New Fuel Tank/Sending Unit

Yesterday I dropped my gas tank. Had a new tank and sending unit that's been sitting in boxes for a while now. Once it was all installed I put about 3.5 gallons of gas I had in the shop and put it in to get me to the gas station. When I was done and cranked the truck the gauge was still sitting at dead empty. Not gonna lie, I was super frustrated. I knew the wiring was correct because the electric fuel pump was working. Figured I'd go fill it up in case the float was stuck or not reading the small amount of gas I had in there or, worse-case scenario...run out the tank and pull it back out to re-check everything. Leaving the gas station the needle just started working it's way up real slow. By the time I got back in the shop at my house it was reading all the way full. Anyone else have issues with these sending units being finicky?

I replaced both pumps and sending units in mine. I had to adjust the float positions on both senders to have them read mostly right.  Mine both swing to almost full and are on E with 5 gallons left.

They're finicky and I got tired of dropping the tanks so close enough for me.

Ok thx bud, appreciate it!

Do you have a 5 pin fuel selector switch ?

If you do then on your truck, the sending unit's resistance value goes through the selector switch. If the selector switch has a high resistance it will make it look like the fuel tank is empty. After a while the switch may settle causing the sending unit's resistance to be sent through.

You can either replace that switch or open it up and fix it.

Here is my video that talks about it a bit
https://youtu.be/8RJJYuAFSgs?si=pUoFMYAg_U-jtZa6

This is not true.
The selector switch should have negligible resistance.
Resistance comes from the sender.

The gauges work by heating up a bimetallic spring, just like a choke cap.
This damps movement, preventing wild swings from gas sloshing around.in the tank.
Hysteresis is built into the gauge itself, because it can't heat or cool instantly from the tiny power it gets from the ICVR.

Edit: response, to a deleted post....

Yes the selector switch has negligible resistance but as the switch ages, it will develop resistance and that will prevent the sending unit resistance value to reach the gauge. In the picture below the link in the circle, when that has a high resistance it will completely overwhelm any sending unit resistance. Worse case if the switch is fully open, the sending unit is pretty much useless

Edit: I deleted because I wanted to double check the wiring diagram. I am sure of it now!!

My truck is 37 years old and I don't see that my switch makes the readings bad when I test with a meter.

But I have old senders and a new style cluster, so my gauges read backwards and I run out when they reach up to 5/8 on the dash.

Metermatch is useless for me.😖
I'd rather have some indication than none at all.

Down here in the dust bowl area, we have those switches full of junk when I open them up. I opened and fixed the 78 Bronco's tailgate switch the other day. It was reading about 80 ohms in the down position. No wonder the window wouldn't go down. After I wire brushed the internal contacts, everything was honky dory

I think in your 87 truck's switch, the sending unit is hooked up to the gauge through the selector valve right ? As long as the switch commands the selector valve properly it should be good.

I don't have Hot Fuel Handling, or an electric pump,  but I do believe that my 6-port valve switches senders.
I'll have to find my EVTM as it's supposed to continue raining at least into Sunday.

Are y’all talking about the tank selector switch? If so mine is a single tank.

I definitely am.
But evidently that's all in response to Chris having mentioned his fuel tanks sending units plural directly underneath your original post.

I apologize for the tangent, and should have known to more closely read the subject and not just correct the responses.

No worries!

I am sorry as well for assuming it was dual tanks... thats what started all this :) :)

But I am curious. Was your truck single tank from factory ?  Meaning your dash doesn't have a selector switch at all. Don't think I've seen a bullnose with single tank but I know they exist as the EVTM has wiring diagrams for single/dual tanks. If its single tank only, the sending unit is wired straight to gauge.

All good man. Yep…8’ bed with single tank.

heck I have three single tank trucks in the garage.

If equipped with single tank, I assume rear tank and what capacity ?

Rear tanks are all 19 gallon, except Bronco's.

all three of the single tank trucks are "midship", front, or side mounted. three terms of the same position. the two long beds have 19 gallon while the short bed has a 14/15 gallon depending on the listing of the maker. I can choke 15 in if I run it out. this is the reason why I took out the c6 and installed an aod . however, it has not proven to help much around town. I need a good road trip to do a good test

Wow those capacities were seriously low. Anything from 300 - 351 probably only got 150-200 mile range in the city.

That said, I'm sure it was very much acceptable for cars/trucks back then.

about correct on range. with the 300 I'm getting almost 14 but with 32/11.5/15s. that is with c6 and 3.08 gear. now it is 4.10 with overdrive and rpm is at least 400 less at 70 so I'm trying tuning, drive style to see how much better I can get. but I'm always driving different vehicles while my personal truck sits for weeks.