#21  
Old 03-29-2009, 06:46 PM
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Quote:
Originally Posted by "QUICK-SILVER" View Post
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?

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  #22  
Old 03-29-2009, 07:10 PM
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I believe, sending unit should read 0 (empty) to 90 ohms (full).

  #23  
Old 03-29-2009, 08:41 PM
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Originally Posted by "QUICK-SILVER" View Post
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.
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.

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Last edited by Greg Reid; 03-29-2009 at 08:51 PM.
  #24  
Old 03-29-2009, 09:00 PM
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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.

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Old 03-29-2009, 09:52 PM
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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


Last edited by "QUICK-SILVER"; 03-29-2009 at 09:58 PM.
  #26  
Old 03-29-2009, 10:25 PM
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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!!
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  #27  
Old 03-30-2009, 04:31 AM
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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.

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Last edited by Greg Reid; 03-30-2009 at 04:52 AM.
  #28  
Old 03-30-2009, 08:24 AM
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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!
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  #29  
Old 03-30-2009, 08:43 AM
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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.

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  #30  
Old 03-30-2009, 11:50 AM
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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

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  #31  
Old 03-30-2009, 12:31 PM
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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..
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Old 03-30-2009, 03:34 PM
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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.
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Old 03-30-2009, 04:46 PM
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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.

  #34  
Old 03-30-2009, 05:16 PM
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Quote:
Originally Posted by "QUICK-SILVER" View Post
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..

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Old 03-30-2009, 06:09 PM
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Quote:
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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

  #36  
Old 03-30-2009, 07:47 PM
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In the '74 MVMA spec. book, the fuel tank is a 20.2 gal. (US) refill capacity.

  #37  
Old 03-30-2009, 11:29 PM
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Quote:
Originally Posted by Held for Ransom View Post
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.

  #38  
Old 03-31-2009, 12:18 PM
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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.
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  #39  
Old 04-01-2009, 02:09 AM
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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'.

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  #40  
Old 04-01-2009, 06:52 PM
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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.

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