Gas Gauge Dilemna

[fatherdoug]

I have developed a problem with one of my gas tank sending units.

Here's what I have:
'78 Ford, two gas tanks - midship plastic and an aft axle metal.
One gauge on cluster that is electrically switchable between both tanks.

Here's my problem:
The midship tank does not indicate fuel level. The last time the gauge for the midship tank worked, it was indicating 1/2 tank of fuel. This has been an intermittent problem for about a month now. Problem comes and goes, but now seems to be permanent. Gauge level reads below the 'E' marker on the gauge.

Here's what I have done.

1) Switched to aft axle tank. Gauge indication goes up to full, which is where it should be. Tank was filled the other night and has not been used.

2) Pulled connector for midship tank and grounded connector. Gauge goes up to past full.

3) Checked resistance across pins of sending unit with connector disconnected. Resistance is 77 ohms.
The resistance range is about 60 ohms to 120 ohms between full and empty?
So the resistance value is indicating 1/2 tank. Is all this correct.

I was assuming that the float was full of gas, and would thus read 120 ohms, but that is not the case. I'm confused because the resistance value of the sending unit is pretty close to correct for 1/2 tank of gas, which I know I have.

I don't want to have to order a sending unit, when all I need is a float.

 

[mm_0_mm]

I thought 70Ω was closer to normal empty reading and full was closer to around 10Ω.

 

[DNFXDLI]

Doug....I haven't looked, but do the manuals give a resistance range?

 

[DNFXDLI]

What mm_0_mm said is what should be

 

[m78]

I never tested mine but it did have a hole in the float. I soldered it and put it back in.

 

[fatherdoug]

Thank You, all. Duncan, I remember reading those values when I scanned the manuals, and now I can't find it! If 70 ohms is a reading corresponding to empty, then that makes sense. Looks like a titanic float needs to be replaced.

 

[fatherdoug]

Pulled the sending unit today, and the float was no longer a float. I found an old float from a previous sender that was in better shape, soldered it up and I am back in business.

I can't find the resistance values for the sending unit in the manual. Oh Well.

 

[DNFXDLI]

Pulled the sending unit today, and the float was no longer a float. I found an old float from a previous sender that was in better shape, soldered it up and I am back in business.

I can't find the resistance values for the sending unit in the manual. Oh Well.

I found them last night..or whenever the hell it was you asked..I wrote it down...and i can't find it.
I do take solace in the fact that I'm old and can remember stuff, but damned if I can remember where it was

 

[fatherdoug]

I found the values in the '79 electrical troubleshooting manual that you posted. FYI. Thank You.

 

[DNFXDLI]

I found the values in the '79 electrical troubleshooting manual that you posted. FYI. Thank You.

I always thought I was good for something..thanks!

 

[BuzzGun79]

just a quick question, is it normal for the dash fuel guage to fluctuate on the 79 F250? I notice it more at like 1/2 tank,coming to stops,and taking off from a stand still driving up or down inclined surfaces.Now i do not notice this in other vehicles that i drive or have had.my thinking is it is moving more than it should,sometimes 1/4 of a tank or more then it resets to level.but since it was not working,i dont know the characteristics of it,or age of the sending units.Grant it i know fuel is going to move around in the tank.it seems to be readiing correctly when the vehicle is at rest,or driving on level roads.

 

[fatherdoug]

Mine flucutuates about one needle-width either way, but it depends on whether or not you are on a grade, Driving up and down hills, I see some difference also.

These old tanks have no baffles in them to keep them from sloshing.

Also, are the other gauges moving around? If they are, that is indication of either the voltage regulator on the cluster printed circuit board(5 volts) or the voltage regulator for the truck.

 

[Mil1ion]

Okay so I'm late getting here but

How this system works

The fuel sending unit receives a 5 Volt pulse and if grounded securely the level of fuel will show up on the gauge.
If you have a moving gauge needle then you have an issue with the ICVR weakening or the pulse needs to be cranked up a bit

Your issue IMO is the sending unit no longer has a secure ground thus the 5 volt signal feedback doesn't register on the gauge or the float is full of fuel

 

[BuzzGun79]

Doug I have the idiot light cluster from a doner vehicle here that worked, with the temp guage which i dont use.I went to aftermarket guages for oil,temp,volts.the charging system has been completley replaced with rebuilt alt..new voltage regulator.along with a very good used cab wiring harness.the volt meter shows just shy of 14 volts,i do notice it will flicker just a little when turn signals are in use for whatever reason.I have had no discharging issues once the harness was replaced.the truck sits most of the week untouched till the weekends for the most part.I have recently started to drive the vehicle long distances even at night with great reliabilty,no prolblem there.
Dennis...I have cleaned the chassis ground for the guage.i will have to take volt readings to double check to your input..but i am suspecting the sender or float as well.from what ive determined of this trucks past it has sat for a long period of time in the hot Arizona sun and heat for it has had a negative impact in wiring and some of the rigs components as well that i recieved. Thanks for the information!