Hello gentlemen and ladies. I'm new here and this is my second F-Series truck I've ever owned. My first being a 1989 F150 XLT 4x2 Short Box Dual Tank with a 4.9L 6 and 5 speed manual. My current one that I jus' purchased is a 1988 F150 XLT Lariat 4x2 Long Box Dual Tanks with a 5.0L 302 V8 and 5 speed manual.

My truck has a brand new Fuel Selector Valve and Front Tank Fuel Pump and screen. My problem is my fuel gauge says I am Empty on the front tank but flip the switch and it says that the back tank is Full (which it isn't). I know I need to replace the rear tank's fuel pump. Is this a ground issue, connector, or possibly a fuel sending unit problem? Any advice would be greatly appreciated.

 

Well, thanx for nothing guys. I was tryin' to figure out this issue before contacting someone and shelling out big $$$$$ on something that could be simple and something I can fix. As I said this is my 2nd Ford F150 Truck. My first one was only 6 yrs old when I got it so I didn't have ANY issues with it as I was only the 2nd owner of it. Haynes and Chilton haven't been any help so far. I guess I'll start replacing parts till I correct the problem. Again, Thanx for nothing.

 

Sorry to see no one pony'ed up to help you out earlier. On the flip side, your bitterness will get you nowhere.

A fuel gauge that always reads empty is typically caused by a short in the sender circuit or a bad sender. A gauge the read full (and passed full) is caused by an open circuit.

This diagram from OldFuelInjection should help you out:

Tank empty should be ~22.5 Ohms. Tank full should be ~145 Ohms.

Invest in a Digital Volt (and Ohm) Meter (DVM). It will pay for itself in a short time. The wire colors in the diagram for the sender should be the same for your 1988. The diagram is for a 1991.

If you feel the need to reference an actual diagram for a 1988 click here: http://ww2.justanswer.com/uploads/ba...tor_wiring.JPG

 

rla2005, I appreciate very much the wiring diagram. I think I'm gonna have to see about replacing both sending units because I've gone through the fuses, wires, etc. I am not able to take the bed off the truck or drop the tanks and test these so I guess I'm gonna have to shell out the $$$$ to take it to someone who can replace both of them at once along with the rear pump. I apologize if I sounded bitter, but this is frustrating having to throw $10 to $20 in the tank to make sure I don't run out of gas going to work across town every two days or so. Again Thanx for the help!

 

Measure the resistance of the senders at the Selector Switch on the dash. That will tell you what is going on without ever loosening a bolt on the bed or tank straps. Could be something as simple as a faulty switch.......

 

On the front tank, you can reach up on top of the tank and unplug the
electrical connector. Then turn the ignition On. The gauge should read
full.

If it does, the most likely cause is the float has a pin hole leak
and is filled with gasoline. This keeps the float arm at the bottom of the tank
so the gauge shows empty. The float snaps on to a wire arm so it's easy
to replace.

Problem is they don't sell the float by itself. You could find a junk fuel pump
assembly and swap the float or find the pin hole and solder it closed.


For the rear tank if the connector was unplugged you would get a full tank
reading all the time. You can't access the connector plug on the rear tank
unless the tank is dropped down a couple of inches.

Most likely the resistive material the sender arm scrapes across
has worn away causing an open in the circuit. Only fix is to replace the
sender assembly. You could use the float out of the rear sender and use
it on your front tank sender if that's the problem with the front sender.

All you would need to purchase is a new rear sender assembly and
fuel pump.

 

clean the prongs on the sending unit

 

does the gauge read full or does it go over full? If it goes over there is an open in the sender circuit either the wiring to the sender or the sender itself

 

Mine does this too,where's the sending unit?
Just did some work to mine,installed a 5th wheel hitch,knew the rear tank leaked so we just removed the tank,unplugged the wiring.
I've tried to follow the wiring underneath and really don't see any problems?? But as the previos guy reported,mine reads way past full.I'm considering buying a regular gas gauge and hooking it up along with mechanical temp,oil,and amp gauges.where or how do I hook the gas gauge in to the existing wiring?
Thanks for any help here.
By the way,I'm new here and just bought this 1987 F250.

 

The sending unit is part of the fuel delivery module inside the tank. Your truck has a low pressure fuel pump and a sending unit inside each tank. Outside the tank(s) is a single or dual function reservoir that feeds into a single frame mounted high pressure pump.

 

I just fabricated some frame mounts for a 5th.wheel hitch,since i was told the rear tank leaked,we just removed it.Dropped the tank down and I cut the 2 wires coming out of the sender.A yellow one and a black ground.While we were under there,I found 2 more wires back close to the tank,that had already been cut,A red & green,twisted together(like a barber pole or candy cane)Those ran up to the motor area but didn't get a chance to follow them any further to see where they went,it was getting dark.The front tank wires,black and red I think ran into a 4 or 5 flat plug the was hooked into the tank selector or or whatever it is,on the frame rail right in front of the main tank.Since I cut the rear tank wires,what should I do with those?(yellow and black ones)
I'd like for my dash gauge to work but I'm not against putting in an aftermarket gauge,just not sure where to start.
By the way,thanks for your reply to me!

 

The rear sender uses a Yellow/Light Blue wires for the signal back to the dash gauge. The Black wire is ground for everything related to the tank pumps and sending units. The EVTM diagram above is for a 1991, I also had a link for a 1988 fuel system. Ford is well known to change color coding from year to year occasionally, perhaps your '87 is unique.

I am a little fuzzy on what you are trying to accomplish. Are you re-installing a rear tank or planning to run on the front only?

 

Run on front tank only.Basically just using the rig to tow our camper to a camping/ATV resort about 30 miles away and back home.
I'm pretty sure the rear tank only had just the 2 wires(yellow/black)black was tied into ground wires.
I just want to get the gas gauge working as I'm sure you know what a Pain in the butt it is not knowing how much fuel you have. Right now the gauge shows pegged or way past the full mark.I've pulled the fuel wires from the back tank,out of the plastic wire loom,up to the second tank wires and continued up to the motor when I ran out of daylight.I just rolled the yellow wire up and put a nylon tie around it till I know what to do with it. That yellow wire didn't wire into that flat connector that plugs into the (i'm assuming)fuel selector unit that sits in the frame rail right in front of the main tank.
Also as I mentioned,there were those other 2 wires(green/red) that were already cut that were back by the rear tank.I aslo pulled them from the plastic wire loom along with the yellow wire up to the engine.

 

According to the diagrams I have there is no yellow wire running to the rear tank. There is a Yellow/Light Blue. It is the signal wire back to the Dash switch then onto the gauge. I also do not see a green or red wires for the fuel pump circuit either.

Perhaps Ford has changed wire colors for 1987 and older trucks. Everything I have is '88 and newer. The wire colors are the same for that time span. I would suggest taking a look at the wire colors on the fuel selector switch to see if any of the wires you mentioned are the same. In reality if you are only using the front tank there is no need to do anything with the wires running to the rear tank except cap them off to make sure they do not ground out to the frame.

 

Sorry for the delay as I do appreciate your help here.It's been raining off and on the past couple days so I wasn't able to mess with the truck.
You're correct on the yellow wire w/blue stripe for the rear tank. I grounded both sending wires and it took the gauge back to empty on both wires.I've follwed the wires from the fuel selector up to the fender well but I'm not sure which wires plug in where before they go through the firewall. One plug is about 1 1/2 square ,there are 6 wires going into it from the wire loom coming from down below,white/red stripe,green and red on one side,yellow /white stripe,red and black on the other side.This plugs in and goes into the main wire loom(through the firewall)
Now,according to above post,here are the red and green wires twisted together,I mentioned,that were already cut back by the rear tank area.....these red and green twisted wires are the same ones going into this plug I'm talking about here.I'm curious what they were for?? I unplugged these 6 wires and it didn't seem to change anything.I thought they might have been for rear lights or something but everything worked,tail,blinkers,backup lights.
Next to this square plug on the fenderwell w/6 wires,is a flat,retangular plug with multiple wires from down below,perhaps the backup light switch,tailights ,etc. Are my fuel wires from the tank and or the fuel selector switch in this group or the other plug?
Hope I'm not confusing you!