i have an 85 f 250 6.9 diesel truck with dual tanks. i swaped the bed out with another bed and put two new fuel tanks out of an 87 f 350 gas truck. the tanks were the same and the tops of the send units and wiring looked the same. i didn't pull the units out of the tanks. i can't get the guage to read fuel with 3/4 full on both tanks. the guage is reading empty. i replaced the guage and selector valve and sitll nothing. i have resistance at the sending units but no reading. i removed the old sending units out of the old diesel tanks and hooked them up to the donor truck with the gas motor and they read on the guage on the 87 truck. when i take the old sending units to the diesel truck and hang them under the truck the guage reads. please help me if you can. thanks

 

Sometime in the late 80's(looks like it may have been 87), they changed the gauges around.

The early units had the regulator on the instrument cluster, and with the fuel gauge, empty was 90 ohms and full was 10 ohms.

The later units did away with the voltage regulator in the instrument cluster, and they went with a different type of guage. I don't know the ohm values, but it is now reversed with high ohms being full and low ohms being empty.

If you have a ohmmeter, you can put it on the sending units and move the float up and down and compare the readings if you take one of the later model sending units out of one of the tanks. I am not sure what year they did this, so this test would make sure that's what the problem is.

 

thank you for your response. i checked the ohms and i can't remember what i came up with. i won't be able to check until monday. thanks again and any further help if this isn't the problem would be great. luke

 

On my '85 the tapers wore out on the sending units so they produce all sorts of inconsistent readings. I fixed the problem with a pocket notebook: In it I keep a record of mileage and fuel pumped. (This is a good idea anyway because sudden changes in fuel economy are an indication of other problems...)

This 'fix' only works as well as your odom does though, but a speedo cable is cheaper than the senders are.

 

I did a similar thing with a 92, and discovered the tank resistors are different than my 86. I at least was lucky enough to find that out before I had the tanks in, I checked it out first. 87 was the first of the new dash, so likely different senders.