My fuel gauge doesn't work at all, always shows empty, the needle never moves. I disconnected the wires from the sending unit and turned the key to 'on', I see movement on the oil pressure gauge, but nothing for fuel. I tried grounding out the sending unit wires with a nail, stuck it into the holes on the connector, and scraped those against the frame. The connector has two holes, I tired both, no movement on the gauge. I tried using a multimeter on the sending unit. Set the meter to Ohms. No matter where I touch the leads to the sending unit, I get the same reading (I do get a reading). The unit has two bolts where the wire connector goes, I assume one for the gauge wire and one for ground. Regardless of which lead touches which bolt (or anywhere else on the sending unit) I get the same reading. Moving the float also doesn't change anything.

The temp gauge also doesn't work, always sits all the way to the left. I suspect the battery gauge also doesn't work, but it's always stuck right in the middle. The speedometer does work. I haven't done any testing on the temp gauge yet, but haven't had any overheating problems.

 

All this testing with the test light is with the key to run.
With test light on the sender wire(s),light to a good ground dose either wire make the test light blink?
Does the temp gauge work? Use the test light to a good ground and the temp wire dose the light blink?
Test the oil psi sender wire the same way dose the light blink?

They all should blink if none of them do then the IVR on the back of the dash gauge cluster could be bad.
You have checked the fuse? Pull it and clean the ends of the fuse & holder and see what happens.
If you get say the oil & temp to blink we need to check the harness that runs down the frame rail.
There is a plug under the booster see if you have a blinking wire (yellow I think) there.
Dave ----

 

What year is your truck? My mind-reading skills have dropped off as I get older (and crankier). Single or dual tanks?

Wiring diagrams here, I guessed 85 but believe all years are similar:

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

Scroll to the second page for troubleshooting tips.

More info here, be sure to click the tabs:

Gauges - Gary's Garagemahal (the Bullnose bible)


in a nutshell, the oil pressure, coolant temperature, and fuel quantity gauges all operate on the same specs. 10 ohms = high, and 73 ohms = low. Those three gauges are basically the same inside, but have different labels on the face. For troubleshooting, you can use long test leads and connect the wire from one gauge to a different sensor and see what happens.

Ignore the ammeter (battery) gauge for now. It's a totally different critter, often doesn't work, and is on a completely different circuit than the other three.

 

Right, sorry. It's a 1984 F-150, inline 6, single tank.

 

Should I test the wires by puncturing the wire with my test light?

 

Other than the fuel gauge does the temp & oil gauges work?
I am trying to see if it is a common problem with all 3 gauges or just the 1 as that will tell us what path to go down.

I would start at the plugs that fit on the senders first, fuel, oil, temp.
Is the tank at the rear or side? Guess it does not make a difference LOL
Now if it is just the fuel gauge and shorting it to ground and or don't get a blinking light we need to move down the harness to the next plug and find the fuel gauge wire to test there.
The next plug is on the frame rail below the master / booster. I would try and pull the plug apart and check it to see if it has junk in it to keep from making a good connection.
Then test to see if you have blinking there. If not it gets harder as I don't know of anymore plugs till the back of the gauge cluster.

But lets test them other wires for blinking and go from there.
Dave ----

edit: you can do that but I would use a pin to go thru the insulation

 

The temp gauge does not work. The oil pressure gauge does seem to work.
It's a rear tank.

Is there a way to check if I'm clamping onto a good ground? My frame has a lot of surface rust... now that I think about it, my testlight ground cord should be long enough to clamp onto the ground bolt on my starter. I'll try everything you've mentioned. Thanks

 

Okay, I picked up one of these LED test lights. Used it on the sender connector, which has two holes. One hole does give a blinking red light, which i guess means it's getting power. I don't know why it's blinking though. When i use it elsewhere, such as on a battery post, it gives a solid red light. The other hole does not light up. Does that mean it's not grounded?

I tried the temp sensor wires. Again, one hole lights up red, but it wasn't blinking, and the light was very dim. The other hole did nothing.

I tried out the test light on my fuses. I have previously pulled all the fuses and they look good, but they are not all lighting up with the tester. In fact, only four of them light up, the rest I get no light at all. What could be going on here?

 

Back to my diminishing mind-reading skills: Maybe if you stand next to a window, I can pick up a stronger signal. But until then, I have no idea how your LED test light is supposed to behave. Can you please post a link to the product, so we can understand how it is supposed to work? I don't know what a red light is supposed to indicate.

The blinking you observed may very well be normal. The oil pressure, coolant temperature, and fuel quantity gauges all get power from a common Instrument Voltage Regulator (IVR). This is a thermal device, similar to a turn signal blinker. It takes a steadyish 12V input and chops it down around 5V for these three gauges. But it's not a steady 5V output, it's actually 12V pulsing on and off, for an average of 5V. By design, the gauges take a few seconds to respond, so you never really see these pulses unless you've got your nose glued to the panel.

Also, be careful troubleshooting with tester like that. I'm guessing the LED draws only a tiny amount of power. Was it advertised as "computer safe" or something like that? If the circuit being tested has a marginal connection, such as only a few wire strands making a connection inside a crimp, you only need a very minute flow of current to illuminate the LED. But then when you put everything back together and try to power the gauge, the current flow required is much higher and circuit can't flow enough electrons. This is a big troubleshooting gotcha, so always be skeptical of a circuit testing good while unloaded, especially if using a meter or test light that barely draws any current.





Ignition on or off? Many of the fuses are only powered with the ignition on. Only a few of them have power at all times.

 

There you go, use that for your troubleshooting. Set your LED test light aside for now. You have a known-good gauge and a known-good sender in the oil pressure circuit. Maybe not 100% accurate, but for the moment let's not dig that deeply.

Fabricate a pair of long test leads with small alligator clips at each end. You want something long enough to reach from the fuel tank to under the hood. It sounds like you already have access to the tank's sending unit, either via the tank or bed removed.

Under the hood, remove the wire from the known-good oil pressure sender. When you insert 10 ohms of resistance between that wire and ground, the corresponding gauge will point to H. If you insert 73 ohms, the corresponding gauge will indicate L. Approximately 30 ohms should put the needle somewhere in the middle. You already know the gauge responded more or less correctly when the oil pressure sender supplied the resistance. Now use the fuel tank sender instead.

At the fuel tank sender, connect your long test jumper to the terminal previously connected to the yellow wire. I'm talking about connecting to the sender, not the vehicle harness. Connect the other end of this jumper to the wire you removed from the oil pressure sender. Back at the fuel tank sender, connect the second test jumper to the terminal previously connected to the black wire. This is the ground. Connect the other end to the battery's (-) terminal. (Note I didn't have you use the frame as a ground, more on that in a minute.) Turn on the ignition (you don't have to run the engine) and the oil pressure gauge should now indicate what the fuel tank sender is seeing. Move the sender arm up and down and you should see the gauge respond. Give it a few seconds to catch up.

If your known-good oil pressure gauge doesn't do anything when connected via test jumpers to the suspect fuel tank sender, then the tank sender is bad.

Likewise, you can test the other two gauges by jumpering them to the known-good oil pressure sender. You will have to run the engine to get oil pressure so the gauge moves.

Forget the frame. Don't get sidetracked. None of the three gauge circuits use the frame as a ground point. Pretend the frame is made of plastic for the purposes of troubleshooting. Look at the wiring diagrams again, linked in a previous message. The oil pressure and coolant temperature senders are grounded via the engine block, which in turn is grounded via a jumper between the engine and battery's (-) post. The fuel tank sender is grounded by a wire running all the way back into the cab, connected to ground point G701 behind the radio. Look for a ground strap from the center of the firewall to somewhere on the rear of the engine. If this is missing, the cab won't be properly grounded, nor will G701 and many circuits grounded there.

 

The blinking is normal. Some of the fuses are not going to have power unless you turn the key to run. One of them won't have power unless you turn the headlights on.

 

Blinking is good, the other port on the fuel plug would be a ground.
For temp and oil they should only have 1 port unless you have electric pumps in tank then the oil could have more ports.
Yes watch LED yest lights as they can give false feed back I like the old school test lights that are getting harder to find.
Being you had a blinking wire for the fuel gauge test the other port for ground. If it is then the sender is bad.
We're the temp & oil gauges working before you started testing with the test light?
If I don't see the need for more testing but that's me.
Dave ----

 

Well I wish I would have purchased an old-school light. They had those at the store too. I guess I could spring for another and still get use out of my LED for the newer vehicles.

To answer your question, I don't get any light when using my tester on the ground port for the fuel sender. I do for the power port.

 

Okay thank you for all the detailed instructions. I'll be running these tests tomorrow.
I can verify that the ground from the engine block to the firewall is hooked up. It's not the OEM strap though. That is on the other side of the engine and broken off, dangling from the firewall. There is another ground cable running from the driver side of the engine, to the firewall.

 

I want to make sure you are testing this plug right.
With the test light clipped to ground you have 1 that blinks (fuel gauge sender wire) and the other that does not blink (ground?).
Now take the test light clip and go to the one that does not blink (ground?) and then the light to the one that does blink, does the light now blink?
This would prove the "hot" & "ground" for the gauge are working.

1 more test would be to run a wire from the one that blinks to ground. Now turn the key on and the fuel gauge should move to full.
All you should need to do is hook the blinking wire to the float arm resister, the ground to the ground of the sender and all should work.
Dave ----