Gas gauge problem/question

[SnowNeck]

The other day I replaced my rear mounted gas tank in my 85' went from a 25-31 or so, the depth of the tanks were about the same so I used the same sending unit. After it was all finished I started the truck up to notice the needle on the gas gauge was bout a nick past full at bout 180 degrees. Doesn't seem that it's moving at all either. Put some more gas in it and drove it, no movement, now is this a sending unit issue? Ground ? Or should I get a new gauge being that I went up in size.

[hatzie]

If the tank depth is the same then you'll be just fine with the same sender. The sender measures fuel level in inches from the bottom to the top not gallons of capacity.
The stock GM fuel sender and gauge are calibrated to not start dropping for around 1/8 of actual capacity to give the perception of better fuel mileage.

If you hooked up the sender ground from the tank ring to the frame then you probably have an expectation issue... your expectation is calibrated for the 25 gallon tank so you're expecting to see needle movement in 20-50 miles.

Since the depth is the same... 1" of float drop on the 31 gallon tank = more gallons than the old 25 gallon tank. The fuel gauge needle should behave noticeably different because it takes more miles to burn more gallons.
Assuming 13mpg and a 31 gallon tank you should see around 1/2 - 5/8 on the gauge once you drive 200-210 miles.
On the old 25 gallon tank again assuming 13mpg you would only go 160-170 miles on 1/2 tank...

[SnowNeck]

So I need to check my ground wire,and then go from there. The original tank was actually a 15-18 gallon tank that was replaced by the previous owner. The last owner moved it to the rear of the truck and then recently it started to leak. I then replaced the 25 gallon tank with the 31. I feel my gauge has always been off slightly. However, I'm going to be driving a little bit today so I'll see if it moves at all. Also I forgot to add, I filled the tank up and the gauge didn't move from where it was, at the 180 degrees position, to the right past full. That's why I'm worried that the sending unit may not actually be as far down in the tank as I thought it was.

[hatzie]

180 degrees... as in needle at roughly 3:00 on the face?

A 3:00 needle, if this is the symptom, usually means... in order of probability.
#1 the tank ground is not making a connection or is missing.
#2 the sender isn't plugged in.
#3 there's a break in the wiring between the sender and the gauge.

[SnowNeck]

Originally Posted by hatzie

180 degrees... as in needle at roughly 3:00 on the face?

A 3:00 needle, if this is the symptom, usually means... in order of probability.
#1 the tank ground is not making a connection or is missing.
#2 the sender isn't plugged in.
#3 there's a break in the wiring between the sender and the gauge.

Thank you very much for being so helpful. And yes the needle is at the 3:00 position exactly. On my day off tomorrow I'll go through those three things and see if I can figure it out. I highly appreciate all your help sir.

[motornut]

there are different depths for the the rear tank,at least what i had to order for the Jimmy
this is what I used to replace in the 79,mine was the smaller
not sure if the pickups have a "join" in the sender wire like the jimmy has an "extention" to the rear, along the frame,maybe off?