Topic: Fuel gauge not working....

[cjshaker]

Hi guys. First post here.

I have a '69 R-code Mach and have been having a fuel gauge problem that is driving me totally nuts. I did a total rotisserie restoration on it several years ago and my fuel gauge at first would start to work, then it would creep back to empty, fluctuating slowly. It went on like this for a while until it just stopped working. With the engine out for a refresh and some power upgrades, I took the time to go through and finish a few details that were bothering me. First and foremost is my fuel gauge.

First, knowing that restorations can lead to bad grounds with painted surfaces, I was careful to double clean all grounds when reassembling the car.

As for the fuel gauge, I checked the sending unit and it is giving good ohm readings at half and full. Right now it reads 15 ohms with a full tank. When I grounded the wire the gauge does not respond at all, so next I went to the dash. I replaced the circuit board and voltage regulator during the restoration, so I looked there first. The circuit board seemed ok but I replaced it with a newer reproduction anyway that seemed a little nicer than the older one I had. No change. All my dash lights and other circuits work but I can't check my other low voltage gauges because the engine is out. The oil pressure and temp gauges worked before the engine pull though, just not the fuel gauge. Of course I checked power to the fuse and the fuse itself. They are good with power coming on when I turn the key.

Since it's nearly impossible to get to the circuit board with the cluster plugged in, and since I have NO confidence in reproduction parts, I replaced the voltage regulator with another one. Still no change (which I expected since the other gauges worked). Next I tested the gauge itself. It reads about 14 ohms which indicates it is good as all of the low voltage gauges I've tested read the same. I have cleaned every terminal, ground, contact (including the contacts on the circuit board feed AND the circuit board. I can even see the "contact" points where the connector is making contact with the circuit board connections). I have temporarily double and even triple grounded all grounds to make SURE that is not causing my problem.

I noticed in my wiring diagram that it is a resistor wire that feeds the cluster, but when I check the wiring harness "tang" at the plug-in I get 12.6 volts. What is the resistor wire for if I'm getting 12 volts at the plug in? Further, when I pull the cluster out with it plugged into the harness and with the key ON, and with the connector at the regulator unplugged, I get 12.6 volts on the "hot" side of the connector (the 9volt battery type connector). BUT when I plug the connector back onto the regulator it then drops to a millivolt reading??

What gives here? Is my ignition switch giving me a "ghost" reading when the load is applied? What suggestions do you guys have to go from here? I apologize for the long post, but this is driving me absolutely insane!! I need to get this figured out before I drive it again (already ran it out of gas once.....and that is ONE TO MANY TIMES!

Thanks for any help you guys can give me. At this point I'm ready to fly in some electrical guru to help me figure this out....if I could afford it.

[midlife]

My first thought is that you had a leaking fuel float in the tank, but when you said you grounded the sending wire, it did not respond with a peg.  That says that there is an open circuit (very high resistance) somewhere between the CVR and the sending unit wire.  I'd check your three-prong  connector under the dash that goes to the tail-light harness that contains a yellow/white wire.

Now then, let's talk about the CVR and the resistor wire (which should read 9.5-10.0 ohms).  Without any load on the circuit, the CVR will read 12V, not a lowered voltage.  The purpose of the resistor wire is to lower the voltage to the CVR to help it last longer (much like points in the distributor, although I'm somewhat skeptical of this).  When you plugged everything back in and found mVolts only, that says there's an awful lot of voltage drop across the resistor wire compared to the rest of the circuit.  That is puzzling, as it suggests a short circuit beyond the CVR. 

Here's what I would do: measure resistance between the yellow/white pin at the dash cluster connector to the sending unit wire at the back of the tank.  This eliminates the gauge, and focuses upon the one piece that's hard to measure.  I've found high resistance at times at the dash cluster pins (crimps unloosening) and across molded plugs.  I'd also measure the resistance at the sending unit itself now that it is installed in the tank.  You know how much gas is in it now?  You can estimate the resistance and compare (high resistance - 73 ohms equals empty tank; low resistance ~ 13 ohms equals full tank).  It is not linear, however.

Break the problem into smaller pieces to see where the problem may lie/lay/be (damn English teachers never taught me good...).

[cjshaker]

Midlife,

Thanks for the reply. The sender is and has been in the car. I checked it by adding 5 gallons of fuel at a time and got good readings at all levels. No, not linear, but resistance dropped as I added fuel. It reads 15 ohms now with a full tank, and that should be a good reading.

I will check the areas you suggested. I am also going to power up my gauge cluster with 3 D batteries in series and check them that way. I can then check the gauge by shorting it with varying resistance (I have resistors to do this, although just shorting to ground shortly (pun intended) should tell me right away. I can also jumper to the sending units wiring and check the whole secondary side of this circuit without the main harnesses resistor wire feed being a possible issue.

That way I can break this down into 3 separate areas, or circuits, and determine where the fault may lie. Just needed to clear my head and approach this from another direction.

Thanks for the resistor wires function and resistance. I could not find that anywhere in ANY Ford manual or schematic. That will be a big help.

[ClevelandKid]

I'm having the exact problem with my 70 Boss fuel gauge. Gauge goes up and down. I did the test with a test light and confirmed its not the gauge. So I'm suspecting its the sending unit float. New floats are cheap and readily available so thats the first thing I try and hopefully the fix.