P-14/15 Fuel sender issue. I need a new theory.

[Joe-Bob]

I'm frustrated. The fuel gauge in my '42 won't register full, or anything else accurately.  The backstory is that I converted to 12 volts and completely rewired the car myself about 15 years ago.  I installed a NOS fuel gauge with a Runtz reducer.  I installed a new fuel sender and grounded it by wire to the trunk floor.  I'm confident everything is correct, because if I remove the sender from the tank and raise and lower the float the gauge reads correctly.  I decided the float's angle of the dangle was off so I monkeyed around with that to no avail.  In fact I have it set so that the float hangs straight down and would touch the bottom of the tank long before it reaches the end of its swing.   The tank is eight inches deep and has about five inches of gas in it.  The gauge reads almost empty.  I don't see where the float is restricted in any way.  And here's another weird thing; if the ignition key is set to accessory, and the fuel sender is out of the tank and in my hand, the gauge reads just over 1/2 tank when it should read full.  If the car is running, it will read correctly as full. I don't understand what that would have to do with anything.  The float does not appear to be gas-logged in any way.

 

I did recently have a 318 installed in the car with the idea to take it on longer drives.  This is the main reason the gas gauge is more important to me than it was before.

 

What in the world am I not doing or understanding?  Any helpful advice?

[Sniper]

Not sure why you thought you needed a Runtz resistor.  The gauge works on current flow not voltage.

 

I don't have a 42 service manual, but I do have a later one that covers that style gauge, there is a while troubleshooting section in it.  Have you gone thru that?

[soth122003]

17 hours ago, Joe-Bob said:

I decided the float's angle of the dangle was off so I monkeyed around with that to no avail.  In fact I have it set so that the float hangs straight down and would touch the bottom of the tank long before it reaches the end of its swing. 

How? A single bend to the float rod? When I installed my new tank unit, I had to do a double bend on the arm so the float would not reach higher than the body of the sending unit. Bend the arm down near the sender, then move down about 3 inches and bend the float up till it's just below level with the sender This gives the arm full swing (basically shortening the arm to match the tank)  showing me the full reading when the tank is full. With just a single bend the tank would only read about 1/2 tank when full and show an empty tank with about 1/2 a tank of fuel left. With the double bend done, I had to measure the float drop and make sure it was about 7 and 1/2 inches to show an empty tank with enough fuel left in it to get to a gas station. It's not perfect, but it shows full when full and has about an 1/8 of a tank when it shows empty.

 

17 hours ago, Joe-Bob said:

And here's another weird thing; if the ignition key is set to accessory, and the fuel sender is out of the tank and in my hand, the gauge reads just over 1/2 tank when it should read full.  If the car is running, it will read correctly as full. I don't understand what that would have to do with anything.

When you hold the sender in your hand, it is not properly grounded and will not read correctly according to the trouble shooting guide. This is assuming you have the thermostatic fuel gage system. (2 Wire) read the manual section on instrument gages and it will explain it. Hope this helps.

 

Joe Lee

[Joe-Bob]

I had read the manual some time back and just read through it again.  It's a  little "techy" for my old brain, but it doesn't answer the question of why my gauge works with the sender out of the gas tank with the engine running, doesn't work correctly with the ignition switch on accessory and the engine off, and doesn't work when installed in the tank with the engine running or off.  I've heard both that I need a runtz and that I don't need a runtz, but what difference would it make? Stock was 6 volts and with a runtz it's 6 volts... And, when I adjusted the float to drop far too low in the tank, it should raise sooner and send the gauge higher sooner, but it doesn't.

 

After reading the manual, I switched the wires on the sending unit and started the car.  With the car running I have 17.5 ohms on the left terminal of the sender and less than one ohm on the right terminal.  The gauge did move to a little over half full.  I'm still confused because earlier, with the wires unswitched and the sender out of the tank, grounded, and operated by hand, the gauge worked as it was supposed to. I can't seem to locate a constant that might allow me to adjust things without just being random.

 

[Joe-Bob]

Soth 12203 -

That I can understand, but I have a ground wire from the sender connected directly to the trunk floor.  

[Joe-Bob]

Soth 12230- 

To adjust my float I loosened the set screw and rotated the entire float rod straight down and tightened the set screw.  At this point it will drop to the floor of the tank and raise to a point several inches lower than the top level of fuel in the tank.  since the rod is longer than the depth of the tank it seems like it should never reach empty.

[61spit]

I would think you want the float rod travel in the tank to match the full travel of the sending unit. When the arm is at the bottom of its travel in the tank it should also be at the bottom limit of travel on the sending unit. Should be showing empty on the gauge. When the arm is at the top of the tank it should also be at the upper limit of travel on the sending unit. Should be showing full on the gauge. If the arm doesn't travel the full range of the sending unit it can't send the proper signal to the gauge. Just my thoughts on this.