Fuel Sending Units - Problem

[Dino L.]

Given a sending unit, fuel guage, 12 vdc battery and a set of jumper wires, what is the schematic to bench test the assembly?

[Gene M.]

Re: Fuel Sending Unit- ohm readings

Dino, I would suggest to verify the variable resistor using an ohm meter. Swing the arm thru it's travel and see the ohm resistance change if it does it's a good part.

[Dino L.]

Re: Fuel Sending Unit- ohm readings

Some time back on the board there was a posting on a repo part that could cause an explosion , because of the faulty wiring. I am restoring the car and have the gauge and sending unit out and would like to bench test the entire circuit.

[Michael H.]

Re: Fuel Sending Units - Problem

Put 12VDC on one side of the gauge, the other side goes to the sending unit. Ground the sending unit frame. If it doesn't work, check the resistance of the sending unit lead to ground. On my '68, the resistance is around 95 to 105 ohms fully up (full tank) and around 10 ohms at empty (possibly vise versa).

[John H.]

Diagnostic Substitute for Sending Unit

Radio Shack sells a great little 0-100 ohm potentiometer for about $4.00 that makes a perfect substitute for a sending unit to determine (before tearing the car down) whether it's the gauge or the sending unit that's at fault. It has three terminals - solder a short wire between the center one and either end one and that becomes one connection; the other terminal becomes the other one. Insert it between the gauge terminal for the brown wire (from the gas gauge to the tank unit, disconnected temporarily) and ground, and turning the shaft on the pot will make a (good) gauge deflect full-scale. If the gauge works with the pot in place but not without it, your sending unit is history; if the gauge doesn't work with the pot in place, the gauge is history.

John

[Jack H.]

Re: Fuel Sending Units - Problem

Circuit topology will change with model year. Straight axle cars and Sharks used a simple ammeter reading gross current flow through variable resistor in sender to ground. This made readings (SA cars only, Sharks stabilized supply voltage) fluctuate based on absolute supply voltage (is generator pumping current and driving battery above its normal rails?). So, here the setup is simple: Battery supply voltage feeds fuel gauge input, output of fuel gauge (S=sender terminal) connects to sender, and sender connects to battery neg terminal completing the path.

With mid-year cars, circuit topology changed. Guage AND sender were constructed to be elements of a Whetstone bridge to eliminate the effects of supply voltage variation. Here, have correct dual ground termination (gauge AND sender) is essential for the circuit to work. When properly connected, fuel gauge reports %deflection of the sender independent of actual supply voltage level(s). Follow the setup shown in the Assy Instruction Manual's wiring diagram paying particular attention to the need for BOTH the fuel gauge and the sender to be grounded to complete the circuit properly.