Ok trying to hammer this one out to see if it is the sending unit, harness or guage. PO simply said he forgot to hook it up, and was going to wait until he took the bed off to hook it back up. Found out it was hooked up.

The tank should have a good bit of fuel in it. I topped off less than a hour from home.

Disconnected the harness.

Read across the fuel sending unit terminals. 85 Ohms

Decided to shoot black wire to multiple grounds. Open. It's my understanding.the yellow hits the sending unit, then the black goes from.the sending unit to ground. Big potentiometer.

So black wire should go straight to chassis ground, question is where is it.? This scheme is showing G701 granted it goes through two splices and one connector as well.

For giggles with the line hot I also shot the yellow wire to ground, intermittent resistance and voltage. No movement on the dash obviously because I didn't put a load on it. I do have a pile of resistors I could make a test box with but I don't know the Ohm range for the fuel gauge.

 

If the sender is working, this would indicate that the float and lever are all the way down in the Empty position.

Float could be saturated or the lever is stuck in that position due to corrosion.

Did you measure any voltage on the Y/W wire?

 

With key in, read from the yellow wire to ground wire and to the tank and to the frame and got intermittent voltage last night.

Will try to get more solid numbers or video here in a bit.

 

What year is your truck? I guessed '86 and selected this diagram:


Fuel Tank Selector & Gauges - Gary's Garagemahal (the Bullnose bible)


Scroll down to page 100 for troubleshooting tips. The sender varies from 10 ohms (Full) to 73 ohms (Empty). With your reading of 85 ohms, that's just a little beyond empty. That means the resistance windings are intact. If the sender ground is bad, that would cause the gauge to show empty, actually slightly below. Rig up a test jumper from a known good ground and see if that helps.

A quick test of the gauge itself is to turn on the ignition and momentarily touch that yellow wire (at the sender) to a good ground. This is the same as 0 ohms across the sender, which should drive the needle slightly above full

If this checks good, I'd guess the float has come loose from the arm or the sender arm is physically jammed.

The fluctuating voltage you measured at the sender is normal. Read the description at the link above to see how the IVR (Instrument Voltage Regulator) works. Basically it cycles full power on and off, for an average output of 5v. The gauges move slowly by design and won't show this normal fluctuation.

 

I'm going to say the sender is bad then. It should be reading about 3/4 a tank which would be well under 73 ohms.

And yes it is 1986.

What had me concerned is I had my daughter in the cab last night when I jumped it thinking it would ping F. She may have missed it.

The PO told me he dropped the tank, cleaned it out, ran new fuel line and forgot to hook it back up. Translation... I broke the sending unit.

Unfortunately the fuel line he has is some sort of plastic line he has running to the tank then shoved over the prevent steel just below the driver floor pan.

 

Rock auto doesn't list a sending unit for a 1986 4.9l with side mount 19gal tank but they do for 1984... Was there a change?

 

This isn't a daily driver yet, right? I'd suggest pulling the sender and making sure the float didn't come loose, or fill with fuel and sink. If nothing else, it would sure be a lot easier to shop for a new one with the old one in hand for comparison.

Sorry I can't help with the variations in sending units. I don't know much about the differences.

 

This is a good idea. I want to change the tank, it's looking rough, and re-plumb the system properly. Also plan on putting a inline fuel filter close to the tank on the frame while I am at it. While it's all out it will give me access to the frame here to clean and seal.

I saw LMC sells just the float. So if your float craps out you can get a new one.

 

The PO may have replaced some fuel line and it may have even be the line in your picture. But that blue poly line is factory Ford and short of mechanically damaging it, should not have anything wrong with it. There should be no reason to replace it.

Do not ever buy that worthless float from LMC. If you buy a whole sender/pickup unit from LMC, take the float off and throw it in the trash. They are made of some inferior material that will fall to ash in just a few months. I have just installed an HDPE (same material gas cans are made of) float that I purchased from Amazon; just type "plastic ford fuel float". I cannot speak to longevity yet, but I am confident it will last longer than the LMC float. I have a 1986 float that I just removed from the rear tank and it had barely formed a pinhole.

 

Good to know. I will order the float and attach it to the sending unit before installing anything.

I'm curious if you could put a thin coat of polysulfide fuel tank sealant on the float or if it would affect the buoyancy too much. A brush seal of some class A polysulfide would seal that float until Armageddon.

To bad they didn't make a resistance based fuel sending unit. No floats needed as it measures the gas based on how many pf of resistance there are in the tank.

 

I had actually considered that same idea. Not sure if it would make it too heavy, but on this truck I just don't have the time to find out the hard way.

 

Did you test to see if the rest of the gauge system works? Did you short the sending wire to ground turn the key on and have it go to full?
Do you have duel tanks? I hear the switch can be an issue. Bad contacts in the switch will change tanks but not let the gauge read right on either tank.
If not the float & sender may be good. I always try the easy way first ...... I am lazy!

I still need to check if my gauges work for my tanks as all I have done is install everything and hopped so far. When I pull the bed back off for painting I will have to remember to do the testing then.
I do have new sending units in both tanks but we all know they can be bad out of the box
Dave ----

 

Yes sending unit is reading too high for the amount of gas in it. When I pull the bed I'm going to take care of it.