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Main tank sending unit troubleshooting

flynylon

New Member
Joined
Oct 26, 2009
Messages
33
Loc.
Boerne, TX
So here's the deal: PO reported that the PO before him said he hadn't used the main fuel tank in my 71. I tracked down and called 2nd PO and asked him why he said that. He said that "the switch or something wasn't working," so he just went to using the AUX tank.

The main tank is darn near empty, and when I hooked up a vacuum pump to the line at the mechanical tank switch, I could draw out a little funky gas. I could hear a sucking sound inside the tank, so it seems that the lines are decent. I haven't found any sort of leak indication, and the tank looks pretty darn good for being almost 39.

I flipped the dash switch to MAIN with the ignition on, and the gauge ran up to FULL. I went underneath, disconnected the connector, and ran voltage on it. It was at .20 volts, and the gauge still read FULL. I found about 120 ohms of resistance between the leads on the sending unit (tank is empty). I also found a compromised wire near the tank, but it hadn't run to ground.

So, as a relative dilettante in automotive electronics, what do you think I have? A sensor wire that's grounded further up the line and/or a bad sending unit?

Thanks in advance!
mike
 

oleguy74

Bronco Guru
Joined
Mar 26, 2008
Messages
2,034
Loc.
calif city ca
flip switch to aux tank.disc wire from main sender and check for a ground.should be an orange wire.if you see a ground repair it.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,355
Yeah, I would think the gauge should have gone to empty when you disconnected the plug from the tank.
I would think it's also possible to have a bad dash switch, that would allow some feedback to the gauge even when it's switched over.
Both are known for going bad before 40.

When I bought my '71, it was only 6 years old and neither sending unit worked properly. Nor did the switch.
Replaced the switch, pulled the senders and, instead of replacing them, cleaned and re-adjusted them.
Front is still working perfectly to this day. Rear has been replaced with a new, larger main tank. BIG improvement by the way! Love not having to look for gas stations near as much. Not as good as getting better gas mileage, but still worth a grin or two.

Good luck.
 

fungus

Bronco Guru
Joined
May 27, 2009
Messages
1,548
Loc.
Kaneohe Bay, Oahu
I've got this very problem with my aftermarket 12 ga Aux tank so I'm interested to hear the fix! Flip the switch to Aux and gauge pegs over full w/ an empty tank... Good ground, new wiring from sender to switch.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,355
In your case fungus, your Bronco just got tired of you spending all that time in the junkyard (thanks again by the way) hunting down parts. It was probably either jealous, or afraid you were just going to keep replacing things until it was all new!
Typical GF/wife/Bronco-thing... ;D

And now, while you're out sunning, swimming, surfing and luauing (official word), it's increasing it's rusting exercises. Copper wire? Sending unit? Hah! Salty island air knows no boundaries!

Paul
 

fungus

Bronco Guru
Joined
May 27, 2009
Messages
1,548
Loc.
Kaneohe Bay, Oahu
That could be true Paul, however, the 'Ol girl is getting used as a DD right now hauling me (& the 4 kids!) to work/ school/ daycare every morning! Not sure how she could be jealeous as she's getting more attention and job satisfaction that anything around my house.

As for the rust on the sending unit/ wire (clean), been having this annoying prob with that tank for several years and I attempt to fix it every now and again w/ the proper motivation to no avail. The only things I haven't replaced are the switch (got a couple spares so need to try one) and the sending unit itself which was brand new w/ the tank 5 years ago. It worked for a couple trips then started pegging to the right all of a sudden (several years ago...).
 

Viperwolf1

Contributor
electron whisperer
Joined
Aug 23, 2007
Messages
24,316
not to hi jack your thread but how do you check for a ground?

With an ohmmeter. Zero ohms is a perfect connection with no resistance. Infinite ohms is a perfect insulator. In reality everything lies somewhere in between those. Your orange wire to ground should be closer to the infinite ohms reading.
 

Viperwolf1

Contributor
electron whisperer
Joined
Aug 23, 2007
Messages
24,316
I've got this very problem with my aftermarket 12 ga Aux tank so I'm interested to hear the fix! Flip the switch to Aux and gauge pegs over full w/ an empty tank... Good ground, new wiring from sender to switch.

Disconnect the sender at the tank and see if it still pegs. If not the sender could be bad. Check ohms from sender terminal to ground.
 
OP
OP
flynylon

flynylon

New Member
Joined
Oct 26, 2009
Messages
33
Loc.
Boerne, TX
I just got back from my very first trip to BC Broncos. Thanks to Jason, I've got a brand spanking new sending unit (plus a free loaner drive shaft in case mine blows apart soon. I also stopped at Autozone for a new spool of orange 12 ga wire, so I will fix this puppy! Thanks for the directions on finding out if the orange wire is run to ground somewhere. I'll let y'all know how it goes.
 
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