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Old November 10th, 2009, 06:02 PM
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Default Stupid Fuel Sending Unit

Welp, got the tank cleaned, painted and installed. I examined the sending unit and based on the looks I decided to reuse it. It was clean and the arm moved freely.
Anyway, after busting my lip from a falling tank bracket hanger and mashing my fingers with the ATV jack I got all the installation complete. I decided to move the truck out into the front yard and cleanup around the garage. When I turned on the switch I checked the fuel gauge and it read chock full, off scale full too. The kicker is there isn't a drop of fuel in it!!!!!!
Dang, sounds like the sender is bad to me. Any ideas?
My search skills suck so if anyone has a PN for the 24v fuel tank sender I'd really appreciate it.
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Old November 10th, 2009, 06:28 PM
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Default same here

Pulled my tank. Checked the sender. Found a broken wire that I might have broke removing the tank. Got it all back together and still reads full all the time. Porkchop said his was doing it and it was the gauge. Trying that next.
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Old November 10th, 2009, 06:46 PM
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If you do need a sender, I replaced mine with an aftermarket one (needed for the 12v gauges), and have the original sitting in my spare parts box. I believe it works well, but as I never had working gauges, don't know for sure - it reads 1 to 36 ohms with no gaps as I move the arm. I have no idea what it's worth, so the cost of shipping and a couple bucks for packing it and it's yours. Some rust on the top 4" or so.

Make sure the wiring harness isn't damaged, too. I think beyond full is the reading if there's an open circuit to the gauge, so damaged/disconnected wiring, bad ground, etc.

--Randy
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Old November 10th, 2009, 07:49 PM
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No ground to the sending unit will do the same thing (indicate full). Shorting the center wire to ground should show empty and ruin the terminal end.

Higher the fuel level, higher the resistance from the sending unit. Current travels from the ignition switch through gauge to the sending unit and out to ground.

First part I replaced on my truck was a fuel sending unit. Memphis Equipment has a universal sending unit that works great.

If we knew the correct resistance of the sending unit at the full position, any universal sending unit could be used. (probably 30 or 90 ohms)
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Old November 10th, 2009, 08:02 PM
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There is a civvy GM car unit, pre 65...that has the same scale as our mil unit...only backwards...

The MIL scale is 0-30 ohms...with 30 being empty and 0 being full.
The GM car, pre 65 is 0 empty and 30 when full.

So you can use the pre 65 g car unit...and read the gauge backwards...
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Old November 10th, 2009, 08:08 PM
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Should be easy enough to make some kind of adapter... a quad opamp chip, a mosfet, and a handful of resistors... adapt any sending unit to any gauge. Might need another mosfet depending on how much current to run though the sender...

--Randy
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Old November 11th, 2009, 03:30 AM
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Man, you guys are well over my head with the tech stuff.
The wire harness is brand new from Wire harness of Maine (whatever the name is) and the gauge is brandy-newy too. I will say tho' that the sending unit looks slightly different from the stock one I had before. It looks military but has a different production look than I'm used to seeing. It may be a replacement from a different app.

Randy, PM sent.

I guess I'll drop the tank again but this time I'm going to check the ohms readings before install and wear a mouthpiece and gloves when I put it back in.
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Old November 11th, 2009, 05:49 AM
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You might want to do a bit more troubleshooting first, before dropping the tank!

Disconnect the plug to the sending unit at the frame rail, and jumper it straight to a good ground... Does the gauge now read empty? If it still reads full, the problem is the wiring or the gauge. If it now reads empty, it could still be the tank ground, not the sending unit.

If you're sure it's the sender, pm me again, and I'll get it in the mail.

--Randy
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Old November 11th, 2009, 06:03 AM
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I should also add, your new gauge, it is a military gauge, right? The standard civilian aftermarket gauges (33-240 ohm) will also always read full if connected to a military sending unit (1-30ish ohms).

--Randy
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Old November 11th, 2009, 12:41 PM
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take the wire that goes to the sending unit, and with the key on, ground the wire, then pull the wire away from ground, the gauge should full sweep from no ground to ground, if it doesnt, you need to see if the wiring or the gauge is bad, if it does, that you need to diagnose the sending unit.

Duane
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