and went to the gas station to fill it up.  Every 6 gallons I stopped pumping and observed the gas gage reading.  It was very linear up to full (18 gallons added).  It read just over full for about 20 miles and then started falling normally.  While the previous sending unit (not original) appear to be working ok I would only be able to get 16 gallons in it while filling it up when on E.  I believe the extra time during the install will ensure accurate readings and the ability to get full range out of the tank.  Good luck!

[scotth3886]

Perfect, just what I needed. 

The refurbished unit was supposed to have been from a 66 so it should be bent to reach the bottom of the tank.