willysmjeeps.com

First, I am not an electrical engineer.

I have a question about the in-tank fuel level sensors.

On the M38 and A1, as well as the CJ, the fuel level sensor mechanism is located inside the liquid/vapor area of the tank. The level is determined by a float moving up and down with the fuel level, actuating a lever which the upper end is wiping back and forth across a spring resistor.

On the M38 that spring resistor assembly is at the bottom of the sender assembly with a wire running up into the cap on top of the tank, which is wired to a Douglas connector for the sender wire.

On the CJ the spring resistor is on the underside of the cap, and a screw terminal on the outside top connects to the sender wire.

As I understand it, the sender is acting as a variable ground. As the level changes and the wiper moves across the spring, the resistance changes, which is then sensed at the meter.

My question is this. As there is voltage going to the sender and the spring resistors are installed in the liquid/vapor space of the tank, what prevents sparking and eventual fire or explosion?

A follow up is if the tank is primed and painted properly, and simply clamped to the tub and sitting on the anti-squeak gaskets, there is no suitable ground from the tank to the tub/frame/battery.

How is the whole thing getting grounded?

Thanks,
Brian

okay the fire thing is simple there is not enough current flowing through the system to cross an air gap, no physical contact no flow. I am not sure on the ground not that far along yet.

The fuel itself is part of the ground, as is the plumbing, the engine and the chassis. But this is not always a very good ground hence the advice is to add a ground strap at tow places. The tank to the tub and the instrument cluster to the tub. As an extra precaution you can add a ground from the tub to the frame.

The screws also ground the sender to the tank, so you have to get good contact between the screws and sender. Mine isn't moving anymore, so I am getting ready to do some troubleshooting myself - lost ground somewhere - I hope it isn't another bad sender!

When these jeeps were built the paint was omitted where the bare new screws rested their heads. For restores who love to paint their jeeps very detailed there is a tool that is a piloted tiny wire wheel which can be used to remove paint around the screw holes neatly. It is technically referred to as a bonding pilot since it is used to insure rivets get a good bond with the bare metal they are fastening. It is available at most aircraft tool supply outlets. Try ATS (Aircraft Tool Supply) or Brown Tool Supply or Yardstore.

Nice! There are so many tools out there that I never knew existed. Beats the crap out of trying to use a screw driver to clean the area!

I will implement Wesk's suggestion of additional grounding wires between the instrument cluster and tub and also between the fuel tank and tub. My question/situation is this.

Fuel sending unit out of tank, resistance measures 1.5 Ohms to 35 Ohms depending on where I move the float lever. When installed in the vehicle with a FULL tank the resistance is 10.5 Ohms and the gauge reads 1/2 full. Since the sending unit values seem correct, do we think this is just a grounding issue or is my gauge also suspect? With the connector removed from the sending unit, the gauge goes to full FULL deflection.

Thanks,
Wade

Before we chase our tail here tell us the exact numbers you have from the Army's test bulletin. We have no idea where your float is when it is in the tank. It may be reading half full because the float has excess moisture in it(weight) and only rides half way up in the fuel.

TB 2300-228-20 when utilized correctly (FOLLOW THE STEPS) will tell you the problem. As always it's free and in our Downloads Section:
http://www.willysmjeeps.com/v2/modules. ... tit&lid=46

Thanks for the link to the troubleshooting guide. I'm sure it will be helpful in running this down.

Prior to posting my inquiry I had confirmed that the float is intact and working properly. The float arm pivots with very little mechanical resistance and the float is not compromised by a leak nor has it gained weight due to stray sealants, coatings etc., being deposited. There are no objects in the tank that preclude proper movement of the float.

You weighed the float? Just a 1/2 Oz of fuel inside the float will hold her down a bit in the tank.. The pin hole that let the fuel in can be very small and undetectable without a glass.

I was going to suggest check to make sure it is clocked correctly
but I remembered my earlier post comparing the CJ sender vs
the M38 and that all five holes are not equally spaced and
so it should only mount one way in the tank.