Fuel gauge problems 88 7.3 idi

[jhamil25]

I just bought this truck from my nephew. It's a 88 f250 7.3 idi and my fuel gauge doesn't work on either tank. When my nephew had it I paid $1800. for a shop down in festus, mo to fix it. They put in both sending units and said they had it tracked up to the dash. I pulled the switch today and rang it out, it was ok. The gauge was reading 3/4 of a tank. I unplugged the 6 port valve and checked for a reading going to the sending units but when I plugged it back in and checked the gauge it pegged past full. I can't afford too many more visits to the shop!
Can anyone give me some advice?

 

[79jasper]

The selector valve could be bad.
Can't buy new sending units for these trucks(IIRC), so not sure what the shop did there.


Sent from my iPhone using Tapatalk

 

[Clb]

Fwiw
My 88 does this .
The front is the biggest liar , will run 50 miles before reading a loss then in about 10 its at a 1/2!!
The rear reads closer to true! Both have 1/4 tank misreads I figure the rheostats are bad!?

 

[jhamil25]

OK, I checked the terminals on the valve. They checked out like the drawing said. If I were to short the gauge to the front tank sensor shouldn't I get the tank reading? On the valve terminal 1 was -12vdc, terminal 2 was +12vdc, terminal 3 went to the front tank, terminal 4 went to the gauge, and terminal 5 went to the rear tank. next I will do the following, posted by franklin2 on 02/14/2014
You can get to the front tank sender.

Switch the dash switch to the front.

Then go under and unplug the sender wiring plug.

Get a piece of scrap wire and find a good ground somewhere or if it's long enough, stick it on one of the battery neg posts, and run it back and underneath to the sender wiring plug you just unplugged.

Turn the key to run, but do not start the engine.

In the wiring plug find a darkblue/yellow wire. Touch your good ground wire to the darkblue/yellow in the plug, and watch the fuel guage.

The fuel gauge should swing full scale in the full or empty direction.

Then take the ground wire off and let the plug just hang in the air. If the fuel gauge swings the opposite way from when the ground wire was touching the darkblue/yellow, then your wiring, fuel tank selector valve, and the fuel gauge are working, you have a problem with the sending unit.

I'll post tomorrow on what I find

 

[icanfixall]

Welcome to the forum. As posted. These sending units are usually not correct reading most of the time. Also a note about running down past 1/4 tank. Most sending units have the suction shower head broken off so we run out of fuel at 1/4 tank. Thats the lowest we can draw fuel from the tanks if the black suction head is broken off. The easy repair is a piece of 3/8 fuel rated hose about 3 or 4 inches long clamped to the end of the broken suction line. On that 5/16 return line is a red flapper valve similar to whats found on a ski boat to automatically drain the bilge when your underway. Do not remove that valve. It keeps return fuel from siphoning back to the tanks.

 

[jhamil25]

Well talk about feeling stupid! I just bought this truck and drove it about 60 miles home. I noticed the fuel gauge not moving off 3/4 tank. My nephew told me he thought it was about empty. Well check this out!!! I pulled the 19 gallon tank to find about 15 gallon of fuel in it!! Let's do the math here, I'd say that was about 3/4 tank. The tank was full of pin holes so I bought a new tank and for $65 I bought a new sending unit. The back tank was leaking too so I went ahead and ordered a new rear tank too.
It looks like I might have this so called problem figured out. I did find some solder joints on the cvr that looked suspicious under my bench magnifier. so we soldered them again and cleaned the contact, Sooo I'll keep you informed on what happens. Thanks to icanfixall for your help. I really like this truck!

 

[ericwade381]

How did you make out?? My trucks doing this same thing

 

[Idiforlife]

Where are. You getting new sending unit

 

[IDIoit]

IIRC, LMC trucks had the front sending unit but not the rear, maybe thats reversed?

 

[jhamil25]

Well I used the same sending unit in both tanks. I know they test in reverse but it worked perfectly. I made sure the fuel pick up was near the bottom and before I installed it in the tank I had to adjust the float because it was set too high. It's been near perfect ever sense. Of course now that I say that, It will probably be screwed up when I go out this morning. Now that I think about it I think I pulled and used the old float and used it on the new sending unit. Also I cleaned the wiper on the old unit and I'm all most sure that's all I would have had to do. There was a film on the surface that kept it from reading. I could be wrong about all of this, but I think I'm right. Lots of luck! I ended up pulling the bed after I dropped both tanks a couple time!!