Fuel gauge always full

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mellis18
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Fuel gauge always full

Post by mellis18 » Wed Apr 27, 2016 10:42 am

Hey pros...question about my fuel gauge. Right now the gauge reads full all the time. In fact it spikes all the way past the full mark. It was working fine before I laid the car up for the paint, body and interior work. I emptied the fuel tank while I was doing this so the tank sat empty for about a year. Would this have affected my sending unit? Can the sending be adjusted and/or cleaned? How hard is it to remove and replace? I like to do a repair as opposed to a $160 replacement if it is the sending unit.
Robert Ellis
Saratoga Springs, UT
1969 SRL311


1969 SRL311-07332
Matching numbers
1966 Mustang Coupe

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msampsel
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Re: Fuel gauge always full

Post by msampsel » Wed Apr 27, 2016 12:15 pm

If the resistor on the sender is shot or rusted then I doubt repair is worthwhile.
It should range from 10 - 84 ohms. Max current with 10 ohms. I notice the 69 - 70
wiring diagram does not list the ohm range.

Which IIRC max current is a full tank the ohms are minimum. You can measure the ohms at the sender to see what it is.
It is not too hard to remove, just remove the pannel in the trunk and then tap the snap ring
I use a 3/8" punch to do this around
until it comes off. Once it is off then the checking of ohms for the up and down position is easy.

Take a picture of your wires before you detach them

Ring looks like this
2567.1.jpg
Here is a note to Jon Frampton (AKA caraddict). A noted guage expert on the ohms expectation.
83 empty and 16 full, bet yours reads less than 16 all the time.

viewtopic.php?f=3&t=23210&p=206943&hili ... ge#p206943

Wishing my fuel issues were just the gauge :cry: :)
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spyder
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Re: Fuel gauge always full

Post by spyder » Wed Apr 27, 2016 5:29 pm

Does the temp gauge operate normal? If so, I would look to see if the gas sender wire is pinched or shorted somewhere. Disconnect it from the sender abs see if the gauge goes back down. Make sure the wires are nor reversed, grounding the sender wire.

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mellis18
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Re: Fuel gauge always full

Post by mellis18 » Wed Apr 27, 2016 6:14 pm

spyder wrote:Does the temp gauge operate normal? If so, I would look to see if the gas sender wire is pinched or shorted somewhere. Disconnect it from the sender abs see if the gauge goes back down. Make sure the wires are nor reversed, grounding the sender wire.
Actually - now that you mention it - the temp gauge never got up to the normal level on my way home from Solvang. It was always registering really cool.

I just thought I had an amazing cooling system...maybe that's not the case. :oops:

Are the two related?? :smt102
Robert Ellis
Saratoga Springs, UT
1969 SRL311


1969 SRL311-07332
Matching numbers
1966 Mustang Coupe

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msampsel
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Re: Fuel gauge always full

Post by msampsel » Wed Apr 27, 2016 6:35 pm

mellis18 wrote:
spyder wrote:Does the temp gauge operate normal? If so, I would look to see if the gas sender wire is pinched or shorted somewhere. Disconnect it from the sender abs see if the gauge goes back down. Make sure the wires are nor reversed, grounding the sender wire.
Actually - now that you mention it - the temp gauge never got up to the normal level on my way home from Solvang. It was always registering really cool.

I just thought I had an amazing cooling system...maybe that's not the case. :oops:

Are the two related?? :smt102
The voltage regulator keeps a constant voltage
For both temp and fuel gauge.

The one with the red and yellow wire on the following diagram.
http://www.311s.org/PDFs/1968%20Datsun% ... 20rev1.pdf
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punkzdriver
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Re: Fuel gauge always full

Post by punkzdriver » Thu Apr 28, 2016 8:22 pm

I dont know if you tried this yet, but there are two wires that attach to the sending unit. If you have them reversed, the gauge will spike as you described...reverse the wires and see what happens. My bet is it fixes the problem.
Waiting for the bugs to be worked out...

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msampsel
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Re: Fuel gauge always full

Post by msampsel » Thu Apr 28, 2016 9:55 pm

Yes, if the wire which should see variable resistance from the sender is tied to a fixed 0 ohms (i.e. ground) then
the maximum current will flow through your gauge and a perpetual full tank is there.

You can measure the resistance of the sender while you try the switch. Might take of photo of the
way it should be if this fixes it.
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mellis18
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Re: Fuel gauge always full

Post by mellis18 » Sun May 08, 2016 7:31 pm

Okay...I got a used voltage reducer for the gauges and installed it. At first I thought I had it fixed...but alas...NO. On my drive to Morro Bay both gauges acted weird. At first they were just like before. The temp gauge registering "freezing" and the gas gauge registering "over" full. I kept watching them for movement but I didn't see any for awhile. All of a sudden I looked down and the temp gauge was a little beyond the half way mark and the gas gauge showed just under full. Then, as kept an eye on them, the temp gauge started to go down and the gas gauge started to go up. After a few minutes they went back to there usual positions..."freezing" and "over" full. As I was driving back home the gas gauge began acting even stranger. As I would slow for a light or stop sign and then come to a stop I noticed the gas gauge registering below full. As took off the gauge would rise again and end up "over" full. Are the gauges possessed!!! :smt077

Is the voltage reducer supposed to be grounded? By that I mean when you mount it under the dash does the screw securing it provide a ground? Right now mine is just hanging by the wires (the way the PO had it set up). I really want to get this fixed...especially the temp gauge. I can live with running out of gas but not with an overheated situation.
Robert Ellis
Saratoga Springs, UT
1969 SRL311


1969 SRL311-07332
Matching numbers
1966 Mustang Coupe

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notoptoy
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Re: Fuel gauge always full

Post by notoptoy » Sun May 08, 2016 9:48 pm

YES Grounding is VERY important. Previously I just cleaned my voltage regulator mount up and my gauges worked perfectly again. It should be securely screwed onto the metal part of the dash.
"When all else fails, force prevails!" Ummm, we're gonna need a bigger hammer here.

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msampsel
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Re: Fuel gauge always full

Post by msampsel » Mon May 09, 2016 6:53 am

If you get sick of Nissan made VREG here is a thread on making a more reliable solid state one.
Cost was about ten bucks. It has to be grounded too. I got the idea from Nissanman ... so far works fine for me.
Assumes the proper voltage for the 69 gauges is still 8 volts.

viewtopic.php?f=3&t=24789

My temp gauge is a mechanical 270 degree sweep. I race my car when it runs :oops:
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78 911 SC Targa (Severe Oversteer but still loved)

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mellis18
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Re: Fuel gauge always full

Post by mellis18 » Mon May 09, 2016 11:30 am

notoptoy wrote:YES Grounding is VERY important. Previously I just cleaned my voltage regulator mount up and my gauges worked perfectly again. It should be securely screwed onto the metal part of the dash.
Thanks - will do that this weekend.
Robert Ellis
Saratoga Springs, UT
1969 SRL311


1969 SRL311-07332
Matching numbers
1966 Mustang Coupe

Pierre
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Re: Fuel gauge always full

Post by Pierre » Wed Jun 26, 2019 5:59 am

mellis18 wrote:
Mon May 09, 2016 11:30 am
notoptoy wrote:YES Grounding is VERY important. Previously I just cleaned my voltage regulator mount up and my gauges worked perfectly again. It should be securely screwed onto the metal part of the dash.
Thanks - will do that this weekend.
Hello,

same problem here.

did you mange to solve the problem?

Pierre

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Re: Fuel gauge always full

Post by JT68 » Wed Jun 26, 2019 6:39 am

If the gauge is pegged full, the tank wire is likely shorted to ground somewhere, or the two sender wires are reversed. If you disconnect the wires at the sending unit, the gauge should read empty. If not, check the yellow wire for a short to ground.
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