My same fuel gauge issue
 

Hey guys,

I finally got around to looking into my fuel gauge issue. It reads waaay past full and lays over fully to the right. It worked periodically a couple of years before it quit all together..now it just lays at the 2-3 O'clock position.

Using my volt meter I have power at the sending unit in the truck...also, I have power into the gauge and out of the gauge...Is it fair to say I need another gauge?

I took the gauge out of the pod and checked it well for good contact re-assembled and still the same result..when you plug it in it moves to the right from around full until it stops..Have I missed anything here?


Thanks!

Grounding the sending unit wire in the trunk, should make the guage go to empty.
Try it and see what happens, that'll really narrow things down.
Straight to ground want hurt it, same thing the sending unit is suppose to do at empty.

If it does bring the needle back down, sending units in trouble or tanks lost its ground.

Check it out, post back, and we'll go further.

Thanks QS,

My single ground wire from the sending unit is grounded well to the inside of the tail pan. I have triple checked it. It has a 100% clean metal to clean metal contact.

Jay, he means to ground the signal side. In other words, the 12v wire that goes to the sending unit in the tank. If that is grounded, it's the same as the sending unit reading 'empty'. The gauge should go all the way left.
When you take it off ground, that is, leave it disconnected from anything, it should peg full.
If those two things happen, your gauge is good and the sending unit in the tank is probably the culprit.

Greg,

Seems like that would just blow the fuse...

No, the gauge has enough resistance to prevent that. Remember the gauge windings are between the voltage source and the ground you're applying to the tank end of the circuit.
Basically, it's what the sending unit does at it's low fuel postion. It's just a variable resistor and when the tank is empty, the resistance is zero....Basically, a short to ground.
Second opinions welcome though! I may have forgotten something in my old age. :old:

PS- By the way, you can do the same check at the instrument panel but by doing it at the tank, you'll also be checking the wiring from the gauge to the tank.

Thanks Greg,

OK,

I grounded the power wire in the trunk(while plugged to the sending unit). Sure enough the gauge went over to empty
as soon as I touched the body. But, if grounded unplugged without the sending unit plugged in it does not go to full...grounded or not it just lays on full.

Now what?

BTW, Is there another ground besides the tail pan ground? Maybe one under the dash?

Greg,

I also just plugged in my spare fuel sending unit...Same result.. I even tested it down on the end for power(where the float arm connects to its body).It has power at the top and the bottom.The gauge still shows past full.

So if I have power into and out of the gauge....power all the way thru the sending unit..and a good ground at the tank(tail pan)..The gauge appears to work(plugged in)..whats left?

A dash ground maybe?

I'm not following you here Jay.
With the switch on, grounding the line that goes into the sending unit from the dash takes it to Empty as you said...That works. All you need to do after that is take that same wire off of the ground (just let it hang) and it should go to Full.
If it doesn't, I'd say your gauge is faulty.
By the gauge going to empty when you grounded it, that pretty much shows the wiring is good.

Sorry,

I am assuming that I leave the tail pan ground from the sending unit during this test..correct? I may have this wrong..

I am conducting your test at the plug that goes thru the body into the trunk. At that plug (while plugged together) if you ground it the gauge goes empty immediatly. However, if I unplug that same plug and ground the hot side back to the gauge I get the same full pegged result...not the empty movement.

Both with the switch on

My test ground or the tail pan ground? Either way I get the same ..Full

That's about it as I recall without having a print in front of me. If you have all that and the gauge is all over the place, I would suspect the gauge.
It's a single line series of circuit.

Simplified, it's kind of like-
12V-----(+ gauge -)----(^^+sending unit-^^)--->ground.

Varying the resistor adjusts the current flow through the series circuit, moving the gauge.

Only three or four things to go wrong if voltage is present;
Bad connection, Bad gauge, Bad resistor(sending unit), bad ground.

We were posting at the same time Jay.
Look at the "schematic" I drew. With switch on...BEFORE the gauge..Where it goes into the trunk sounds like a good spot...unplug the wire. Should be full. Ground the wire...Should be empty. You're taking the sending unit out of the circuit to test the gauge.

[QUOTE=Greg Reid;3609711...unplug the wire. Should be full. Ground the wire...Should be empty. You're taking the sending unit out of the circuit to test the gauge.[/QUOTE]


Its gotta be the gauge,

If I unplug the wire it shows full...if I ground it (unplugged) it stays full..only plugged in and grounded does it go empty.

I found one on ebay..should get it this week.


Thanks Greg!

Something is wrong here
 

If grounding the sending unit wire makes the guage go to empty, plugged or un-plugged wont make any difference, ground is ground.

Something is not getting checked right. Ground the tan wire going to the guage and what happens? No other ifs, ands, or buts.

The connector has nothing to do with this test, just dead ground on the wire TO the guage.

If the guage goes to empty, sending unit or tank ground is bad.

What you're describing is kind of stramge. You ground the wire while the circuit is complete and it goes empty but you ground it with the wire unhooked from the sending unit and it stays full?
I'm thinking it should have gone to empty either way.

Very strange indeed
 

Unless the guage IS bad, but then the first empty reading wouldn't have happened.
Unless another possibility, a broken wire in the ribbon running from front to back and wiggiling the harness/connector had it making and breaking contact. But it doesn't really sound like it though.

Think of the fuel guage as an ohm meter, thats only capable of reading 0/zero (dead ground) to 90 ohms. And 0 to 90 is all the sending unit can produce. 0=empty and
90=full.
With the connector unplugged, on the back/trunk side, you can test the sending unit with an ohm meter. Sending unit wire to ground should read 0 to 90 ohms if it's good.

You can perform the same test on the spare sending unit and move the float up and down to see the 0 to 90, if it's good. If it is, hook it to the car harness and see if the fuel guage works.

I just double checked the fuel gauge needle..

It was wedged against the gauge body (at 4 oclock).I had to get it free. So,it will go to empty now...plugged or unplugged.It being wedged on the unplugged test messed us up. So now we are looking at the sending unit or ground to it?


QS,

I just bought a basic multimeter..still need to figure out how to check ohms or resistance.Do I use both probes on the lead/plug I am checking in the resistance mode?

BTW, I checked the exact power to the sending unit and it is around 8.7 volts.

You use both probes to read across or through something, one probe on each end.

Get your spare sending unit......
Put one probe on the wire terminal and the other againt the mounting plate. If it's good you'll get a reading. Move the float up and down, down should read 0 ohms resistance, and up should read 90 ohms. Readings should be close anyway.

On the car, tan wire to tank and ground will be your touching points, with connector unplugged. With a bad sending unit or tank ground, meter should give an infinite reading. About the same as just holding both probes up in the air, not touching.

Touch your probe tips together to see what it's zero reading is. Most wont show a true zero, usually a .0? number.

Try checking some plug wires from end to end. You'll find that interesting.

Check the difference from contacts to base on a 1157 bulb. You can see the different resistance for bright/dim-brake/tail.

It'll take some trial and error to learn the quirps of the meter, but it's fun and can be very usefull.

PM or E a number if you have any questions.
Clay

Thanks Clay,

I tested some stuff...

an old plug wire gave me .23
touching the probes together gave me .03
a 20 amp fuse gave me .02
my spare sending unit .0L
the sending unit in the car gave me .0L
an 1156 bulb gave .06
an 1157 gave me 3.3 (on bottom) and 2.8 (side to bottom)

I guess both sending units are bad.Any other tests on the sending units?

I believe, sending unit should read 0 (empty) to 90 ohms (full).

We're exactly on the same page as far as how it works. I haven't looked at a fuel gauge in a while and don't remember the colors of wires, etc. but it's a very simple, one line series circuit. 12v, gauge, variable resistor {sending unit}, ground.

Keep going Jay. You're close. As Clay said, clip the ohmeter leads to the sending unit and move the lever up and down. You should read a variable resistance from 0 at the bottom to 90 at the top.
OL means it's out of range. I don't know what meter you have but I wouldn't think that 90 ohms would be out of range.

Greg,

I guess I am out of tests to try. I need to find a sending unit that works or get my 2 fixed. I just saw a few minutes ago where a NOS one sold on ebay for $280.00+

Any Ideas?

My meter is a Sperry Pocket pro. I have my spare sending unit here trying to get something but I just get the 0L.

Nothing to lose
 

Carefully take the spare sending unit apart. There's a couple of different places for failure. There's a spring loaded contact moved by the float arm and a wire wound rubbing block. Wires can't rub and twist but so long. You may be/maybe not able to solder something back together.

If worse comes to worse;
I've repaired several using parts from different sending units. Even created/fabricated a few for cars where there was no replacement availible.
Marinas carry universal (make it fit) sending units. You can silver solder the mechanism to your pickup tube. Just has to be a 0 to 90 ohm model.

FWIW anybody checked to see how much Robmc's new large tube units cost....?
Just bumped it to the top in street.
Clay

PROGRESS!

I took my spare sending units lower box apart and cleaned the contact points then reassembled it.. I also noticed I had power only to the pin on the body..The flat ribbon wire that goes from the body to the box had no power. So, I ran a jumper from the body to the box....and my spare now works. It will need new end fasteners somehow though between the ribbon wire. It even shows the correct level on my car gauge in testing.

We also now have 91.3 ohms at full and 6.1 ohms at empty.

I will pull my tank unit tomorrow and see if that is the same issue on that piece.

The second pic shows the area that power is lost to the pin (and below). No idea how I will duplicate this connection without damage. But luckily my in tank sending unit has NO rust at all on it. This spare was good for a guinea pig anyhow.


Thanks guys..I owe you both!!

Jay, looks like you've found the problem. I haven't had a fuel tank sending unit in my hand in years so I'm not sure how that ribbon is attached to the body. Looks kind of like it's captured under that stud with a type of ring terminal?
Anyway, no rust is a good thing if you're planning to solder a jumper in. Seems like a viable solution unless there's something special about that ribbon.
Quick Silver obviously has some experience here...

I would think that sending unit would be available from NAPA...Maybe even GM dealerships. They must have made nearly a million cars that use that same one?

If not, maybe you can find a clean used unit from another car in the classifieds. I wouldn't think there'd be any difference over quite a few years of mid '70s GM cars....At the very least, F body cars for '74.

I felt like I needed to repair the spare before I dropped my tank just in case. So, first I bead blasted the pin fastener on top of the body and on bottom. I checked it and no ohms. Then I took a pocket size screwdriver to clean out under the fastener. It had a bunch of rust between the fastener and the insulating rubber.I dug all the rust out then tested it...and it works now! It now has the same amount of ohms the jumper gave me.

I know its not 100% clean under the fastener(see pic) but disassembling that fastener would do a ton of damage to the rubber and the ribbon.Without a rebuild kit that I know of this should last awhile. Maybe I can get some wd40 under there as well.

Now I gotta drop the tank. Not looking forward to it.I will let you know what I find.

Thanks again!

Looks good Jay...Your call but it looks do-able as far as getting that off to clean the connection really good...but you're a better judge of that since you have it right in front of you.
Definitely the kind of job you only want to do once if you can help it.

Jay -
That looks like a normal spring fastener you should be able to find at Lowe's or Ace hardware in their hardware dept. You can probably pry that one off; if your worried about the rubber washer, that's probably available as well. Then you can clean all of it properly, and it'll all be like new.
cm

Thanks Chuck,

That may be what I do with my main unit. I pulled it out of the tank. Despite looking nearly new (compared to my spare) it has the exact same issue as my spare.

Hmmmmm..

OK,

Its back together...

I sucked out and poured back in exactly 6 gallons of gas and the gauge shows just below 1/4 tank. Not sure how accurate that is. When you turn the switch off the gauge goes up to a 1/4 tank.

I am pretty sure I have an 18 Gallon tank..I need to see how much it takes to get to 1/2 I guess.

FWIW
 

I recently had to go take new pics of my car for the insurance co.. It was in storage and out of gas. I carried 15 gallons to put in it. First 5 brought the guage up where yours is. Next 10 moved it all the way to full.

Mark,

That makes sense. All the searches I have done show it has a 15.4 or a 15.5 gallon tank.It took 3 or 4 gallons just to move the gauge for me.

I am going to get some more fuel tomorrow and see what it does. Its race gas so I gotta tote it..

What car do you have? Your in Ga? I have family in Menlo..

79 10th TA stick car, got the back up engine for the race car in it, right at 11.5 CR 400.
So know what you mean about fetching fuel.

In Lafayette, right up the road from Menlo. Been here about 25 years.

And a whole bunch of other Pontiac junk; fiebirds, TAs, big cars, some run some don't.
Enough stuff to build a couple of nice ones, if I was able. Knowing what I know about Pontiacs and barely being able to walk is bummer. I reckon I've done a little bit of everything with them in the past. Past 200 a couple of times, on the highway, scarry in a bird, but not really. 100 mph 1/8th cars, 130, 150 mph 1/4 cars. Done a bunch.

Next one, if I can, is going to be a 428 or 55 in a tunnel boat. Back flip might just sraighten me out 'NOT'.

Had a shop before I got busted up and had customers come from everwhere. Fixed anything but specialized in carburetor and electrical work.
Getting a computer and finding PYs been a blessing after being down 13 years.
Broke as hell but got it made as long as you don't talk to my Dr.s.

Glad yours is working
Clay

In the '74 MVMA spec. book, the fuel tank is a 20.2 gal. (US) refill capacity.

15 gal. brings the guage up to full in the 79, probably does take another 5 to fill the tank. The top 1/2 a tank last a while but when it drops below 1/2 you'ld better be close to a fuel supply. Guage to tank ratio being off has pretty much been true for every firebird I've ever had. The float arms length, arm travel stops, and tank shape are just not right to correct it. If the tanks were perfectly square, the guage tank ratio would probably be close.

I went and bought 10 gallons...

6 gallons show (a little less than)... 1/4 tank
11 gallons show ..........................1/2 tank
16 gallons show...........................3/4 tank

I guess its a 20 gallon tank.

Jay, I just checked page 8-25 in the '74 GM Service manual and it shows the F-Body fuel capacity as approximately 22 gallons. That would make 11 gallons exactly 1/2 tank.
Looks like your's is pretty accurate now.
I think most cars will show empty with about 3 gallons left in the tank for 'reserve'.

Clay and Greg,

Drove the car a little bit over the last couple of days and the gauge is slowly coming down so everything looks great. I wanted to thank you both again for staying with me and leading this blind squirrel to a nut.

Good deal Jay and glad we were able to help. I've gotten a bunch of good info on this site from a lot of good people including you.

Mods. This needs to be a sticky!

Yes! I have the same issue with mine, however slightly different.
 

Yes! I have the same issue with my 68's fuel sending unit.

I am pleased that I have resolved all of my electrical issues of the car. . . . . except for this one. Put in the cluster and the fuel gauge needle was near empty, then turned the key to the accessory position and the needle has disappeared past the Full indicator and has stayed there since.

My attempts:

• Pulled the feeder wire to the unit and made a loop off of the battery = good, quick and bright - no break in the wire.

• Media blasted the insides of the tank straps, related hardware and filed trunk support hangers for clean metal.

• Sanded sending unit's eyelet grounding spot at trunk.

• Used a screwdriver probe with an internal light to check the harness at the back of the car with ground to the bumper. It took about a second to light and the bulb was quite dim versus my other points of contact.

• Pulled the harness at the dashboard clip and probed there, still the same dimness. Not getting enough voltage?

• Polished the printed circuit and needle gauge screw threads at the back of the cluster with 0000 steel wool.

• Media blasted a spare 90 ohm sending unit, (resistance stamped into the armature cover), and have tested with an Ohm meter. . . the reading is everywhere! I have not swapped the current unit in the tank for this one yet.

Not sure what else it could be?

Thanks for the thoughts and insight
Alex

Pull the pinch fastener on bottom side of the sending unit off carefully with a small pocket screwdrive...slowly. You can reuse it. Under that is where your ohm's are losing contact and jumping around. Rust..oxidation and just crap build up under there. It will come out as a dust.. careful with the rubber washer as well.

Reassemble and check the ohms again and see if you did some good.

Good Luck.

Use 1qikta's jumper method to test sending unit (1st pic in post 26). Good thinking 1qikta!



Charles

I think I'd narrow down the search first.

Unplug the body/ribbon harness in the trunk.
(1) Ground the tan sending unit wire, going up front to the guage, and see if the guage drops back to empty.
(2) Check sending unit wire from tank to ground, with an ohm meter, and see if the sending unit is in the 0~90 ohm operating range.

With this info you'll know what direction to head in, up front or underneath.

FUEL SENDING Unit issue resolved!
 

Want to pass along a sincere thanks to those who provided their insight on the matter! Fuel sending unit issue resolved!

Details:
First, removed the push-on retainer cap and cleaned this along with the brass contact pin of the sending unit. Next, grounded the sending unit harness to the bumper, turned the key to the accessory position and within a ten second count, the needle reappeared and had rested at empty. Drop the tank, remove the existing sending unit and replaced with spare. Grind trunk supports for clean metal to make contact with topside of tank. Grind a bare metal spot for sending unit ground. Disconnect harness plug at trunk and turn key, needle began to rise back to full, shut off key, re-connect and test. . . .works!

Main task to consider before giving up or paying someone else to to this - clean all points of contact and remove all rust to ensure a good ground. This is, if not all, the majority of electrical problems this era of cars have. When I started, only a quarter of my electrical system worked. It was all there with no rewiring foolery. Went through and pulled all of the blade connectors and polished. As I progressed each section of the electrical was returning.

Hint:
Repressing retainer cap onto brass pin - used a small quarter inch drive ratchet socket on a screw driver

Don't it feel good?
Good deal.

Glad it's working, but......
 

Curious about all the media blasting, cleaning points of tank to body contact, and bare metal.

Tanks usually have something to insulate them from the body. Tar strips on top or something.

IMO Sending unit ring should have a ground wire to the body to complete the circuit. Not tank to body contact.

Just worried about rubbing, rust, and noises. Stuff that might lead to premature tank failure.

Just concerned a little bit

I agree with above, wholeheartedly.