Sending unit ohms/volts & ground

[RSG]

Can someone help me out? I've searched the net and not had much luck.

I'm looking for the fuel sending unit resistance and corresponding voltage at empty, full, and points between. I also recall 2 wires on the fuel tank cap going to the sending unit, and so I guess this means the tank itself is not grounded?

The deal is that my stock gauge shows full but my aux pump is not shutting off; it is supposed to shut off when the signal from the sending unit is around 2.55V and turn on around 1/2 tank, supposedly at 1.65V if I can read my handwriting from the vendor phone call. I want to check if the sending unit is behaving correctly...maybe I damaged it when I pulled it a few weeks ago. Or the controller I bought is screwed up, or maybe I didn't ground something properly when I put the tank back up.

I measured these values up under the dash from a wire spliced into the OEM tank wire (white/yellow stripe) when I pulled it down:

Gauge reading Voltage
Really Full!..........3.25
Full....................2.95
7/8 or so.............2.43
1/2 tank +/-........1.93
​

Thanks!

 

[BJS]

Ford uses resistance for the fuel gauge not voltage. If memory serves me right I remember reading the range is 15-160 Ohms.

are you checking voltage with the engine running or not? voltages change when the alternator is spinning.

 

[RSG]

Thanks...i know it's a resistance (potentiometer) but it is converted to a voltage since they drive the circuit with a current source. I should have measured it when I took the sending unit out for my harpoon job but I neglected to do that.

So, I measured volts between the sending unit wire and ground...and you're right, the source voltage will change but the current source is fairly stable so the voltage on that circuit shouldn't be changing much -- especially with my new alternator..

 

[DaveBen]

So, I measured volts between the sending unit wire and ground...and you're right, the source voltage will change but the current source is fairly stable so the voltage on that circuit shouldn't be changing much -- especially with my new alternator..

That doesn't make a whole lotta sense RSG. To get a change at the gauge, the voltage should change. That is the whole reason behind the Potentiometer.

Dave

 

[RSG]

Ok, let me rephrase. I can see what you mean Dave, not very clear what I was trying to say.

BJS pointed out that voltage in our trucks fluctuates depending on if the engine is running or not (agreed) and I also noted that it depends on rpm and alternator condition. All true and good...

The sending unit has variable resistance. 15 ohms empty and 160 ohms full according to BJS (thanks!!!)...which is what I was hoping to learn. What I meant to say was that the fluctuating voltage (depending on charge system) doesn't hardly affect the voltage readings taken from the sending unit, because the circuit is driven at a constant current...and the voltage from the sending unit is derived from V=IxR where I is the current source, R is the sending unit's resistance, and V is what I'm measuring. The current, I, is pretty stable regardless of engine state. That's what I was trying to say. Otherwise your gauge would give you different fill readings depending on if the engine were on or off (ACC) or if the alternator was running at 14+V or not charging.

Anyhow, I wanted to know R at full fill, to match V at full fill, to estimate I...and it looks like it is I=3volts (full)/160 ohms (full) = 19 mA. So now, knowing an estimate for I, allows me to estimate at other values. For example, at a half tank I measure V=2V, I think I=20 mA, so R=2V/0.02A=100 ohms. That is what I was trying to work out without taking my tank down (again).

Now I can go to my auxiliary tank transfer controller and see if it is programmed wrong, or my sending unit is screwed up. The controller switches on when voltage drops below a certain value and is supposed to switch back on when voltage goes up equivalent to around 3/4 tank...which should be around 130 ohms according to these calculations. I would like to verify this from a service manual or if anybody happens to have their fuel tank on the floor!

My problem is the controller never switches off (automatically, that is)...coupled with the harpoon mod, I've been running on "Full" since last week...I've never filled her that full o diesel in 10 years!!

Reading this post, I can see I'm about as clear as mud, I don't think I helped much... But I know what I meant

 

[DaveBen]

Good enough for me...

Dave