gas gauge problems

[automart 10]

My 70 Mach I fuel gauge will not read above 1/4 tank when full. I changed the sending unit but that wasn't the problem. I took the dash apart and checked the circuit and it seemed to be ok (it's all new) the car if freshly restored so everything is new or rebuilt.

[musman 10]

My 70 Mach I fuel gauge will not read above 1/4 tank when full. I changed the sending unit but that wasn't the problem. I took the dash apart and checked the circuit and it seemed to be ok (it's all new) the car if freshly restored so everything is new or rebuilt.

 

 

Count me in this problem too....just finished the resto, just like you everything is new, tank, flood, sending unit, the dash was checked and everything else works fine, but the gauge won't read beyond 1/4....i haven't had time to check that particular problem cuz i've got more urgent ones to solve right now...but i'll be reading others input on this one for sure...

[Pakrat 1,043]

New does not mean working, everything must be verified. If you just keep changing items that may be the problem you will go nuts and broke.

 

With the ignition switch on, remove the wire from the fuel tank sending unit and ground it to the unit housing. This removes the resistance supplied by the sending unit, and should cause the gauge to swing over to the 'Full' position. Does this happen?

 

Yes

The gauge and wiring are good. The problem is most likely with the sending unit. The sending unit must be removed for further testing.

 

No

Ground the sending unit wiring to a good chassis ground (instead of the sending unit housing) and watch the gauge. Does it now peg out to the 'Full' side?

 

Yes

The problem is with the sending unit ground. Either the unit itself is bad or the float/arm is binding/missing. The sending unit must be removed for further testing.

 

No

The power supply to the gauge has been interrupted, (check fuses), the gauge is defective, or the wire from the gauge to the sending unit is open or has a break in it.

 

 

 

Testing the Sending Unit

After the sending unit has been removed from the tank, check to make sure the float is present, intact and still floats and that there is no binding with the float arm. Reconnect the gauge wiring to the unit and ground the unit. Move the float arm up and down. If the gauge reads full in the full up position and empty in the full down position, then the sending unit is good. Verify by hooking up an ohmmeter to it and making sure it reads 8-12 ohms full and 70-73 ohms empty. If not, the sending unit needs repaired or replaced. You can easily disassemble the head of the sending unit to clean and/or repair.

 

 

 

 

 

 

 

[musman 10]

Thanks for the tips, i will do that when i get to that problem, but for now i've got a power brake fluid leak at both rear wheels.....man will this ever stop....

[automart 10]

Nope, never stops. I've got my check list, I'll work on it tomorrow. Will let you know. (It can't be the sending unit since I've already replaced it and it's doing the same thing)

 

By the way, Pakrat, how did you figure all that out?

 

Thanks!

[musman 10]

Nope, never stops. I've got my check list, I'll work on it tomorrow. Will let you know. (It can't be the sending unit since I've already replaced it and it's doing the same thing)

 

By the way, Pakrat, how did you figure all that out?

 

Thanks!

He probaly went through the same problem......who did not anyways with these old cars...

[bnickel 10,004]

Nope, never stops. I've got my check list, I'll work on it tomorrow. Will let you know. (It can't be the sending unit since I've already replaced it and it's doing the same thing)

 

By the way, Pakrat, how did you figure all that out?

 

Thanks!

 

 

actually, it could very well still be the sending unit, or rather the float needs adjustment.

[Pakrat 1,043]

Nope, never stops. I've got my check list, I'll work on it tomorrow. Will let you know. (It can't be the sending unit since I've already replaced it and it's doing the same thing)

 

By the way, Pakrat, how did you figure all that out?

 

Thanks!

 

Never say never. Here's a perfect example, old style xmas tree lights. If one bulb was bad the whole string went out. You usually had start replacing each bulb one at a time with a test bulb until you found the right one. If your test bulb was also bad though you could never find the culprit. You could have very easily replaced a bad part with a new but also bad part or the ground to the tank could be bad meaning no matter how many new ones you put on it won't work. This is why you should always diagnose before you replace, even if you feel certain what it might be. My best friend in my tool aresnal is my multimeter. The largest gremlin in the Mustang is often the ground and a good multitester is cheap compared to replacing the solenoid, volatge regulator, coil, starter, etc.... and still not having a running car. Yet there seem to be way more people that do this than those who own a tester, lol.

 

BTW, yes, I had this same problem many moons ago and this is one of the oldest tests for one of the oldest problems out there. I usually never forget something once I learn it and I still am always learning something new about these cars all the time (and usually from BNickel). Now you can pass it on to the next person as well.

[automart 10]

Hey Guys,

 

Well, I grounded the fuel wire and yes my gauge moves all the way to full :), I have a full tank of gas and will try and burn most of that out and drain the rest before I pull the sending unit out....again. I'll need to buy a tester, any suggestions?

 

Thanks

[musman 10]

You can get a mutimeter at any car or electronic shops for very cheap....mine cost me $17.95....and has all the goodies on it...

[Pakrat 1,043]

Correct. A regular analog unit is less that $20 and you can spend hundreds on a digital one, all ranges of quality. All of my equipment is the vintage Dixco stuff that hasn't been made in years but even after 25-30 years it all works great still. I have the Pro-Tester 7 Automotive Analyzer. It does:

 

High Tach (0-8000 rpm)

Low Tach (0-1600 rpm) both for 4, 6 or 8 cyl cars

Dwell 0* to 8*

Points condition (if you are still running them)

High volts (0 to 20)

Low volts (0 to 2)

Amps (0 to 100)

 

With that you basically can do over 30 different types of tests on the car.

[Pakrat 1,043]

BTW, did it peg to full when grounding to the housing or did you have to find a frame ground?

[automart 10]

I put a wire in the fuel gauge wire and grounded it to the leaf spring. The gauge moved all the over to full. So my next step I assume would be to remove the sending unit, which I've already replaced (it was a used one but very little.)

[SoCal69GT 10]

I'm on the same boat here. I'll try Paks tips.

 

Thanks,

Jun

[Pakrat 1,043]

I put a wire in the fuel gauge wire and grounded it to the leaf spring. The gauge moved all the over to full. So my next step I assume would be to remove the sending unit, which I've already replaced (it was a used one but very little.)

 

 

Ah see, I'm glad I asked then, you skipped a step. If you had tried to ground it to the housing itself first and it did not peg to full as it did on a chasis ground then it could just be a bad ground at the sending unit and not a faulty unit. Since you had this happen with two units it could be the case and why I suggested it first. The sending unit actually grounds itself via the tank to the chasis. So if the unit is not making metal to metal contact with the tank or if you have say way too much caulking between your tank lip and the trunk floor then the unit is simply not making a ground and why it won't work. This is common to folks running a plastic fuel safe for the first time. You end up having to run a seperate ground wire to your sending unit because the plastic cell insulates it from the chasis.

[automart 10]

Well, grounded the fuel line to the unit itself, gas tank, and frame again, each time the gauge went to full. Now what? Thanks

[Pakrat 1,043]

Than it is 100% the sending unit so you might as well remove it. Refer to the tests listed and have at it one at a time testing the arm and float and so forth while it is uninstalled but still connected and ignition on.

[automart 10]

ok, well I haven't purchase the ohm meter yet, I tried the other sending unit, not in a tank, the one I thought was bad, and it still will not go further than 1/4 and 1/2 tank with the float all the way to the top. I took the unit apart and cleaned the contacts but it didn't change anything. The sending unit in the tank is doing the exact same thing. Do I still need to purchase the ohm meter or is it something else?

 

Thanks

[Pakrat 1,043]

That's totally your call and decision. Are you sure the arms move freely and aren't binding? It would be good to know the ohm reading for certain and everyone with an old car should some type of multi testers anyway, they always come in hand for trouble shooting. I suppose you could just buy a brand new unit if you want to and hook it up outside the tank and if you have the same issue then you could still return it and keep looking else where.

 

Because you have the same issue with two units my first instinct normally would be to look somewhere else but because you replaced the thought to be bad unit with a used one it is not impossible that both are indeed bad.

[Carguy960 10]

Thanks for the tips mine reads 3/4 tank when full & 1/4 tank when empty.

[automart 10]

I ordered a new sending unit from Mustang Unlimited Tuesday. I'll let you know what happens.

[automart 10]

I installed the new sending unit, that wasn't the problem.

 

I'll get the meter but what will it tell me, a short in the wiring, gauge not working properly?

 

I guess replacing the gauge will be the next step. I do have the factory tach and working. The fuel gauge will probably be hard to find.

 

Thanks,

 

Ronnie

[Pakrat 1,043]

I'm not sure where to find a stock 69' gauge actually but I really don't think that is the probelm. Honestly I don't know what to think at this point. The odds of 3 bad units is not very likely. If the gauge was bad it shouldn't have pegged when you grounded it out. That should rule out a bad fuse, gauge or a break somewhere in the line from the gauge to the sending unit. It's a simple system and there really aren't any other culprits.

 

The only thing I can suggest at this point is to trace back the power line and look for any kinks or breaks in it.

[automart 10]

It's fixed! It was the "IVR", instrument voltage regulator or the back of the dash panel. It also works the temp and oil gauges. I've already messed up my water temp sending unit on the engine. I'll have to remove the alt, brackets, etc to drill out the old sending unit.

 

Hopefully this will help you others.

[Pakrat 1,043]

Are you talking about the printed circuit board? So how did you finally determine it?