Trying to help my son fix his truck !

Its a 1988 f250 w/duel tanks, fuel gauge stays passed full regardless of
which tank is selected. Truck runs great, selector switch works. I did
find 1 wire going to the front tank that was chaffed but it wasn't broken, fixed that but still have same problem ?? Any help ?

 

Probably the fuel sending unit? I have the same problem with my front tank. If you do a search on here there is a way to test the wiring to the sending unit to make sure it's not just bad wiring? From what I have gathered on here is a common problem with these trucks. Hope this helps.

 

The truck is new to us so I don't know the total history. I was thinking the
same thing but I didn't understand why the gauge would peg past full on both tanks ? what's the odds of both sending units being bad ?
I'll try to find the thread for testing...
Thanks for the comments,

 

This diagram for a 1991 F-series should help you out:


Use a DVM to measure the sender resistance for each tank. An empty tank will measure ~22.5 ohms. A full tank will measure ~145 ohms. You can measure these at the fuel selector switch.

Notice there are two sets of contacts on that switch. One for pump power, another set for the sender. How did you verify your switch was "good"?

 

I do not have the resistance values off hand but the above post is backwards.
When the fuel level is high the resistance is low.
When the fuel level is low the resistance is high.

What rla2005 post is right for the trucks in the 1990's but wrong for the 1988 truck like the one towerwork is working on.

Sounds like you have a wire shorted to ground.

 

This will help, thanks for all the info.. trying to at least find the problem before todays race LOL !!!

 

So to check you need to measure resistance between the sender wire coming from the tank and ground at the fuel selector switch? If it is in close to the correct ohm range for the amount of gas in the tank the problem is between the switch and the gauge. If not it is either in the sending unit or wiring to the switch. Is that correct?

 

I found those values in my Ford service manual. Granted the diagram was for a 1992, but Ford is very well known to use the same circuits year after year.

 

All measurements are referenced to ground, correct. Your questions are correct. If it measures good running to the switch then you need to start tracing wires between the switch and gauge.

I have been in this circuit before. The values I found came straight from my Ford service manual. I feel very confident they are correct. I'll dig around to find a diagram closer to the model year of your truck.

Here is a digram that subford has posted before for a 1988 model year F-series truck:



It does not have the resistance values. You should notice the fuel sender circuit is exactly the same as the one I previously posted. The fuel pump circuit is a bit different but you are not chasing a fuel pump issue.....

 

The 1988 Ford service manual says what I posted above.
Also the 1988 fuel pump I put in yesterday works as the 1988 manual says.

We have been through this before about the measurements being opposite
between the 80's and the 90's on the fuel gauge.

From the 1988 F-series E&VTS manual:






This diagram above from the 1988 E&VTS manual is wrong and does not match the diagram in the the 1988 shop manuals.
The bottom of the resister in the fuel sender does not connect to ground as in the diagram above, it is open. Only the wiper goes to ground.

/

 

Also note if the wrong sender is installed you will get a full reading also.

 

I'm also having a similar issue on my 88 although my needle "dances" on on my front one and reads full up to a certain point then reads empty after the fuel level gets low enough although it won't read it accurately and my front tank always reads empty

 

_____________________
Welcome to the forum. Is your truck "New to You" ?
Do you have a volt / ohm test meter ?
Something like these:
https://www.fluke.com/en-us/products...al-multimeters

Hobo

 

I've had this truck for two years the gauges have never worked properly when I first bought it u thought there was no life to them but the dancing had me wrong I don't have any testers just a test light

 

Understood... We have all been there.
Let's see what we can come up with.
Where are you, what city ?

Here is some reading for you to do as far as testing:

https://www.ford-trucks.com/forums/1...auge-test.html
https://www.tanksinc.com/index.cfm/p...cat/cat159.htm
https://www.fordf150.net/forums/viewtopic.php?t=129658