Fuel gauge out of whack

Coelacanth

Coelacanth

Active Member
Joined
Jul 7, 2014
I imagine something like this has happened to one of you guys already...so, I'm driving the car on a late-summer spin. A day or two ago I put in just over a half-tank of gas. I'd been driving it quite a bit and keeping an eye on the fuel gauge, thinking "Gee, I'm getting REALLY good mileage...hmm" as the needle isn't dropping much down from half a tank. Every once in a while, it bounces back up to a half tank. When I park it that night, it's reading about an eighth-of-an-inch below half, but should be damn near empty by my estimation.

Not wanting to "break down" embarrassingly somewhere due to running out of gas, I go to the gas station and put in 50 bucks...it should probably read 3/4 of a tank or so. I thought fueling up would sort out the fuel gauge needle, but now the needle points way ABOVE "Full"! Not a hair or eighth-of-an-inch but as far right as it goes, I imagine. After driving for about half hour or so, it's still above "F".

I assume if the sending unit failed, it wouldn't work at all...any ideas what's going on?
 
My gauge goes past full and takes quite a while to really start moving. At 1/2 tank I'm looking for a gas station and usually find one by 1/4 tank. It rarely goes past 1/4 without getting filled up.
 
I "adjusted" the fuel gauge reading on a 79 trans am by pulling off needle after I filled the tank and put it a tad above full. Just a thought
 
Thanks for the comments, guys. I'm no stranger to the car, my father bought it new in '86 and I acquired it from him about 8 years ago...drove it many many times in all those years! The fuel gauge needle never did this before. It should've been close to empty but was reading just below half a tank. I'm used to the car's needles wobbling and being somewhat inaccurate at the best of times, but it's never been THAT far out.

That's a good tip, regan. I'll have to look into this.
 
Just went thru this...Gauge would read erratic. Seems the "PO" had put a new pump in, and a hotwire kit. {Original hanger}:rolleyes:
Put the snap on ground wire over the undercoating.(n) The plugs were dirty, had no di electric on them. In trunk ground was totally corroded, had bad ohm readings.
Dropped the tank.. Same shitty quality work there.. Hose clamps loose.
Tested the sender for continuity. Forget that. It was all over the place.
A new tank, RT ss sender/new pump is is in.
There is now a grnd from the ear on the tank, to the frame. The in trunk grnd is now clean.
Next stop, the dash gauge. {Should the issue still be present}.
I posted the issues I've found, so you may start looking at those locations.
Last off, Pete @ Kirban's has a new dash ribbon.. That may be the next item.
 
I am also having an issue with my fuel gauge staying way past full. I have replaced the factory gauge with another factory gauge and nothing changed. Today i removed the tank and sending unit. I cleaned all contacts best i could. Grounds seem good. had my friend turn key on and moved the float up and down by hand and still read way past full. Also unplugged everything and turned key forward and nothing. I cut a hole in the trunk to access the unit easier next time. Not sure what to do.

I do have an after market gas gauge and i was thinking or hooking it up right at the rear direct to purple wire and see if it reads any better. for testing purposes.

D
 
EVIL said:
I am also having an issue with my fuel gauge staying way past full. I have replaced the factory gauge with another factory gauge and nothing changed. Today i removed the tank and sending unit. I cleaned all contacts best i could. Grounds seem good. had my friend turn key on and moved the float up and down by hand and still read way past full. Also unplugged everything and turned key forward and nothing. I cut a hole in the trunk to access the unit easier next time. Not sure what to do.

I do have an after market gas gauge and i was thinking or hooking it up right at the rear direct to purple wire and see if it reads any better. for testing purposes.

D
Click to expand...
AIRC, a continuous full reading can be caused by the signal wire being shorted in the harness.
Have you checked the harness for "issues"?
When you say the gnds seem good, did you actually ohm them out, clean and add some di electric grease? I use electronic component cleaner in a spray bomb, to help clean.
Any extra grounds/ body to frame, etc? Engine to body grnd at firewall clean and tight?
Same for the small screw grnd in the trunk? Clean that plug, too?
 
I did not ohm the grounds.
I will do that though.
I did think of replacing the signal wire. Figured I'd test the sending unit by getting the feed right from it with at the tank first.
It's enough to drive a man crazy.

thanks
D
 
Another common problem is a digital dash sender used on a analog gauge the analog is 90 ohms and the digital is 120. This will cause a reading of full for a longer time and then a more rapid decent at 2/3 tank and below.
 
Analog sender: 90 ohms FULL, zero ohms (grounded): EMPTY
Digital sender: 120 ohms FULL, zero ohms (grounded): EMPTY

The resistor wire senders are very inefficient and the accuracy doesn't even exist.

So, if you place a jumper between ground and the center pin of the weatherpack at your gas tank, you should read EMPTY.

Use a 47 ohm resistor in place of that jumper and you should read half a tank (analog).
Use a 56 ohm resistor in place of it for the digital car, and you'll read somewhere around 8 gallons.

You can use an analog sender for the digital car, but when full, the digital gauge will show around 14 gallons instead of "F". As you consume fuel, the digital display becomes more accurate.

You should make it a practice to keep no LESS than half a tank of fuel at any given time - lower than half, the fuel pump runs hot and wears out prematurely. It's the gasoline that keeps the fuel pump cool, and in top operating condition. (corrected)
 
Last edited:
The original GM sending unit was a piece of crap when new, and after 30 years why would anyone think it still works right?

These units which was probably cost $2 bucks when GM used them in the 60's-80's and were never accurate then, and worse now.

Chuck is really good at what he does, but like me, electrical stuff is not his strong suit, as NO ground in a GN should EVER be attached to the frame, it should go on a direct path back to the battery?

No way a rusty, low carbon steel frame made up of many pieces will conduct electrical current as well as a proper stranded, copper wire.

The body runs a close second to the frame as a bad ground.

Lights, horn, radio and other non-critical items can get away with a body ground, but important items like the fuel pump, spark and other functioning parts need a REAL battery ground!
 
Their all wacked out, when the needle hits half that's My "E" as in empty.
 
Nick Micale said:
Chuck is really good at what he does, but like me, electrical stuff is not his strong suit,
Click to expand...
No, but I have a lot of really smart friends!:smuggrin:

"There is now a grnd from the ear on the tank, to the frame. The in trunk grnd is now clean."
FWIW, I ohmed the grounds, and checked the + and neg sides for volt drop...All's well.
One of my really smart friends told me how to do this.:smug:
 
Can you have you friend come to MA for me? If he can fix it I'll give him a years supply of ham.

D
 
I use a simply fix. Set the trip odometer after each fill up. Run 200 miles and assume you are out of gas. Time to fill up. Works all the time and costs nothing and never have run out of gas in twelve years.
 
BRAD_PADGETT said:
I use a simply fix. Set the trip odometer after each fill up. Run 200 miles and assume you are out of gas. Time to fill up. Works all the time and costs nothing and never have run out of gas in twelve years.
Click to expand...

This is what I do, but with e85, I try to keep it around 150 miles.

I've had sending unit out twice, a brand new sending unit with the racetronix DP set up, cleaned/added grounds, cleaned contacts at the back of the gauge and it still has a mind of its own
 
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