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70 Javelin - Low Fuel Warning / Fuel Gauge Issue

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worm611 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote worm611 Quote  Post ReplyReply Direct Link To This Post Topic: 70 Javelin - Low Fuel Warning / Fuel Gauge Issue
    Posted: May/27/2023 at 10:55am
Hey all,

Been fighting with my fuel gauge, Low Fuel Warning Light is basically always on.  Read some other forum posts, but didn't exactly answer what I think I need.

Current state:
- Low Fuel Warning Light starts blinking about 2 seconds after Ignition is ON
- Fuel Gauge starts to come off of E, then drops down to below E shortly after ignition is turned ON, about the time the Low Fuel Warning Light starts to blink.

My original sending unit was bad, the float was full of fuel when I pulled it out.

Replaced the sending unit, and while I was at it I put a new fuel tank in it as well.  At the Yellow Wiring going to my fuel gauge and ground I get 22-24 Ohms, which should be about correct since put in 5ish gallons of fuel.

I don't fully understand how this Low Fuel Warning Relay works, is there a sensing circuit in the relay that measures the Ohms, and if it sees +/- 1/4 tank, which is about where I'm sitting now, opens the sending unit circuit?  It is a bit confusing to me.

I pulled the relay off the harness and the Fuel Gauge never moved, so I guess it needs to stay in the circuit, or is there a way to bypass it?  I was under the impression the Yellow Wire from the sending unit ran straight to the gauge but I guess that is not exactly true.

So basically, how does the relay internally function, and why is it so quick to react when I turn the ignition ON?  Its like it doesn't give it enough time for the gauge to read the proper level before going into "Warning"

I'm leaning towards it being a relay issue, and not a gauge issue, but I have no clue how long that old sending unit has been bad for, so maybe something got damaged.

Thank you,
Jeremy
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304-dude View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: May/27/2023 at 11:15am
The low fuel warning system bypasses the gauge from the printed circuit. The IVR volage input of the gauge has a plastic washer to insulate from conductivity, under the speed nut. The low fuel warning module power draws power from a pin near the IVR. There is a plug from the IVR to power the gauge. To remove low fuel warning system, unplug the wire from the printed circuit and remove the plug and plastic washer from gauge connection. The gauge should properly function without it. The modules wear out eventually.
71 Javelin SST body
390 69 crank, 70 block & heads
NASCAR SB2 rods & pistons
78 Jeep TH400 w/ 2.76 Low
50/50 Ford-AMC Suspension
79 F150 rear & 8.8 axles
Ford Racing 3.25 gears & 9" /w Detroit locker
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worm611 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote worm611 Quote  Post ReplyReply Direct Link To This Post Posted: May/27/2023 at 12:34pm
And that IVR is the rectangular one with 3 prongs next to the Temp Gauge and Fuel Gauge, I assume?  Which I did test with my fluke meter and seems to be working ok.... I can set Min/Max and log it.  I even pulled it out, cleaned up the tabs which looked good, circuit board looks good.  I can't say I noticed any plastic insert, or anything out of the ordinary, but can't say I was looking for this little insert either.  

I will check tomorrow, greatly appreciate the info!
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Post Options Post Options   Thanks (1) Thanks(1)   Quote rodhhrod Quote  Post ReplyReply Direct Link To This Post Posted: May/28/2023 at 12:47pm
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worm611 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote worm611 Quote  Post ReplyReply Direct Link To This Post Posted: May/29/2023 at 9:09am
So the plastic washer, is part of the bulb holder for the Low Fuel Warning light, I guess that is what had me confused.

So basically what I did, was pull the green wire off the pin as you suggested, and then also removed the bulb holder (washer), now I am reading half a tank, which is probably a little high, but at least I am reading something!

So, what is the actual issue?  The low fuel warning relay I assume?

Thanks again!

Jeremy
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