Fuel gauge aux. & main inoperative

[BCH BRCO]

My gauge used to intermittently work when switched from aux. to main. Now they do not work at all after installing a new selector switch. There 3 wires going to the switch. I assume the middle wire goes to the gauge and the other two go to each tank. Is there a ground wire somewhere that could be loose and causing the inoperative gauge when selecting either tank?

 

[DirtDonk]

Yes, but no ground wires at or near the switch. Only down at each sending unit. So unlikely the issue.
Though not impossible! Our Bronco histories are full of "coincidental failure" stories.

Either you got a bad switch (very possible), or you wired it incorrectly (certainly possible, even if not probable), or the wires themselves are just getting old and did not like being shuffled around.
If you look back by the switch (and if this is still original wiring) then you should see two Orange wires with strange looking 90° connectors on them. Separate them from the switch, and simply connect the two 90's together. One is male, one female.
If the gauge still does not work, then you have a problem elsewhere. If it does, then it's in the switch wiring.
The Orange wire and the two 90's are essentially connecting directly the gauge to the rear/main tank.

Paul

 

[BCH BRCO]

Yes, but no ground wires at or near the switch. Only down at each sending unit. So unlikely the issue.
Though not impossible! Our Bronco histories are full of "coincidental failure" stories.

Either you got a bad switch (very possible), or you wired it incorrectly (certainly possible, even if not probable), or the wires themselves are just getting old and did not like being shuffled around.
If you look back by the switch (and if this is still original wiring) then you should see two Orange wires with strange looking 90° connectors on them. Separate them from the switch, and simply connect the two 90's together. One is male, one female.
If the gauge still does not work, then you have a problem elsewhere. If it does, then it's in the switch wiring.
The Orange wire and the two 90's are essentially connecting directly the gauge to the rear/main tank.

Paul

Let me restate what I have experienced over the years with my selector switch. Often I would have to cycle the switch back and forth to get the aux. and main tank to read on the gauge. Recently cycling the switch didn’t work so I took the original switch out and cleaned the spade connectors on the switch with a small wire brush. I put it all back together carefully assuring the wires were put back properly. Lo and behold everything’s worked fine and both tanks registered fine on the gas gauge. After a couple weeks went by the switch would no longer register either tank on the gauge. So I bought a new switch and installed it thinking the old original switch was corroded internally. No luck. the gas gauge wouldn’t register for either tank. So after reading on this forum I did a couple things. I pulled the sender connection off the main tank and grounded it with a jumper wire. I turned on the ignition switch and with the switch in the main tank mode the gas gauge went to the full mode. I thought that must the problem so I cleaned the terminals on the rear tank float, installed some Ox-Guard (diaelectric grease) and put the connector back on the main tank. I also found a ground wire eyelet adjacent to the aux. tank and wire brushed the eyelet & installed a new ground screw. (The only ground screw that I could find was at the the aux. tank and none at the main tank). Anyhow I then turned on the ignition switch and nothing registers in either tank mode. I don’t think I can get the aux tank connector off the fuel sender to ground it as I tested the main tank. I can’t imagine both fuel senders would be defective at the same time. Any suggestions on what to do to chase this problem to resolution would be appreciated!

 

[4xfun]

The issue could be in a couple of areas. Check the following items:

1. unhook the battery. You don't need power for this test. You could check volts, but what you are checking is the quality of the transmission of electricity through the wires. Set your voltmeter to resistance. touch the leads together and you should see a value of zero.
1. Hook the black wire of a voltmeter to a good body ground and text the resistance on the wires to each tank, downstream of the switch. Someone on here will be able to tell you the values you should see. The value changes with the level of the float. If your values are not in the correct range. There is an issue between the wire and tank float.
2. Use the voltmeter to test the switch resistivity. Place the black wire on stud for the gauge wire and use the red wire on the tank stud. You should read a value of zero between the stud for the gauge wire and the tank selected. Check both tank sides. Also check to make sure that you do not have a value when you check the resistance between both tank studs on the switch.
3. Check the wire between the gauge and the switch. You can use the voltmeter and resistivity between both ends, but I think it is easier to use a cable/circuit tracer (short finder), which allows you to follow the line and find where a break is located.
4. Hook the black wire back to a good ground and check the value between the dash and the ground. A higher number means you have a poor ground.
5. I assume you are using factory gauges. Hook the battery back up, switch the vehicle on but do not start it, and check the voltage on the gauge. It should read around 5 volts. If it is significantly higher or lower, the IVR on your gauges is bad (you should also be having issues with the oil pressure and engine temperature gauges). Aftermarket gauges will usually read 12 volts.

 

[DirtDonk]

I did not keep them handy, but there are charts and graphs created by members here showing the full range of motion between the float arm and the expected resistance.
They are not linear either, so even though the range might be 73 ohms empty and 10 ohms full, half a tank is not going to be at or around 40-ish ohms.

Paul

 

[Oleguy 74]

the range of the gauge is 10-73 ohms.10 ohms =full 73 ohms =empty.if oil and temp gauges read ok then ivr is good.ivr (insterment voltage regulator).the output from ivr is a pulsting 5-8 volts,when testing just turn ign sw to accseeary.check the tech section on this site.there is a diagram of the tank switch.

 

[Oleguy 74]

empty =73 ohms, 1/2=25 ohms, full=10 ohms.

 

[BCH BRCO]

Anyone on here have a link for schematics showing the wiring from fuel gauge to selector switch to the aux. and main tank and location of the related ground screws?

 

[Oleguy 74]

one side of the sender is grounded.the wire goes from the sender to the frame

 

[Oleguy 74]

go online. seabiscuit68.tripod.com/details

 

[Oldtimer]

The ground screws are within a foot of the sender units, just follow the black wire from the sending unit harness plug.
Main tank ground is on frame (I think), and aux tank ground is on body.

With a single factory tank, connector "A" would plug onto post on meter.
Connector "B" also contains wires for rear turn/brake lights and rear running lights.

 

[BCH BRCO]

Thanks guys for the schematic & info. Hopefully I can find the main tank ground to frame. I found the aux. tank ground. Just have to get access to remove the aux. tank wire connector from the float switch as access is tight.

 

[BCH BRCO]

Well I have some partial good news. I have the aux. tank side of the gas gauge working after putting in 6 gallons of gas. The gauge indicates about 7/8 toward full. I feel somewhat stupid as it appears the tank was nearly empty. i do keeps a log of the gallons of fuel purchased for each tank and mileage on the odometer. Both tanks had been around 7/8 full and I had driven 64 miles. (Historically I average about 7 miles per gallon). I believe most of that mileage was on the aux. tank. Now for the not so good news, I only put 2 gallons of fuel in the main tank when it gurgled and the pump shut off indicating that the tank must be nearly full. Prior to this I did a continuity check at the rear tank whereby I pulled the plug connector off the fuel sender and used a jumper wire between the orange side wire in the plug and put the other end of the jumper wire alligator clip in the ground side of the fuel sender plug. I then turned the ignition switch on and the gauge pegged to full. I also did another continuity check where I used a 12 volt test light continuity tester from the orange side of the fuel sender plug to the ground side of the plug WITH THE IGNITION ON and the tester lighter flashed on and off with a pulsating frequency. This indicates to me that the circuit from the IVR (instrument voltage regulator) at the cluster via the selector switch to ground is okay. (btw the main tank ground is directly above the fuel sender secured to a frame crossmember)
So here’s my situation. It appears there may be something wrong with the main tank fuel sender float switch that currently will only register on the lowest mark on the fuel gauge. When I turn the ignition on, the needle covers the lowest “empty” mark. When I turn the ignition off the needle retracts slightly left of the lowest empty mark.
is there a test I can do to the fuel sender while it is in the tank that will verity that the problem is with the fuel sender? Thanks to all of you that have contributed to this saga thread, any further suggestion will be appreciated.

 

[Oldtimer]

The brass floats get a hole in them and sink.
I have changed mine a couple of times in the last 50 years.

1966-1977 Ford Bronco Brass Sending Unit Float,-Broncograveyard.com

Brass construction for durability. Composite: JLIAFF5559011

shop.broncograveyard.com

Also get the seal (and retaining ring).

1966-1977 Ford Bronco Sending Unit Seal-Broncograveyard.com

shop.broncograveyard.com

EDIT:
And with the lock ring tool you don't have to pull the rear tank.

Fuel Tank Sending Unit Lock Ring Tool-Broncograveyard.com

FUEL TANK SENDING UNIT LOCK RING REMOVAL TOOL 2 1/8 ID - Tool enables easier removal and installation of lock ring when replacing the fuel sending unit - Handle provides additional leverage without risk of damage that could otherwise happen if using hammer and punch - Correct tooth pattern...

shop.broncograveyard.com

 

[BCH BRCO]

Thanks Oldtimer for the advice and parts links. Guess I will get those ordered. Can any gas remain in the tank when pulling the fuel sender?