Fuel Gauge:

[Lazy Jake]

Let me preface this with the fact that I did a search and I did read the fuel gauge troubleshooting guide, however... Maybe someone would like to tackle this:

My fuel gauge doesn't work. I bought a new sending unit from LMC. I measured the resistance at a solid (no fluctuation) 75 Ohms when in the empty position and 9.4 Ohms when in the full position. That seems to be within specs. My original measures close to the same but, the float was cracked so I bought a new one just to have a spare.

My gauge will read full when I hook it up but, will not indicate anything below 1/2 full. Or 1/2 empty depending on how you look at things.

The gauge will drop to empty when the key is off, ie, it doesn't have 12 volts. As soon as I turn the key on the gauge goes up to about 1/2 full - even when the sending unit is disconnected and the wires (fuel gauge and ground) are not hooked up.

Do you think maybe the fuel gauge wire (the orange one on my '68) could be shorted somewhere?

Or, maybe, I need to replace the fuel gauge itself.

Dale

[cghstrojan08]

I have a near same type of problem with my truck. When It is full, it is at 3/4, then it drops to 1/4 and just sits. I have tried for a long time to get it fixed but have never gotten any further. I just fill it up all the time. If you can figure it out, I would like to know to.

[Lazy Jake]

Yeah, I'm going to work on it some tomorrow. It really is a simple circuit and I don't want to try and overthink the problem. I have the wiring diagram and will try to nuke it out in the morning.

In the meantime, if anyone has any ideas it looks like at least two of us could profit from a little expertise.

Dale

[kid]

The circuit has too much resistance in it. That causes the high reading.

I know you have a sixty eight but this is so close as to be it.

http://www.fordification.com/images/sch ... rging2.jpg

You have three connectors and a ground. If any of those are corroded it raises the resistance.

[Jake11]

It's not gonna have 12volts. Theres a regulator on the back of the
cluster that cuts it down to around 5 or so. If its messed up your guage
will not work ok. Kinda hard to test with a digital voltmeter. A testlight
hooked to the wire on the sending unit should pulse off and on. If not,
replace the CV reg. Their pretty inexpensive. If ok, when you ground
the sending unit wire, the guage should move to full.

[Lazy Jake]

Kid & Jake11,

I'm going after it this morning. I've read your post and the Tech. article on trouble shooting the fuel gauge. I have the '68 wiring diagram and a pot of coffee. I suspect high resistance. From looking at the wiring diagram it appears that the regulator reduces voltage to the temp sending unit and the oil pressure gauge also. These are working fine on my truck. Nevertheless, I will test everything, clean up the contacts, run the tests, etc, etc...

I'll get back this evening with the results.

OBTW, I removed all the wiring and the switch for the aux tank in the C.S. So that doesn't even enter into the picture. If I get the main tank working I plan to go after the aux tank. This thing burns the good stuff and I can't afford to fill the aux tank and the main tank at the same time but, they might as well work.

Dale

[kid]

The voltage regulator (chopper) only does the fuel gauge. That is to stop the gauge from swinging it only allows sampling of the fuel sensor. It works by heating up and opening and then cooling off and letting electricity through the gauge again. Take it out and your fuel gauge does the Tango.

If the regulator fails it fails either open and no gauge or closed and bouncy gauge.

[68F250]

The instrument voltage regulator drives all the gauges except the ammeter. The IVR, fuel gauge and tank sender are all connected in series. If the gauge reads anything when the sender is disconnected, there is something shorting after the gauge, for reference 1/2 tank = 23 ohms. I'd look where you removed the CS wiring and switch of the aux. tank IIRC there's two orange wires behind the dash, one is used for the cab tank only or the other one is used for the 2 tank option.

[Lazy Jake]

I had to scoot out of town last Friday for a little road trip to California. Before I left I found a little "coil" attached to the "chopper" that was broken. I was trying to fix it with some glue and some solder - the copper wire was broke out of the solder joint. I didn't get a chance to look at the wiring diagram to see what it does but, broken can't be good. I won't be home for a week or so but, when I get there I will go back to work on it and let you know what I find.

Dale

[68F250]

You can take the coil off. It's an add-on choke that reduces AM radio noise interference, that's all it does.

[Lazy Jake]

Barry,

Tks. I just got home from my little jaunt out to California and will get back to work on the fuel gauge. I recieved my headlight harness from Paul today and will work on getting that put in also.

Paul outdid himself on the harness and I look forward to getting it installed.

I'll get back with a post relating the results of my efforts on the fuel gauge.

I believe my orange wire to the fuel gauge was feeding through the radio suppression coil. The wire in the coil was broke and maybe that was causing a problem. Maybe not. We shall see.

Dale

[briscoco]

I think your wife is messing with your mind and keeps adding fuel to your tank. I just can't seem to get my wife to do that please advise were to get a wife that does.

[Lazy Jake]

I wish I COULD get someone to add fuel. At about 9 MPG this 390 isn't helping the war effort at all. Still nothing good to report of fixing the fuel gauge. I believe there is some resistence in the wire between the sending unit and the gauge. The wire is stock routed up under the headliner, down the drivers side "a" piller to the gauge. I haven't found a cut or short yet but, I'm still looking.

Dale

[Jake11]

Ya know, maybee the stupid guage is just shot. It happens.

[Lazy Jake]

Ya know, you are probably right. I went back through everything last night. Used a VOM to trace out all the wiring, cleaned all the connections with emory cloth and electrical contact cleaner, checked both sending units again, etc, etc... Nothing. The gauge still reads pegged out over full when the sending unit is in the top position and about half when in the bottom or empty position. So... I'm going to put this thread to rest until I can get a new or replacement gauge installed. When that happens, and I get a good result, I'll revive the thread or, at least, post the results.

In the meantime, thanks to everyone for their input. I have to admit, I may not have my fuel gauge working yet but, I've learned a few things along the way.

Dale