Desperately in need of a truck ( half way though building our house, living onsite in an RV and my vehicle died ), I found a 1986 F-150, standard cab, 8 ft bed, 4 x 4, manual transmission, 300 six ..... even came with new tires and a working fisher plow .... $1000 ... awesome!!!!!!!!!!!

OK, so I patched the body back together, did a handful of small mechanical and electrical jobs, replaced the muffler ......

I have one annoying problem left .... the fuel gauge does not work.

I have looked through my Haynes book think I might not be looking at the right things ... I don't have a high pressure pump I don't think because I have a carburetor ( might have been a closed loop unit at one time but no wires going to it now and all the wires to the O2 sensors are gone ... even the air pump is gone). There is still a module on the distributor .... might be an engine control module somewhere but I doubt it since I am guessing it would not work with all the sensor wires cut ( oh ... and no vacuum lines ... all are capped off ... even the gas recirculation valve is missing, capped off ) I am thinking I should think about changing the distributor to an older one with a vacuum advance since I may have no advance at all now ???

First, is there a place I can pick up the wire going to the dash ( my book says it is yellow with a white stripe .... have not found it yet) so I can test if it is the gauge or the sender? I assume I can put 12 volts into the wire and the gauge should go to full?

If it is not the gauge then I guess I need to pull the tank to test the sending unit?

Since it runs, I must be getting power to the pump inside the tank.

Any advice would be greatly appreciated !!!!!!!!!!!!!!!!!!!!

Mike

 

I can't answer your questions, but I had the same problem in my F350. Instead of trying to figure out what's wrong with my 30 year old gauge, I just bought a universal sender/gauge. Took less than a hour to run

 

I don't think you have a pump in the tank. But whether or not it doesn't change the gauge. If you have an oil pressure and temp gauge and they work then the instrument voltage regulator works. So go to the tank and pull the wire that goes to the stud on the sending unit. I don't remember the color but there are only two - a ground and this one. Put a jumper on that wire to a good ground, climb in the cab, turn the key on, and watch the fuel gauge. If it goes up, and probably to Full, then the problem is the sending unit. If not, then there's an issue between the gauge and the sending unit.

 

Fantastic! I do have other gauges working so I will crawl under the truck today and ground the wires.

Is it a slip on connector or a stud with a nut and bolt or ??? on the sending unit?

Thanks ..... Mike

 

IIRC it is a stud on the sending unit and a slip-on connector on the wire. But the ground is a 1/4" blade style.

 

I needed to replace the muffler and while it was off, I could get to the top of the tank.

I jumped the two wires in the connector together and the fuel gauge went to full.

Checked the ohms of the sensor .... zero.

Time for a new sensor.

Thanks ...... Mike

 

That'll do it, for sure. Glad you found it. Btw, the resistance on the senfpder should be ~12 ohms at Full to 72 ohms at Empty.

 

I LOVE IT when a plan comes together...

 

Thanks so much for this post. I've had the same issue with my 86 XLT Lariat. My gauge won't move on one tank @ full & the other shows 1/2 a tank at full, then goes to empty after about 20 to 30 miles. My issue maybe different, but at least I have somewhere to start looking now. Thanks again.

 

Showing odd readings with various amounts of gas in the tank is a classic symptom of a failing sending unit. You should confirm that by grounding the wire to the sending unit as Mike did, but I'm sure that's the problem - at least on the one with odd readings. But I'm not sure if the tank that "won't move on one tank @ full" has readings other than at full, and if it has no readings at all it could be a wiring problem. Still, check it the same way.

 

On my truck (82) I had to twice had replace the brass float on the sending unit. it ends up leaking and filling with fuel and then becomes too heavy to float correctly resulting in an incorrect reading on the gauge. The gauge always reads that there is less fuel in the tank then there is, and never reads full.

The first time I bought the complete assembly and didn't realize it was just the float until I had the unit out. The second time I pulled the unit first and was able to buy jut the brass float.

Interestingly, it came (in Ford packaging) in a quantity of two - so now I have a spare for the next time.