Fuel gauge won't read below 1/2 tank, electrical or mechanical issue?

BRetty

Full Access Member
Joined
Jul 16, 2020
Posts
443
Reaction score
911
Location
Los Angeles
First Name
Franklin
Truck Year
1973
Truck Model
c10
Engine Size
350
All,

My fuel gauge has been wonky since I got my 73 C10. (A 73 that actually has an 81 dash and engine/trans)

It reads correctly above 5/8 of a tank up to full. Below 5/8, it just sticks at 5/8. I've run out of gas a few times, mostly because I was in denial about my gas mileage being ~5mpg.

So I replaced the sending unit a few months ago. I checked the new unit before I installed it, it read 0-89 ohms as I moved the float, just like it should.

So I installed it, but I actually had about half a tank at the time, so it just went to 9/16 this time. We had cut out a random loop of extra wire in the sending line.

Same problem. Tonight I finally got the bed moved back and checked it at the tank. It's reading 47-48 ohms, despite I think I am below 1/4 tank.

Is it possible there is a blockage in the tank that is holding up the float?

I will run a ground right from the battery as a last check, but next I guess I open the tank and peer inside.

BR
 

Vbb199

B-rate Hillbilly Customs
Joined
Jan 12, 2018
Posts
8,778
Reaction score
14,457
Location
Salisbury NC
First Name
Vince
Truck Year
89, 79
Truck Model
89 Suburban R1500, 79 C10
Engine Size
350, 502
Wrong tank/sending unit combo?
As in wrong gallons to wrong sending unit.


If you checked and got 0-90 range prior, id think elsewhere.....

The same 0-90 range is used until like, the 90s i think
 

75gmck25

Full Access Member
Joined
Oct 13, 2016
Posts
1,640
Reaction score
1,241
Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
I’m using Autozone sending units in my ‘75 GMC and they seem to work fine with the stock gauge. However, they are not very linear as the fuel goes down in the tank.

I have learned that I can go from 1/4 tank to out of gas in a very short time. However, I’ve never measured the resistance as it drops, so I don’t know how low it goes.
 

BRetty

Full Access Member
Joined
Jul 16, 2020
Posts
443
Reaction score
911
Location
Los Angeles
First Name
Franklin
Truck Year
1973
Truck Model
c10
Engine Size
350
Thanks for the replies, I'm gonna take the sending unit out this aftn and check the travel on it.
 

BRetty

Full Access Member
Joined
Jul 16, 2020
Posts
443
Reaction score
911
Location
Los Angeles
First Name
Franklin
Truck Year
1973
Truck Model
c10
Engine Size
350
So I have found the problem I think. There's a vertical baffle in the tank right where the sender drops in. The float rests on it when level gets to ~half.

this pic shows the black float and the ridge in the tank:

You must be registered for see images attach

There's no way to rotate the sender, there are locking tabs and slots. I could try to cut out the baffle. Flipping around the float is unlikely it's a bent wire that would be destroyed before it could be modified.

Guess I'll look for a different sending unit that gas the float on the other side. This unit is a Spectre.

You must be registered for see images attach
 

Dleslie212

Full Access Member
Joined
Sep 2, 2019
Posts
58
Reaction score
15
Location
Phoenix
First Name
Dustin
Truck Year
1977
Truck Model
K10
Engine Size
5.7L 350
Back when I changed both my sending units, I believe they were sold separately for drivers side and passenger side. Maybe you got the wrong one?
 

Vbb199

B-rate Hillbilly Customs
Joined
Jan 12, 2018
Posts
8,778
Reaction score
14,457
Location
Salisbury NC
First Name
Vince
Truck Year
89, 79
Truck Model
89 Suburban R1500, 79 C10
Engine Size
350, 502
I believe that baffling indicates you have a 87 tank
 

BRetty

Full Access Member
Joined
Jul 16, 2020
Posts
443
Reaction score
911
Location
Los Angeles
First Name
Franklin
Truck Year
1973
Truck Model
c10
Engine Size
350
I believe that baffling indicates you have a 87 tank
Ah, that's entirely possible. Tank is PO replacement, I never deciphered the upside-down yellow paint marker notes on it from the parts yard. Might also be why my filler neck is squished and kinda choked.

I'll look for a sending unit for an 87. Thanks!
 

Albrigap

Junior Member
Joined
Jan 16, 2021
Posts
25
Reaction score
17
Location
South Dakota
First Name
Alan
Truck Year
1991
Truck Model
Suburban
Engine Size
350
Rarely, like never, is a fuel gauge problem in the gauge.
Virtually always the tank sender or occasionally a bad wire or ground.
 

Latest posts

Forum statistics

Threads
37,489
Posts
802,670
Members
27,491
Latest member
grimm106
Top