Rear Tank 1985 Diesel

Although they taught us kids all this stuff in 5th Grade science, I never did quite understand this ohms business --- I could rewire Wolf Creek Dam and it work as it should; but, this ohms business was always a stumbling block.

According to Gas Tank Depot, the correct sending unit for a 1985 DIESEL rear tank:

Ohm range: Full 85-10.5, Empty 63-85

Does this mean that FULL could read anywhere between 85 and 10.5 ohms = seems like a pretty broad range to me.

NOTE: That is NOT a typo on my part; when I compared what they say for the Front Tank, it says 8.5-10.5, not 85-10.5; now, which is the typo on their part ?

And then, for EMPTY, the size of the numbers are reversed, 63-85, for reasons unbeknownst to me.

My sending unit, in my hand, with all contact surfaces clean and shiny and the little wiper bent into good strong contact,

Digital Multimeter set at 200 ohms


Float arm all the way up at FULL position = 89.9 ohms


Float arm all the way down at EMPTY position = 161.2 ohms


"Bench" tested in truck and seems to be working as expected.

Swing the arm up to the top and the dash gauge climbs to FULL.

Swing it to the bottom and it goes to EMPTY.

However, the dash gauge is very slow to respond to changes = that may just be how it is.

It seems there is quite a bit of discrepancy between my numbers and the numbers provided by Gas Tank Depot; yet, the dash gauge seems to be in agreement with float arm positions.

Why the difference between my numbers and the provided range ?

 

Gas Tank Depot data is wrong. Find another source of information for this issue.

This site says about 10 ohms full and 73 ohms empty
https://www.fordification.com/tech/fuel-sending-units.htm#:~:text=The%20fuel%20tank%20sending%20un its,and%2070%2D73%20ohms%20empty.

I don't know why your gauge is working with very different resistance values. Make sure you are properly measuring the resistance of the float resistor. See the photos is the attached link.

 

Thanks - this has been discussed here several times; but, when I try to search for those threads, I just get sent down rabbit holes.

 

See my updated post. Focus on your measurement technique. But it seems your float is working so decide how much work you want to do to determine where the mistake is.

 

Thanks; I will read through all that.

I had to quote your post and then remove the period you put after the link, then copy/paste it minus the period to get it to work.

The period was behaving as if it were part of the link and giving a wrong address.

 

I would not worry about the resistance values you measured. No idea why they seem so far off. However, the final determination is whether the gauge correctly follows the input from the float arm. And in your case, you said it does.

Don't worry about the delayed response. This is intentional, to help limit variations as the fuel sloshes around the tank. I have an old Jeep with a quick-reading fuel gauge. The needle bounces around like crazy when driving. To get an accurate reading, you have to stop on a level surface and let the fuel stop sloshing.

 

I hadn't really thought about that; I have driven vehicles like your Jeep where you would think the needle was going to beat itself to death.

 

My 66 Bronco with aftermarket VDO gauge and sending unit is like that too.
Supposedly you can add a resistor and a capacitor to the sender circuit to slow down the gauge readings but I have never found the time to do so.

 

Just to clarify, the OP is working on a 1985 model and your link is for 69-72 models . Does that make any difference or was Ford still using the same technology 13+ years later?

 

Good catch. I searched for model year specific info but obviously did not read the web page completely.

 

Same basic details, with the 10-73 ohm range at the sender. See page 99 here:

https://www.garysgaragemahal.com/1985-evtm.html

 

They all worked the same on Fords, cars and trucks, from the late 50's till 1987. Then they changed it.

 

Thank you for the information and advice.

I had the sender out again today and retested and got more sensible readings this time.

EMPTY = 72.6

FULL = 10.6

That is pretty close to what you posted above.

I am not sure why I got such wild readings before; however, I got to thinking on this and need to look back through my records to see just when the previous test was performed in relation to me discovering the sender wire broken where it connects to the sender itself.

If I did the test before finding the broken wire, and that wire just barely touching; then, that may/would explain my wild off-the-wall readings.

If I did the test after repairing the wire, then the reasons for my wild readings may forever remain a mystery.

I will seek out those notes I made and post back my findings.


EDIT/UPDATE:
I found my notes.

Both events occurred on the same day; however, by reading my notes on how this played out, I had tested the Ohms readouts prior to discovering the broken wire; so, that wire just barely touching may very well have been cause for my wild readings.