Fuel gauge not working on my '69 Mach....

[cjshaker]

My '69 R-code Mach has 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 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. I have not had a problem with anything else working because of a ground.

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 seemed to work before the engine pull though, just not the fuel gauge. Although I have Autometer gauges that I depend on for temps and oil pressure, I only have ONE 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. 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 gauge 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 a load is applied? I was going to pop the fuse out and feed 12 volts to the gauge side of the fuse and see if that was the issue, but I'm puzzled about that resistor wire in the diagram. A resistor should drop voltage, yet I get full voltage at the cluster plug-in.  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. I have never had such a hard time figuring out a problem like this. 

[ScotiaFE]

The gizmo in the tank is good?
They can be quirky especially old ones .

[jmlay]

If the gauge reads empty then it sounds as if the circuit is open or zero volts. What is the resistance of your sender as measured from the cluster plug to ground?

[jayb]

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 seemed to work before the engine pull though, just not the fuel gauge. Although I have Autometer gauges that I depend on for temps and oil pressure, I only have ONE 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. 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 gauge 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??

I have several comments and suggestions, none of which may lead to a solution, but they may provide a start.

- Measuring the resistance of a gauge is not a good indication that it is functional.  The fact that it measures the same as the other gauges is positive, but I wouldn't rule out the gauge as the problem based on that measurement.

- When you checked the wiring harness tang at the plug in, was it connected to the dash cluster?  If it wasn't, the only current you would get through that wire is through your meter, and so the voltage drop due to that current, across the resistor in the wiring harness, will be negligible.  If you then plug the connector in and only get millivolts at the regulator, there is a whole bunch of current going into the gauge panel, leading to this low voltage at the regulator.  Maybe the resistor in the wiring harness has given up and nearly shorted, or maybe there is some other component that is shorted and causing the problem.

With a bunch of different connections it is nearly impossible to debug a circuit like this with everything connected.  I would start with the gauge, all by itself, to confirm that it is working.  I believe the voltage regulator is a 5V unit; take 3 D batteries, hook them together in series to get 4.5V or so, and power up the gauge with it disconnected from everything else.  If I recall correctly with no input from the sender the gauge should read empty.  Then, ground the sender connection and see if the gauge goes to full.  If it does, next I would try to power up the panel with the voltage regulator disconnected; just power the wire from the regulated side of the voltage regulator, and see if the fuel gauge still works.  If it doesn't, you know the problem is in the panel somewhere.  If it does, it is either the regulator (doubtful since you have replaced it) or the resistor wire in the harness leading to the instrument panel.  Wish I had a 69 wiring diagram; I would kind of like to know the purpose of that resistor.  Anyway, hope this helps - Jay

[ScotiaFE]

The book says to take the tank sending unit out and move the lever up and down,
if the gauge moves properly "the sending (gizmo) unit is defective".
If the gauge does not move properly the gauge or wire is faulty.
I still say the gizmo is defective. lol
I have changed two of them in the last 10 years or so because the gauge would not move.

[cjshaker]

The gizmo in the tank is good?
They can be quirky especially old ones .

I'm sure the sender is good. 13-15 ohms is right where it should be for a full tank. Resistance is higher for a lower fuel level.

What is the resistance of your sender as measured from the cluster plug to ground?

I cleaned all the connections, but I'll make a long test lead and check this. I don't expect to find a problem here though.

- When you checked the wiring harness tang at the plug in, was it connected to the dash cluster?  If it wasn't, the only current you would get through that wire is through your meter, and so the voltage drop due to that current, across the resistor in the wiring harness, will be negligible.  If you then plug the connector in and only get millivolts at the regulator, there is a whole bunch of current going into the gauge panel, leading to this low voltage at the regulator.  Maybe the resistor in the wiring harness has given up and nearly shorted, or maybe there is some other component that is shorted and causing the problem.

Jay, I thought a resistor dropped voltage, but you're saying that it is based on current draw? In other words, the voltage will not show a drop until it has a load on it? And if there is a short somewhere in the system, then the voltage will drop considerably more than it should? Just trying to wrap my head around what you said.

I would start with the gauge, all by itself, to confirm that it is working.  I believe the voltage regulator is a 5V unit; take 3 D batteries, hook them together in series to get 4.5V or so, and power up the gauge with it disconnected from everything else.  If I recall correctly with no input from the sender the gauge should read empty.  Then, ground the sender connection and see if the gauge goes to full.

Yes, it is a 5 volt system, and yes, it should read empty with no contact (full resistance)....or full with little resistance. I have had my doubts about the gauge and had not considered the battery trick. I will try that first. This may be an issue as my other gauges worked, at least best I remember because honestly I never watched them, just my Autometer gauges. But I DO remember my temp gauge working because I compared its intake probe with the Autometer gauge probe located in my thermostat housing to see exactly when my thermostat opened.

I have a 1969 original dealers ring bound book that covers all wiring/vacuum schematics for Ford and Mercury products in 1969 and there is nothing that indicates what the resistor is for. It is simply in line from the fuse block to the gauge cluster feed and affects or feeds nothing else. Although I can find NOTHING in any Ford manual that says what that wires resistance should be To my knowledge, the resistor is only there to help the regulator live a long and prosperous life

The battery trick to test the entire cluster is a great idea and I will do that.....as soon as I cool off and clear my head...lol
Now that I think about it, I should be able to power up the cluster with the batteries and jumper to the sending unit wiring also to check the entire secondary part of the system. That will break it down into 3 separate systems and isolate which one has the problem. Thanks for jarring my head and getting me to think a little straighter about the subject.

edit: According to a '69 Mustang Concours wiring specialist, the resistance wire should read 9.5-10 ohms. So I can check that also. Ford was definitely in a transitional phase with wiring from '69-'70....and it shows. There are several shortcomings in the '69s wiring system. Some that are down right dangerous to the health and well being of your car. That's why I NEVER leave the battery connected while it's sitting.

[cjshaker]

The book says to take the tank sending unit out and move the lever up and down,
if the gauge moves properly "the sending (gizmo) unit is defective".
If the gauge does not move properly the gauge or wire is faulty.
I still say the gizmo is defective. lol
I have changed two of them in the last 10 years or so because the gauge would not move.

I have checked the senders resistance at low fuel level, half full and full. The resistance is correct at all spots, so the sender is not the issue. It was easier to just add 5 gallons of fuel at a time and check it at each level than take the whole unit out

[My427stang]

What happens if you ground the sender wire?  Does it go full?

[ScotiaFE]

Not trying to start an argument, but it's the sender.
Take it out and wiggle the lever. That's why the book even says to take it out.
Although it is just a potentiometer, it is a gizmo in a tank of gas.
It does funny things.
At the end of the day you can change all the easy parts and new wire and spec everything with the meter.
You will still have to pull the gizmo out. btdt

[The Magic Ratchet]

When I worked at the Ford dealer in the 70's, this is the test method we used to use:

Disconnect the wire at the fuel tank sending unit. Connect a common test light in series between the end of the sender wire and ground. Turn ignition key to "on." The test light should blink while the fuel gauge moves to full. If this happens as described, the sending unit is faulty. If the light does not blink, the IVR is bad. If the light stays on steady, there is a short to ground somewhere in the wiring. If the fuel gauge does not move to full and the test light is blinking, the gauge is bad.

Hope that helps.

Lou Manglass
proud owner of "The Magic Ratchet"

[jayb]

Jay, I thought a resistor dropped voltage, but you're saying that it is based on current draw? In other words, the voltage will not show a drop until it has a load on it? And if there is a short somewhere in the system, then the voltage will drop considerably more than it should? Just trying to wrap my head around what you said.

That is correct, this is Ohm's law, pretty much the first thing they taught us in electrical engineering school.  V=IR, where V is voltage (V), I is current (Amps), and R is resistance (Ohms).

So, let's say that your wire leads from the 12V battery, through the 10 Ohm resistor, and directly to ground (0V).  Then the current through the wire will be (12-0)/10, or 1.2 amps.  That's the simple case.

Now let's say that you have connected your voltmeter to the end of the wire, and the other side of the voltmeter to ground.  The voltmeter has a very high internal resistance.  Let's say for the sake of an example that it is 10,000 Ohms (it is higher actually, but we'll use 10K Ohms as an example).  Now, you have a different equation, (12-0)/(10+10,000), or about .0012 amps (1.2 milliamps).  So, the current is greatly reduced.  How much voltage will you see at the 10 Ohm resistor with only this small amount of current going through it?  Voltage drop across the resistor will be (.0012) X 10, or 0.012 volts.  So, with your 12V battery, you will see 12-0.012, or 11.988 volts at the 10 Ohm resistor.

I don't know if that helps; it sounds to me like the test light method described above is probably a pretty good one if it works...

[bn69stang]

Jay and Doug i have copies of the Ford wiring manual , they cover 69 mustang and cougar . when i was in high school i had a buddy that worked at a ford dealer in parts and gave me the master to copy i had a 1970 cougar back then and a 69 mach now .. you guy s contact me at bn69stang@gmail.com  or  ( 505 ) 934-4127 and i can dig it out and make some copies and get them to you .. and my 69 did the same thing as yours and it was the sending unit ..Bud

[cjshaker]

Jay and Doug i have copies of the Ford wiring manual , they cover 69 mustang and cougar .

Bud, I have a factory Ford wiring and vacuum book that covers all Ford products for 1969. Picked it up many years ago for $5 at a local yard sale of all places. It turned out to be very valuable when I restored my car. Here's a couple shots of it...





First, I did as Lou suggested and did the test light test. The light never even hinted at coming on.
Next, I double checked continuity from the plug-in back by the tank up to the cluster plug-in. I got about 3 ohms which seems like a good number considering the footage of wire involved. I checked each connection point (there are 2, a 3 prong plug behind the dash and the single connection in the trunk for the sender wire), visually and by wiggling it to make sure there was no intermittent connection failure.

So I did Jays suggestion next and used 3 D batteries and powered up my gauge cluster. A shot of the battery set-up, minus the leads I used to jumper to the gauges....



Doing this, I got every gauge to work perfectly. First I went across each terminal on the respective gauges. They all moved nice and smooth with no hesitations. Second, I went to the primary side of the regulator on the circuit board then to the secondary side of each gauge. Again, they all worked smoothly. That told me that the circuit board and the regulator seemed to be working fine and was making good connection at all points.

At this point I figured the resistor wire feeding the gauge cluster was bad. But before I condemned it, I played around with the gauges some more to see if I had a short somewhere causing the voltage drop from the resistor wire as Jay suggested might be the case. This is where I got confused again. I got a continuity reading from grounding to the metal cluster housing, then to each terminal on every gauge. This, to me, indicated I had a short somewhere and that that would cause the voltage drop from the resistor wire.

I took all my gauges out and inspected them. All isolating pads are in place as they should be, and in good shape. I even checked to make sure they did not give a continuity reading through the isolation pads. A shot of the pads on the back of  the gauges, and the ones that go on the outside of the cluster to isolate it from the housing....



This one just showing the pads without the gauges being installed....



Next, I checked for continuity from the regulator to the cluster housing. First from the regulator housing to the cluster housing. I got good continuity as I should have. Then I checked on the primary and secondary side of the regulators terminals to the cluster housing. I got 12.78 ohms on the secondary side....then 4.27 ohms (yeah, ironic huh ) on the PRIMARY side. I thought BINGO, I have a short in the regulator! It shouldn't be reading from the regulator terminals to ground.

WRONG. I checked the schematic.....



As you can see, the primary and secondary sides of the regulator seem to be connected to the regulator housing, or ground. So the numbers I got appear to be normal. This also explains why everything worked fine when I powered through the regulator.

So now I am wondering why and how this works. When I jumper the positive side of my batteries to the cluster HOUSING, then connect the negative to the secondary side of the gauges, they work. This just doesn't seem right to me But I am not an electrical engineer, so I may just be wrong. Am I simply feeding power through the ground, into and through the regulator (essentially bypassing it) and on to the gauges? According to the schematic, that seems to be the case.

IF this is normal, then my issue is with the resistor wire feeding the cluster. So now I need an engineer to tell me if this is normal per the schematic.
Is there a Doctor in the house?

[bn69stang]

Thats the same ford manual that my copies are out of , but just have the electrical part of it - but  $ 5.00 well spent lol and it looks and sounds like  its driving you crazy .. GOOD LUCK     Bud

[rcodecj]

Same question as My427stang, "What happens if you ground the sender wire?  Does it go full?"
If it does go full then I would say it is the sending unit.
If you want a further test ground the sender wire through a variable resistor and watch the gauge go up and down.
I believe ScotiaFE is correct, it is going to be the sending unit.