I think I've read what I could here on fuel gauge issues.
I've replaced all the lines (I know that doesn't really matter but it needed to be done.) I've replaced the main alternator ground and the fire wall ground and now I've wasted $50 on a new fuel sender. The problem remains the same, the gas gauge always shows too much gas (when the tank is full it's way past the F).
The oil pressure and temperature gauge seem accurate. I have dual tanks and it seems to read high on both. What's my next step? A new fuel gauge?

 

My '78 has done that for the 30+ years I've owned it.

With the 460 I have I can only wish for too much gas.

 

Yeah I could deal with it by just using the odometer but unfortunately other family members use it occasionally and guess who gets called when they run out of gas....so I want to fix it.

 

The important measure is if your gauge shows properly at just past the E when your tanks are empty.
I do think it is normal for the gauge to show above the F when first completely filled. Mine does that on both tanks.

David

 

I had similar symptoms ('78 F250). Checked the wiring and fuel pickup and both OK. Checked/changed the instrument voltage regulator. No Change.

Added an aftermarket 2 1/8" fuel gauge on the lower dash and it worked.

Sounds to me like your dash mounted fuel gauge is fubar. Getting the OE cluster rebuilt is an option... but slow and expensive. An aftermarket gauge tied into OE wiring has worked well for me. Battery + from a switched source, ground and connect to the OE fuel gauge wire on the engine side of the firewall.

I used an Autometer gauge from Summit though lower cost options are available.

 

Agreed.
But unfortunately it shows above full now (3/4 in there now) and it shows a 1/4 when empty.

 

Where does your needle come to rest when the key is off? It should look like this:

If you know that it is truly reading 1/4 tank fuller at all times, the gauge works, but the needle is off. It's a thin metal needle that can be easily repositioned (bent) to read accurately.

David

 

You could also bend the float arm in the tank to compensate for the inaccuracy. This sounds like the problem to me. If you adjust it to read like a sixteenth of an inch past the Full line when the actual float ball is as high as it will go inside the tank, it might just drop down below the Empty line before it runs out of gas.

Another angle is false resistance between the gauge terminal posts and nuts, and the Instrument Panel Printed Circuit Board.
I had a Mustang in which the temperature gauge would immediately peg full hot as soon as the engine warmed up enough to lift the needle off of Cold. I pulled the cluster, and unscrewed the nuts from the terminal posts, and cleaned the contact surfaces. The gauge worked perfectly after that. Actually got the idea to try that from an article in Motor Trend.

 

Its not the float arm as I just replaced the sender and it's exactly the same. Since it does the same for the cab and rear tank I'm goin to look into the resistance thing. I think you have something there, thanks for pointing me in that direction.

 

Like rougeriver, mine is accurate when empty.

 

NM76 - I'm not sure you and meangreen are on the same page: he is suggesting you adjust the bend in the float arm of your sender. Just a coupla basics:
1) If you ground the wire to your sender back by the tank - what does your gauge do?
2) When the ignition is off does your gauge needle drop below E?
Your sender and gauge both appear to be functional, so you have a few areas to check - sorry, but none of them is instant. First thing to keep in mind is that juice runs from the dash through the gauge, then to the rear of the truck, through the sending unit, then to ground. Think you said you checked the rear ground? Do it again.
A) Using a multimeter check the resistance across the sending unit - should vary between roughly 20 and 70 ohms. If not, your sender may be shot, even though new.
B) Have to drop the tank again for this...look in through the filler opening in the empty tank and see where the float is hanging when empty. If it is hanging in mid-air a couple inches off the bottom of the tank, that may be your problem - it should hang almost touching the bottom of the tank. Close, but not touching.
C) Now that you have checked the float is in the right place, using a dowel rod through the filler move the float up and down- your sender resistance should vary with the float position. If the float is positioned right and the resistance is changing like above, you should be golden.
D) Now, If the answer to #2 is no, you may need to drop the gauge and adjust the needle. If the original answer is yes, so your gauge shows less-than-E to past-F swing, most likely the gauge is OK.
E) If you clean the mounts and check the sender is OK yet still have the bogus readings, you have a bad contact in your wiring harness giving you an artificially high resistance on that front-to-back circuit.

 

HoustonDave, thanks for the thorough reply.
1. Gauge goes full scale right, way past F.
2. Yes
Sender is working fine. To be clear I'm working on the in cab tank (it's easier) but gauge has same issues on both tanks.
I'm going with the resistance issue and pulling the gauge cluster today to clean all contacts.
Will report when done.

 

Keep us posted, especially if you find something no one mentioned. We can all use extra schoolin' and what you find can save one of us hours of %$##%^!!

 

Yeah will do. Of course it's never an easy fix. Pulled the cluster and found the tabs to be crumbling. So reinforced those then as usual with vehicles I purchase the wiring by the PO leaves a lot to be desired. Crimp connectors, cut and taped wires etc (being **** and never wanting a break down due to something I did, I'm more of the solder and heat shrink type) so hopefully tomorrow it'll be back together.

 

Success!
Cleaned all the connections on the gauge cluster and that's about right for the cab tank, judging by what I saw when the sender was out. Rear tank now shows 1/2, really no idea how much is in there so I'll have to go fill it.
Cleaned all the bulbs and the dash lights are adequate now. When I was cleaning the fuses the courtesy light fuse came apart. Had to chase all over town for that one, really? How many of these trucks were sold? And Napa can't carry the right fuse?
The tank selector toggle is broken off so I looked for one of those. No luck either so I ordered one for a Bronco. It's clear now that the Cab tank was added later, at least genuine Ford parts were used.