fuel gauge problem
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Junior User
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fuel gauge problem
Ok, its a 79 f150, with 2 tanks. The fuel guage will come up slightly when the key is on or running but stops just short of the empty mark. At first I thought that it was a bad sending unit but I doubt that both front and rear sending units would be bad. Maybe they are? Changing the front/rear tank switch makes no difference with the gauge at all. Is this a common problem with these trucks and is there a specific place(s) I should look for problems? Thanks
Laughing Gas
Joined: Jul 2002
Posts: 843
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From: USA
There can be problems with the gounds, each sender has it's own. I found a bad lead from my rear tank, it had a bad case of corrosion. spliced in new wire and it worked great, for a while. I just replaced the sender and it's back on track.
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I have only had the truck for maybe a month. At first I thought that they were just empty. I have added 5 gallons to both tanks but the needle doesnt move above the E. I just find it hard to believe that both sending units are grounded out. How much power is the gauge itself supposed to get? I will the multimeter and test a few things.
Cargo Master
Joined: Sep 2005
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Originally Posted by 70_f100
I have only had the truck for maybe a month. At first I thought that they were just empty. I have added 5 gallons to both tanks but the needle doesnt move above the E. I just find it hard to believe that both sending units are grounded out. How much power is the gauge itself supposed to get? I will the multimeter and test a few things.
Laughing Gas
Joined: Mar 2004
Posts: 1,243
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From: Maple Ridge B.C. Canada
i have heard that fords use a 75 ohm unit,so full should read 70-75 ohms,empty should read around 0-2 ohms.to check the gauge you ground the wire (from the tank to the gauge) to the frame,(with the key on,) this should pin the gauge above the full mark.then remove the wire from the frame(grounded spot on the frame)and let the wire hang free (not touching any ground,)now the gauge should read below the empty mark.if this all works as explained,the gauge in the truck is ok.then you need to look at the sending unit and see what it reads.as i said,full it should read 70-75 ohms,empty 0-2 ohms.this is what ive read anyway,i once did this search and this is what i found out.if anyone else has any other info on this matter,please feel free to jump in and add your knowledge too.hope this helps you out.-billy
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Junior User
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They work after all. THe 5 gallons just wasnt enough to register on the gauge. $60 later, the front tank is full and the gauge works. I will have to get paid again before I try the rear tank.
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Cargo Master
Joined: Sep 2005
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Originally Posted by 70_f100
They work after all. THe 5 gallons just wasnt enough to register on the gauge. $60 later, the front tank is full and the gauge works. I will have to get paid again before I try the rear tank.
Posting Legend
Joined: Sep 2001
Posts: 30,255
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Sorry hotroddually, you must be thinking of a Cheby or something. Fords are the opposite. When FULL the resistance will be low, but not 0-2 ohms. A resistance that low would burn out the gauge. People can ground the sender wire momentarily for testing but only power up the system for a moment to avoid burning out the gauge. With the sending wire grounded the gauge will peg the meter to over FULL.
For calibration on a 78 fuel gage use:
-------------------------------------
33-ohm 5W resistor for FULL
100-ohm 2W resistor for 1/2 tank
270-ohm 1/2W resistor for Empty
This information comes directly from the factory service manual.
When testing a dual tank system and both tanks read the same the problem is probably in a "common" part of the circuit not in the individual tanks.
For calibration on a 78 fuel gage use:
-------------------------------------
33-ohm 5W resistor for FULL
100-ohm 2W resistor for 1/2 tank
270-ohm 1/2W resistor for Empty
This information comes directly from the factory service manual.
When testing a dual tank system and both tanks read the same the problem is probably in a "common" part of the circuit not in the individual tanks.
Laughing Gas
Joined: Mar 2004
Posts: 1,243
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From: Maple Ridge B.C. Canada
torque1st-thanks for clearing this up,as i said in my first reply, i recently searched this problem out on this site.i was only repeating that which i had read in the electrical forum,posted by a guy (fredrick),it seemed to be an experienced persons account of this problem,so i was passing it on. i did say for others to jump in ,and im glad you did.the specs are what i was originally trying to find,and you have helped me personally with that.i have wrote the info down, and now can pass it on to my son.(do these specs also work on an85 f250 4x4?) he is the one that has this problem in his 85 f250 4x4.i will say this,there is a wealth of info on this site,you just have to sift through it to get what you need.thanks again-billy
Posting Legend
Joined: Sep 2001
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The values may change slightly but the sender works the same way with low resistance reading FULL.
For an 85 E series Van I have a listing for the fuel tank sender to be 8 Ohms FULL and 85 Ohms EMPTY from the OEM electrical systems manual.
I have the following info for a Fuel tank sender on a 94 Ranger:
=================
The resistance of the sendor should be
15ohms on an empty tank and 160 full. You can test the gauge by using resistors.
-I don't know where the Ranger info came from or if it is accurate.
For an 85 E series Van I have a listing for the fuel tank sender to be 8 Ohms FULL and 85 Ohms EMPTY from the OEM electrical systems manual.
I have the following info for a Fuel tank sender on a 94 Ranger:
=================
The resistance of the sendor should be
15ohms on an empty tank and 160 full. You can test the gauge by using resistors.
-I don't know where the Ranger info came from or if it is accurate.