70 Fuel gage problems replaced sending unit - Pontiac GTO Forum
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post #1 of 9 (permalink) Old 06-14-2013, 01:04 PM Thread Starter
 
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70 Fuel gage problems replaced sending unit

My fuel gauge would show 1/2 full to full all the time.

I noticed the ground wire was broke off by the sending unit so I removed the tank and replaced the sending unit.

Now it always is on empty. It is on the E line plus a hair above. (like half on half off the line)

I don't drive the car often so I don't want to put a lot of gas in it.

Originally I put in 2-3 gallons of gas then put in 4 more gallons in it.
From an empty tank 5-7 gallons should show a little more on the gauge correct?

Besides the gauge what else could be wrong?

Any way to test the gauge?

Thanks
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post #2 of 9 (permalink) Old 06-14-2013, 01:48 PM
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Disconnecting the sending unit wire the gauge should show empty. Ground the wire and the gauge should show full. If it does then the problem is with the sending unit you just put it. If not then the wiring to/and or the gauge is bad.
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post #3 of 9 (permalink) Old 06-14-2013, 04:44 PM
 
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Garage
Im no expert..do a search of the forum on fuel gauge. Any how ive been changing out sending unitis like underwear on my 64. Fortunately on a 64 you can r&r the sending unit without dropping the tank. Anyway i tested resistance with an ohm meter from the tank to the fuse box. Next i unplugged the coupling from the rear harness under the dash just before the fuse box. T tested my new sending unit with jump wires under the dash befor install. In the end it seem my 64 is supposed to want a 0-30 ohm sending unit. They didnt work, after bench testing that they were good with the meter. So i bought a 100 ohm variable resistor for $4 at radio shack. Tested it with jump wires under the dash. My gauge liked that so i bought a 65 sending unit 0-90 ohms and all is well. I guess my gauge was rebuit with a 65 . Any way the point is you can test all day long without putting the unit in or dropping the tank just find the right wires and use a multimeter. Maybe thats obvious to the pros but i figured it out reading posts on the forum and i really appreciate the help.
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post #4 of 9 (permalink) Old 06-14-2013, 10:24 PM
 
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Have fun! I got tired of screwing with mine and bought an aftermarket fuel gauge and aint looked back!
Also 1970 with a replacement sending unit.
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post #5 of 9 (permalink) Old 06-15-2013, 04:40 AM Thread Starter
 
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Quote:
Originally Posted by Rukee View Post
Disconnecting the sending unit wire the gauge should show empty. Ground the wire and the gauge should show full. If it does then the problem is with the sending unit you just put it. If not then the wiring to/and or the gauge is bad.
Thanks,

I grounded the gauge wire and it did go to past full so it looks like it is working.
when I hooked up the wire again it now moved up a hair or two. Now it is barley touching the E.

I wonder if it is working, just with 5-6 gallons of gas maybe the first couple gallons don't register because it can't pick it up?

I didn't want to do it but looks like I need to put more in to see if it works.

Any one know of any cheap pumps that will siphon gas and get into the tank to pickup the gas?

Thanks

Last edited by michaelrj9; 06-15-2013 at 04:52 AM.
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post #6 of 9 (permalink) Old 06-15-2013, 10:09 AM
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5-6 gal should be about 1/4 tank. I think you have a bad sending unit. After you drain the tank and remove the sending unit you can hook up the wire and ground, move the arm on the sending unit and have someone verify that the gauge is responding.
I've used like an external boat tank bulb hooked up to the fuel line on the sending unit to drain fuel into a gas can.
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post #7 of 9 (permalink) Old 06-21-2013, 08:19 PM Thread Starter
 
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I am still having problems with this.

I replaced the new sending unit with another new sending unit and I am having the same problems. The gauge shows a hair above E. I put in 10+ gallons of gas.

@ Rukee
Before I replaced the 1st sending unit, I did what you suggested above and I disconnected the sending unit wire going to the gauge and it showed E then I grounded it to the body and then it went to F or a little past F. Maybe it was the opposite, grounding the wire goes to E and just having the wire disconnected it goes to F. Sorry I don't remember but all I know is it went from E to F doing one of those ways.

That is when I replaced the sending unit.
Then another one.

Now the tank is installed again with the 2nd sending unit & 10 gallons of gas. If I disconnect the sending unit wire the gauge goes to F.


So it seems like something near the sending unit again? The wire that attaches to the sending unit that attaches at the rear body harness I tested for continuity as much as possible by bending the wire around and didn't find any breaks.

So does anyone have any other suggestions?


If I need to drop the tank again I have a couple questions.
The tank isn't rusted but has a slimy film on the bottom. Is it safe to clean and rinse it with acetone and or soap and water as long as I make sure it is completely dry before using it?
Is that going to promote rust?
Can I use the water in the tank and test the sending unit or will that ruin the sending unit or not give me enough resistance (or too much) to test the sending unit?
My garage smells like it will go BOOM with the flick of the light switch

I was just wondering if the float is getting stuck at the bottom of the tank and would be easier to test out of the car with water if possible.
Or I read on a different site and a different car but he was having a problem with the metal ribbon going from the wire post to the resister was touching the tank itself on 2 new sending units.

Any ideas on what could be the problem? and is it OK to clean & rinse with water and use water to test the sending unit in the tank to help with troubleshooting this?


Thanks

Last edited by michaelrj9; 06-22-2013 at 08:24 AM.
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post #8 of 9 (permalink) Old 06-25-2013, 07:00 AM Thread Starter
 
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I'm trying to see if I can get the original sending unit to work but need some help.

Any suggestions on what to use for glue-sealant on the strainer-sock on the fuel sending unit that will be good being submerged in gas?

I have a new sock that was on a different unit.
It's the correct size it just is a little loose on the tube. The part of the sock that slides over the metal tube is some cheap tin-metal.

Thanks

Last edited by michaelrj9; 06-25-2013 at 07:16 AM.
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post #9 of 9 (permalink) Old 07-11-2013, 11:14 AM Thread Starter
 
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Here is an update just in case anyone has the same kind of problem this might also work for you.

I bought my sending units at Auto Zone because they were in stock and I wanted to get it done ASAP. The models were Spectra Premium/Fuel Tank Sending Unit FG91B. I bought 2 of them because the first one did not work

I found out they ended up hitting the expansion volume chamber inside the tank not letting the float go up.

So I ordered a exact reproduction of the sending unit and seems to be working. The ground wire was a little short though and had to change the grounding point on the car.
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