Fuel Gauge Acting Crazy

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Fuel Gauge Acting Crazy

Powerman5K
This past week, my fuel gauge started acting wonky and I figured I'd ask the Bullnose Brain Trust for answers.

Today, I left for work with a full tank of gas. My gauge was reading half a tank. Within 2 miles, it went to nearly empty (where it stayed the rest of the 40 minute drive).

After work, it read half a tank the entire ride home then as I turned on my street it shot up to full.

I replaced the sender within the last year. Is there anything else it could be before I drop the tank (not my favorite thing to do), to swap out the sender again?
1980 F100 Custom. Inline 6. Power steering. 3 on the Column.
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Re: Fuel Gauge Acting Crazy

Gary Lewis
Administrator
You will probably have to replace the sender as that's classic wonky-sender behavior.  But you can test the gauge and all the wiring by pulling the wire off the sender and grounding it.  The gauge should go to Full or higher, indicating that all but the sender is good.
Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

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Re: Fuel Gauge Acting Crazy

BigBrother-84
In reply to this post by Powerman5K
Powerman5K wrote
After work, it read half a tank the entire ride home then as I turned on my street it shot up to full.
My dream!
A Bullnose fuel tank that magically fills up while driving!
Jeff / 1984 F350 Crew Cab 4x4/5.8L w351 4V/ T18/ D50 4.10 front/ 8' bed.
Restored 2019-2022.
Nicknamed «Big Brother 1984», due to its soooo-looong shape & nod to George Orwell's 1984 famous novel.
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Re: Fuel Gauge Acting Crazy

mat in tn
amen
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Re: Fuel Gauge Acting Crazy

mat in tn
In reply to this post by Gary Lewis
i have had even new replacements do this. double check all grounds first. one seemingly strange symptom is if it acts up more on very humid or rainy days. corrosion holds moisture and at times cam work better once dried out. clean the grounds first I have also found it helpful to add a ground strap to the tank. one with the barbed s clip and go to the frame nearest. please don't drill into the tank to run a bolt. this is more for the young guys reading along but hey we see some crazy stuff done to old trucks.
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Re: Fuel Gauge Acting Crazy

81-F150-Explorer
In reply to this post by BigBrother-84
Truck: 1981 F-150 Explorer / Engine: 300-6 California MCU Feedback System / Trans: T-18 - 4 speed / 2.75 Ford Rear Axle Open Diff.
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Re: Fuel Gauge Acting Crazy

Powerman5K
In reply to this post by mat in tn
I'm going to test the grounds this weekend as Gary mentioned. Then go from there. If the sender is truly bad, I'm going to buy the best one money can buy to replace it! I hate dropping the tank! I just replaced it last May!
1980 F100 Custom. Inline 6. Power steering. 3 on the Column.
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Re: Fuel Gauge Acting Crazy

Machspeed
Administrator
In reply to this post by mat in tn
mat in tn wrote
i have had even new replacements do this. double check all grounds first. one seemingly strange symptom is if it acts up more on very humid or rainy days. corrosion holds moisture and at times cam work better once dried out. clean the grounds first I have also found it helpful to add a ground strap to the tank. one with the barbed s clip and go to the frame nearest. please don't drill into the tank to run a bolt. this is more for the young guys reading along but hey we see some crazy stuff done to old trucks.
I've been dealing with a dead fuel gauge for several years now. I put new sending units in the tanks about five years or so ago. Checked them out before I put the bed back on and they seemed to function. After putting the bed back on, noted they no longer worked.....ARGHHHH!!!

Interestingly, my gauge seems to work in rainy weather. I'm sure it's some sort of a groud problem which I think requires dropping the tank or pulling the bed, niether of which I want to do.
John

"Blackie" - 1986 F150 4x4 - Mildly warmed over 351W HO - Original owner
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Re: Fuel Gauge Acting Crazy

ArdWrknTrk
Administrator
John does your tank selector valve work?

The senders ground at G701(the main ground near the radio)

Both senders come together at Splice 154, which also takes the lights from the rear.
And then join again at a major Splice (155)

I'd think if either of these splices were bad you'd have other noticable issues. So if it's both senders that are out I'd think they are individually damaged, or it's the feed wire between the instrument cluster and selector that's gone wonky.






 Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.
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Re: Fuel Gauge Acting Crazy

Powerman5K
In reply to this post by Gary Lewis
Gary, can you describe this process a little more? I remove the positive and negative leads to the sender. Then run a ground to the frame. If the dash reads full, the sender is bad? I don't have to do anything to the power lead? Thanks!
1980 F100 Custom. Inline 6. Power steering. 3 on the Column.
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Re: Fuel Gauge Acting Crazy

Gary Lewis
Administrator
There should be only one wire to the sender, and if it is the rear tank it is the Y/LB wire.

As shown below the power comes from the fuse box in the upper left, through the resistance wire to the Instrument Voltage Regulator.  From there it goes through the fuel gauge and to the Fuel Tank Selector Switch if you have two tanks.  Assuming it is the rear tank it then goes on the Y/LB wire to the sender.

The sender is a variable resistor with the other end tied to ground.  At Full it is a 12 ohm resistor, and at Empty it is a 72 ohm resistor.  But if you pull the wire off the sender and connect it to ground the gauge should go to Full and maybe beyond in a few seconds.  (Don't hold it to ground for a long time as the wiring wasn't designed for that, but a few seconds is fine.)

If the gauge goes to Full or beyond then the wiring, gauge, and regulator are good and the problem is the sender.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

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Re: Fuel Gauge Acting Crazy

Powerman5K
Thanks! I will try this over the weekend. I only have one tank.
1980 F100 Custom. Inline 6. Power steering. 3 on the Column.
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Re: Fuel Gauge Acting Crazy

Machspeed
Administrator
In reply to this post by ArdWrknTrk
Thanks for responding, Jim! And Powerman, sorry to intrude on your post but maybe we can all learn together.

Jim, my switching valve works, and I can switch from one tank to the other. In fact, that is how I keep from running out of gas completely, as I just switch tanks and then go fill the empty tank. I have no lighting issues, or for that matter, any other problems. The ground you speak of behind the radio is good, I inspected it and even cleaned it up last year when I had the dash all apart. It made no difference.

Five or more years ago, my fuel gauge quit working, got nothing from either tank. As I was doing some body work to the truck, I pulled the bed and replaced both sending units. It's been a while, but I recall connecting the electrical connector to the assembly and holding the unit outside of the tank and moving the float assembly while watching the fuel gauge through the back glass. As best as I can remember, I got no movement of the gauge.  I then took a long wire and grounded the sending unit to the frame, moving the float assembly and noting movement of the gauge indicator. I assumed that the assembly needed to touch the tank to make ground. I put it all back together, bed back on only to note later that the gauge did not work.

Strange thing..... I have noted gauge movement in wet weather. Last time was last year after leaving our truck show and got caught in a down pour.        
John

"Blackie" - 1986 F150 4x4 - Mildly warmed over 351W HO - Original owner
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Re: Fuel Gauge Acting Crazy

81-F150-Explorer
Water is a conductor. Check to see if your frame grounds are good.

It sounds like the ground gets wet, and works until it dries out.  (From your description/symptom)

Machspeed wrote
. . .   Strange thing..... I have noted gauge movement in wet weather. Last time was last year after leaving our truck show and got caught in a down pour.
Truck: 1981 F-150 Explorer / Engine: 300-6 California MCU Feedback System / Trans: T-18 - 4 speed / 2.75 Ford Rear Axle Open Diff.
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Re: Fuel Gauge Acting Crazy

Gary Lewis
Administrator
My first '58 Chevy had intermittent wipers - when it rained they worked and when it was dry they didn't.  Never did find the problem, but I knew it was a ground.  However, at the time I didn't really understand about grounds and GROUNDS.  Now I know that a connection can carry enough current to fool a volt/ohm meter (this was before DVM's, which are even more easily fooled), but not enough to run a motor.  So I should have run a voltage-drop test.
Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

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Re: Fuel Gauge Acting Crazy

Machspeed
Administrator
In reply to this post by 81-F150-Explorer
Ohhhh, I know it's a ground somewhere.....but where? I have no other electrical issues and the ground behind the radio is good. Thoughts?
John

"Blackie" - 1986 F150 4x4 - Mildly warmed over 351W HO - Original owner
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Re: Fuel Gauge Acting Crazy

ArdWrknTrk
Administrator
Perhaps the tabs at Connector 208A, circuit 29 or the switch itself?
What else is common to both tanks and exclusive of all other circuits?

'bad ground' doesn't make sense, when everything else that joins the ground circuit at the same spot/splice is fully functional.

... and back to your scheduled program.
 Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.
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Re: Fuel Gauge Acting Crazy

81-F150-Explorer
The common ground on the 1986 is G-701. Since all those circuits work except for the gauges it has to be upstream from that eyelet connection.

Easy way to check if this ground is good is to remove the fuel sender wire at the tank and ground the wire in the connector. If the gauge pegs to full, it's not the gauge or ground.  If it does not peg, and the gauge is good. . .

Splice S-154 would be suspect. Check for corrosion, loose, frayed, wires at this splice. Missing insulation etc. Located near the Front Fuel Gauge sender.  You may have to remove the insulating tape etc. . .

Note: Not all circuits that use S-154 may be malfunctioning for it to have a bad connection on one circuit.
 



If not found there, you will have to remove the frame rail harness, unwrap it, check for abrasions along the wires. Uses a multimeter set to ohms to check for continuity among the different wires / circuits etc.

Why I think it's a ground wire/splice or connection is that it tries to work when the truck is wet. Where does the wires get wet?  
Truck: 1981 F-150 Explorer / Engine: 300-6 California MCU Feedback System / Trans: T-18 - 4 speed / 2.75 Ford Rear Axle Open Diff.
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Re: Fuel Gauge Acting Crazy

Machspeed
Administrator
Yes, think I could probably find a fix for it if I pulled the bed, but just not interested in doing that right now.  Is it not interesting that I've seen the gauge function in the rain....LOL! This makes me think it is a ground related issue.
John

"Blackie" - 1986 F150 4x4 - Mildly warmed over 351W HO - Original owner
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Re: Fuel Gauge Acting Crazy

dionden
Hi John, I am sure you know this; have a dual tank harness off vehicle.
Found splice location S154 on the main harness about 1 to 2 inches towards the front of the vehicle from the junction of the front tank/main harness connection. "It had a repair"
It can be accessed through the opening of the front tank and frame by the fuel filler hose from below.
I have a cheesecake frame, so everything is accessible!!!

1981 F150 4x2 C6 Flareside 302
1986 F150 4x2 Flareside T170RTS 302 Efi


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