Fuel Gauge/Sender ?

[RossL]

I just installed a new gas tank and sender on my 55. The gauge is not moving off of empty. If I ground the yellow wire that connects to the sender (in the trunk), the gauge goes to full. Power to gauge is 6 volts.

I've got close to a full take and the sender is reading 66 ohms, gauge reads "E". Seems like the gauge is expecting to see ohms reading in the 15-30 ohm range.

The car is new to me and the gauge has not been very accurate. The old sender is no longer working so I cant measure the resistance/ohms.

Can anyone verify if my findings of 15-30 ohms is the correct range? At this point I don't know if my gauge or the new sender is out of spec

[ward 57]

I've heard that there are two types of sender resistors. Analog and linear. The linear is for digital gauges and are not compatible the analog gauge. I know somebody on here has delt with this. Some of the suppliers don't label the difference.

[Rusty57]

Is the sending unit properly grounded? Sometimes there is not enough metal to metal connection between the locking ring, the plate on the sending unit, and the tank.

Sometimes the tank is not properly grounded to the chassis. Did you install some type of isolating strip like rubber or fabric between the tank and the hanging straps?

The sending unit should read low or close to 0 ohms when the float is in the full position. That is essentially the same as grounding the wire and getting a Full reading. I’m not sure what the empty reading should be on your sender. It would not surprise me if it should be at least 60 ohms.

In general these style sending units and fuel gauges are not known for accuracy. The basic concept of a float moving in an arc to give a linear change is resistance is flawed.

I’m by no means a TBird expert and especially for a positive ground 6 volt ‘55. If I remember correctly what I have read is that the needle for the fuel gauge on those cars was actually a driven by the by the deflection of a bi-metallic thermal strip. When the sending unit resistance was low more current (amperage) went through the strip which caused it to heat and therefore deflect more. The strip would cool as less current went through it so the needle would deflect less. This style gauge does not respond quickly to changes in current flow. It takes a while for the thermal strip to heat and cool.

Check your shop manual.

[RossL]

Is the sending unit properly grounded? Sometimes there is not enough metal to metal connection between the locking ring, the plate on the sending unit, and the tank.

Sometimes the tank is not properly grounded to the chassis. Did you install some type of isolating strip like rubber or fabric between the tank and the hanging straps?

The sending unit should read low or close to 0 ohms when the float is in the full position. That is essentially the same as grounding the wire and getting a Full reading. I’m not sure what the empty reading should be on your sender. It would not surprise me if it should be at least 60 ohms.

In general these style sending units and fuel gauges are not known for accuracy. The basic concept of a float moving in an arc to give a linear change is resistance is flawed.

I’m by no means a TBird expert and especially for a positive ground 6 volt ‘55. If I remember correctly what I have read is that the needle for the fuel gauge on those cars was actually a driven by the by the deflection of a bi-metallic thermal strip. When the sending unit resistance was low more current (amperage) went through the strip which caused it to heat and therefore deflect more. The strip would cool as less current went through it so the needle would deflect less. This style gauge does not respond quickly to changes in current flow. It takes a while for the thermal strip to heat and cool.

Check your shop manual.

I ran a separate ground wire to the sender, it did not make any difference. I'm reading 60 ohms with close to a full tank. The sender was reading 175 ohms at about 1/4 tank. I'm thinking they sent me the wrong sender or it's defective.

[BJRVentport2]

Common Factory Ohm (Ω) ranges

Empty Full Vehicle Application
0 ohms 30 ohms Most pre-'65 GM
0 ohms 90 ohms Most GM 65-present
16 ohms 158 ohms Most '87-present Fords
73 ohms 8-12 ohms Most Fords before '87 and most Chrysler
240 ohms 33 ohms Use with 3262 sender
10 ohms 70 ohms Ford Bi-Metallic Gauges (pre-1987 F-Series Trucks)
15 ohms 160 ohms Ford Magnetic Gauges (1987 and later F-Series Trucks)

[MarkR]

your gauge seems to be good from your test and you verified good ground. You may have received the wrong sender.It’s happened to me. The replacement senders are sketchy. In fact a few years ago I received one that would not fit through the floor pan.
I always manually index a replacement sender before installation. With the gauge wire attached, Simply run a jumper from the sender mounting plate to a ground in the trunk. With the ignition on you can manually move the float from empty to full and back while checking the gauge. On the four 55 senders I indexed, about 3 3/4” up from lowest point should read close to 1/2 full on the gauge. The tank is 7 1/2” tall. Adjust float for an empty reading at 7 1/2” from the mounting flange.(it’s not a bad idea here to bend the float down slightly to give yourself a little pad at empty) Then move it up the 3 3/4” for half a tank and check the gauge. No two senders were identical but now you have a fairly accurate starting point.
Don’t leave the ignition on, it can burn the windings in the gauge.

[RossL]

your gauge seems to be good from your test and you verified good ground. You may have received the wrong sender.It’s happened to me. The replacement senders are sketchy. In fact a few years ago I received one that would not fit through the floor pan.
I always manually index a replacement sender before installation. With the gauge wire attached, Simply run a jumper from the sender mounting plate to a ground in the trunk. With the ignition on you can manually move the float from empty to full and back while checking the gauge. On the four 55 senders I indexed, about 3 3/4” up from lowest point should read close to 1/2 full on the gauge. The tank is 7 1/2” tall. Adjust float for an empty reading at 7 1/2” from the mounting flange.(it’s not a bad idea here to bend the float down slightly to give yourself a little pad at empty) Then move it up the 3 3/4” for half a tank and check the gauge. No two senders were identical but now you have a fairly accurate starting point.
Don’t leave the ignition on, it can burn the windings in the gauge.

Are the measurements you suggest to the Top, Bottom or Center of the float to the float mounting flange?

Going a bit more than 7 1/2" to leave a little reserve in the tank when the gauge is on "e"?

[MarkR]

I go the center of the float. With these senders, especially the offshore repros we get, accuracy is hit and miss and not knowing exactly where the float freeboards in the fuel adds to the problem. The center just seems the best. To my way of thinking an exact full reading is not critical. Full is full at the pump. The same at half. The empty reading is the important one. . You’ll want the gauge to read a bit less gas than you have and then go by it. Get gas when it reads close to empty.
I started doing this when I ran out with the gauge just above empty in the 57 at last glance. I started up a hill to the house and there wasn’t enough fuel to cover the intake at the front of the tank. Had to walk to the house for a gas can, walk to the station and back to the car.

[ward 57]

I have only run out once many years ago when I pushed the limits. Don't remember where the gauge was reading but always now refill at 1/4 tank. The pump tells me it is pretty accurate.

[MarkR]

Same here. Sold the 57 and have had three new senders in the 55 trying to get it close to accurate. Settled on calling 1/4 tank empty. Taking no chances.

[RossL]

Update after trying two senders, calibrating the gauge, checking ground wires, trying to bend the rod for the float, I installed the CASCO 12v gauge and sender and it looks to be very accurate

That is definitely the way to go if your car is 12 volts

Looks like I have about 2 gallons reserve, gauge didn’t move until I put in 2.5/3 gallons

[Rusty57]

That sounds like good progress. Maybe I missed a post in this thread. Did you convert your car to 12 volts and negative ground?

[RossL]

Yes car is now 12v negative ground. Since my last post I completely filled the tank and the gauge only reads 3/4……. I will call CASCO and see if there is anything to adjust

[Rusty57]

Thanks for that additional information.

That might be the best readings you will get. Even new these sender/gauge systems were not very accurate.

Before you call CASCO I would measure the resistance of the sender now that the tank is full.

I know this is a somewhat messy task but you might need to remove the sender and do some additional testing. I expect that you did some checks by moving the float arm through the full range while you watched the gauge. When you do that again have some type of set-up where you can hold it at different positions. The gauge takes some time to respond.

Another test is to measure the resistance of the sender at each end of the travel. Also take readings at what you would consider 1/4, 1/2, and 3/4 position. That might give you some insights.

The third area is to evaluate the depth of the tank compared to the full linear travel of the float. I know this sounds crazy but you might need to shorten the float arm so the linear travel moves the arm through the full range of the rheostat. Bending the float arm only offsets the reading for a given fuel level. It does not mean that the arm goes the full range. Remember that the diameter of the float is part of the total travel equation.