I have a 78' F100 4.9L 300 3 speed. I was having issues with my taillights and found that it was the trailer connection the previous owner had installed (not very well). I took that off and wired everything back to what it was before. My taillights now work correctly after removing the shotty trailer wires and replaced a few sockets.

Before removing the trailer connection, I decided to take a look behind the gauge cluster. It fell apart as I began unscrewing it. So I replaced it with a new one I found online.

Now it seems my fuel gauge is incorrect. It reads about 50% over what it should. For example, the gauge will read that the tank is half full, when in reality it is closer to empty. Everything else on the gauge cluster seems to be working fine. I looked through previous posts regarding fuel gauge issues but I wasn't able to find a post similar to my situation. If anyone knows of a previous post that I may have missed, please let me know where I can find it, Thanks

 

Yeah, not familiar with such a big change just from changing clusters.
New wiring circuit board/film and IVR too? How do the other two gauges read?

I would not think that there were any official variations on how many ohms the gauges used over the years. Anything that the cluster came out of that would fit your particular truck should have the same value in the gauge. Only exception I could think of is if it was out of a '73, but even then it should have used the same resistance values.

When the tank is filled up, does the needle peg all the way up and beyond? Or does it read normally in that regard? What about where it is when the tank is totally empty? Still near the middle?
What about a dual-tank selector switch? Do you have two tanks and the gauge is running through the switch?

Good luck. Maybe we can come up with some other tests along the way, but just some more details may help. Or at least keep us occupied in the meantime!

Paul

 

Hi Paul,

I used the old wiring circuit board/film from the original gauge backing that fell apart when I attempted to take it out of the dash. The circuit board film seemed fine to me so I decided to reuse. To be honest, costing around $60-$70 did help me come to the conclusion that the old one would work fine.

The other gauges read just fine. A few days ago, I flushed the radiator and didn't put enough coolant back in so the engine began to run hot. The temp gauge worked accordingly. The speedometer is a bit jumpy/wiggly but it has always been that way. It is "generally" correct.

The only thing I really changed was the plastic backing that houses the gauges and light bulbs.

Currently when filled up, the tank is definitely above the usual mark. I haven't ran the tank to totally empty, so I'm not sure exactly where the needle lies when it does.

Earlier today, I stopped to get some fuel. The gauge needle was right between the half mark and the three quarters full mark when I made the stop. I topped off the tank with about 13 gallons. I have a 19 gallon tank. (After thinking my math through, my guess of the gauge being 50% above is incorrect)

I only have the one tank.

Thanks

Casey

 

Hey Casey. Yeah, testing the tank to empty may not be the best practice when there's only one! One of the cool things about having dual tanks actually.

Not sure what might have changed then, other than the new gauge being wonky. It's happened before. Not all systems are created equal sometimes, unfortunately.
The IVR's are often adjustable, but doing so will effect the reading of the fuel, temp and pressure gauges equally. And besides, that's a BIG adjustment to make to bring the gauge back to it's old normal.

You could at least start by testing the sender in the tank. Now that it's pretty much full, it should read approx. 10ohms on a meter between the two contacts. The actual numbers are something like 73.5 to 9.6 ohms, or something like that (info probably available on the internet) but we usually round it off to just "73 to 10" to ease our memories. At the minimal it should read full.
If your sender is reading much less than ten then, you may just have run into the typical "coincidental failure" that happens when we work on one thing and something else decides to take a crap on us.
It's not as likely of course, but still worth checking because it's easy enough to do.

Ford did change ohm ranges for the fuel senders at some point, but pretty sure it didn't happen until the later '80's or so. Still worth looking into any possible variations you might have run into however. Your gauge cluster is for a '78 though, correct?

Next would be checking the voltage output of the IVR to see if it's within spec. Whatever the spec is that is. The older ones (and probably yours too) will put out a pulsing voltage that averages out to something like 5v or 6v because in the end, these are actually 6-volt gauges if I'm not mistaken.

Then you can verify the ohm reading of the gauges. Honestly, not sure just how to do that properly, so maybe someone can school us on that score.
Maybe they'll come up with some other tests as well.

Good luck.

Paul

 

Thank you for the advice, Paul. I will study up on correctly checking the sender on my rig and make that the first step. At this time, my bank account can't take another "coincidental failure."

The gauge cluster is definitely the original setup from 1978.

Thanks again and I'll post an update after checking the sender

 

I don't know, I'm going to bet that the sender is OK, and that the problem popped up because of the cluster back swap. In this case, I'd definitely recommend double-checking your work before digging into any other areas.
You can ground the wire which plugs into the sender, and turn the key to Accessory or Run, and the gauge should peg Full. You can also insert a resistor between the sender wire and ground, and the gauge will stop short of Full. I forget/never knew the resistor values, but there are three different ones for testing 1/4, 1/2, and 3/4 of a tank. But I think I already know the results - they'll all give high readings...

And you say that it's your old original gauge installed into the new cluster shell?

So if the gauge is reading higher than it should, then that indicates that there could very well be too much resistance in the circuit. Something like where the gauge retainer nuts contact the Instrument Panel Printed Circuit Board. If one of the nuts is loose or dirty/corroded at its electrical contact point, then that will create more resistance for the circuit. Same goes for the IC PCB. You can give the IC PCB contacts a gentle scuffing with the finest sandpaper you have, or anything 400 grit or higher.

There is another contact point where the gauge cluster harness plugs into the cluster shell. Same thing there regarding dirt and corrosion, and in addition, it's possible that the IC PCB shifted position slightly, where the connector plugs in, and that could be causing a bad connection.

I have noticed that it's usually temperature gauges which are so profoundly affected by extra resistance, but the fuel gauge is basically the same as the Temp (and Oil) gauge, so it should be prone to the same problems.

 

Which instrument cluster back did you install? 1975/79: There are TWO different types. 1973/74 is not the same!

1975/79 F100/350; 1975/80 Econoline; 1978/79 Bronco:

The instrument cluster backs were made of a composition material that can TURN TO DUST before your very eyes!

1981: FoMoCo came out with hard plastic cluster backs for Econolines. These were also the "service part replacements" for the POS cluster backs. There are TWO types:

E1PZ-10848-A .. Hard Plastic Cluster Back-Use with Oil/Amp GAUGES / Obsolete ~ Available NOS

1981/91 Econoline. Retrofit: 1975/79 F100/350; 1975/80 Econoline; 1978/79 Bronco.
------------------------------------------------------------------------------------------------
E1PZ-10848-B .. Hard Plastic Cluster Back-Use with Oil/Amp WARNING LAMPS / Obsolete ~ Available NOS

1981/86 Econoline. Retrofit: 1975/79 F100/350; 1975/80 Econoline; 1978/79 Bronco.
-------------------------------------------------------------------------------------------------
1976/79: There are THREE different printed circuit boards. 1976/78 with WARNING LAMPS; 1976/79 with GAUGES; 1979 with WARNING LAMPS.

1976/79 with GAUGES also used on 1980/91 Econoline. 1979 with WARNING LAMPS also used on 1980/86 Econoline.

The instrument cluster backs used on 1973/74 F100/350, the printed circuit boards used on 1973/75 F100/350 are different!

 

I glanced at it after I noticed the disparity but I didn't actually unhook the speedometer cable and take it to my bench. I'll make sure to give it a thorough look. Thank you

 

I definitely have the correct back and I'm using the circuit mat originally installed. I triple checked to make sure I received the right part. Thank you

 

I changed my sender now mine does read full when full but actually has 8 gallons left when on empty!

 

I believe the opposite is true, extra resistance will cause a gauge that reads low. To support my theory, look at what happens when you connect the sender wire directly to ground, no resistance. The gauge pegs to the maximum reading.

Notamechanic, You can test the accuracy of your fuel gauge, if you have the cluster removed. Connect jumper wires to the studs on the back of the gauge. Connect the other ends to a 1.5V battery. Wait a minute or two for the needle to rise. If it doesn't rise, reverse polarity. If accurate, the gauge needle will stop the width of the needle past the first line on the gauge. Add another battery for 3V. The needle should stop a needle width past the 3/4 mark. I just confirmed these readings on 4 spare fuel gauges, all read exactly the same. It is possible to bend the needle to make the gauge read correctly. It is also possible to bend the needle when removing & reinstalling the gauge in the cluster, that may be what happened to yours.

 

Pic of testing a fuel gauge. Tried to attach it to my previous post, but it wouldn't show. Angle of picture makes the needle look like it is reading higher than it is when viewed from straight on.

 

Well, it won't let me post a picture for some reason, nor delete a post. Shows the pic when I try to edit, doesn't show when I post. Sorry, no picture.....

 

I guess I should've deduced that... While I'm good at repairing most electrical items on older vehicles, I am still an illiterate hack, unversed in theory...
BUT... My old '82 Mustang GT would shoot the Temp needle straight up to Hot/overheat, as soon as the engine warmed up enough to prompt the gauge to start moving the needle. I cleaned the contacts, and then the gauge worked perfecly for years after.

And this scenario never even occurred to me!


And the site must be glitching again, because I can see your picture of the "Unleaded Gasoline Only" fuel gauge, red and black jumper wires with alligator clips, and Energizer AA battery, in the text field where I'm typing this!

 

Are you seeing the full image, or just an icon? I can't see anything, but if using the quote function I see icons where the images probably are.
Sorry for the off-topic discussion there Notamechanic. But it's good to take a break now and then!

Paul