This may be a dumb question but they say the only dumb question is one never asked. Does the fuel gauge itself need to be grounded to the tank, or is simply grounding the tank to the cab good enough?

The reason I ask is I had a "dead" fuel gauge, no needle movement at all. I noticed the PO did NOT have a ground wire from tank to gauge, but he did have one from the cab to the tanks sending unit mounting screw, which has a rubber gasket so I thought it may be preventing ground so I moved the ground wire to the tanks flange and I got needle movement on the fuel gauge! But it just goes past Empty than stops (with half a tank).

Will pulling the instrument cluster and running a ground wire from tank to gauge make any difference? Thanks.

 

It shouldn't be needed, but you never know!
Generally speaking, the ground is simply a pathway through the sending unit. The gauge itself does not need to be grounded directly to the sender, the single wire from the sender needs to have a good ground so that the small current passing through the gauge first, then the sender, will have a path.

So you have power to one post on the gauge, a wire from the other post on the gauge to one post on the sending unit, then the other post goes to ground.
They are likely polarized, so you need to make note of any markings or follow Autometer instructions. For the sender, making sure the ground has that good connection to the body is critical. So the wire should come off of the sender, the go to the body if an in-cab tank, or the frame if a chassis tank.

Now, since you say Autometer, is this an aftermarket under-the-dash type gauge? If so, you need to verify some things.
First off, is the gauge made for the Ford sending unit? And is it still a Ford sending unit?
The Ford rating is roughly 70-10 ohms, so the gauge needs to match that, or the sending unit needs to be whatever the gauge is. Probably four or five common ohm ratings in the aftermarket stuff, to cover all the usual suspects from the factory.

The other thing you need to verify is where the gauge is getting it's power from.
The factory gauge likely used a reduced voltage. Many Fords were in the 5-7 volt range, fed by an IVR (instrument voltage regulator) also called a "constant power supply" and the gauges were dampened so they reacted quite slow to variations. Whereas an Autometer might be expecting a full 12v to make it work.
You'll need to check those items first.

And important to any vehicle, and important to check the condition of, in older trucks like yours where a PO may have had their way with the wiring, is to make sure you have good grounds from the start.
Sorry if you're already familiar with all that, but I'll run it down anyway.
You need the main batter cable to the engine block of course. Most have that even after a PO has messed with it.

But you also need a body ground right there from the battery. Usually this gets left off after a few changes. So if you don't have a good wire of at least 10ga from the battery negative to the body, add one. This is usually to the inner fender skirt, either up by the battery hold down, or I like to put them under one of the attaching bolts on the starter relay. Grounds the body and the relay in that case, for a good reliable contact for both.

Another critical one is from the back of the engine to the firewall. Usually a bolt on top of the intake manifold (but not an intake-to-head bolt!) and a screw on the firewall is all that's used. Just make sure it and any others have a good clean surface to work with.
If rust is an issue in your area, a good cleaning first, then attaching grounds, then a quick spritz of spray paint over the top to keep rust at bay for a little longer is a good practice.

So that's a couple of things to think about anyway. Let us know what you've got and see if any of that makes sense and works.

Good luck

Paul

 

If the Autometer gauge is wired for a full 12 volts and the sender is an OE unit the 12 volts from the gauge will burn out the sender. The OE senders had the voltage supplied through the IVR which reduces the instrument cluster to 6 volts +/-. If the gauge is wired through the IVR then it is only receiving 5 or 6 volts when it requires 12 volts to operate properly. If the Autometer gauge is wired for into a 12 volt supply, then the sender needs to be changed to a sender that operates on 12 volts with an ohm range that is compatible with the gauge. When I added the second OE tank/sender I researched this quite a bit as I was going to add an Autometer gauge to monitor the fuel level in the second tank. I ended up running both tanks through a SPDT switch to use the instrument cluster gauge for both tanks. I grounded the aux. tank to the cab and that works fine. You may have to pull the sender and test it with an ohm meter to be sure it still works (10-70 approximately). If the PO just thought a gauge could be wired up and work then I guess you get to fix the fix, like most of us get to do on occasion. The PO had hacked the wiring on my Bronco so severely it needed a total rewire, so, I feel your pain.
Mark.

 

You don't need to ground your gauge at the tank. Pick a suitable spot at the dash. As stated before, make sure you have a 12v supply source for the meter and the backlight. The gauge should be specific to the Ford sending unit. Also, be prepared for the levels to bounce up and down on the gauge. There are other threads that discuss this part in detail. Good luck.

 

Thanks for the detailed replies guys. Looks like I have some investigating to do and i'll report back what I find out. Thanks again!

 

OK, here's what I was able to learn in just the last few minutes. The AutoMeter gauge is the correct one for the application part #3515 for the early Fords with the 73-10 Ohm scale. The sending unit is the OEM piece. So in theory this should work, correct?

I am going to look online for the AutoMeter wiring instructions and re-do my quick wiring job from last night when I was only testing the gauge and see where that gets me. Fingers crossed!

 

Yes, as far as reading goes. But you'll need the 12v discussed, rather than the normal output of the IVR to run that instrument.
I had never heard of the original sending units being sensitive to higher voltage like espy was saying, but that's very good to know.
So you may end up having to test the sending unit to verify it's current function just to be sure.

Do you know approximately how many gallons you have in the tank right now? An ohm-meter on the two posts should read approx. 10ohms if it's full, and approx. 70-73 or so when full. If I remember a chart from the other day that someone here put up, you may see about 47ohms at half a tank.
This last bit is not for sure though, as the in-cab tanks were pretty consistently shaped and the under-cab tanks were not always so consistent. So many (all?) of our Ford senders are considered "non-linear" in that they don't just drop their resistance values consistently from full to empty.
This is especially true for odd-shaped tanks like under Broncos and such, so not sure if yours would be the same way or not.

Paul

 

That's exactly what's throwing me for a loop...the 12v issue mentioned here by other members. I have read conflicting info on the matter, some stating 12v will burn up the OEM sending unit and others stating it will not, claiming the OEM sending unit can tolerate the higher voltage. (I have yet to test my sending unit).

Not to confuse matters even more but in this tech article... Fuel Tank Sending Unit Tech - FORDification.com ...the writer claims the AutoMeter fuel gauges are specially designed to work with the OEM sending units. Does this mean the gauges have a built-in resistor to drop the voltage? or when he says "specially designed" is he referring to the specific Ohm readings for various manufacturers? He's not really clear on that.

 

Whenever someone says that the gauges are designed to work with the OE sending units, they usually are talking about the ohm range compatibility. I've never heard of them reducing voltage, but have never checked either.
However, that's an easy thing to verify with a volt-meter. If you have 12v going in to the gauge and anything near 12v coming out, then there is nothing inside that's there to reduce voltage levels to the OE rating.

And that's where I have always gotten my feelings about the old sending units being fine with 12v too. Not that I'd ever heard that specifically, but until now had never heard of anyone having a problem. Or being required to change their sending units out because they were installing an aftermarket gauge. Only if an old one was not working.
That all doesn't mean there isn't some issue, but that it's maybe not that common. Perhaps there have been some failures and they were blamed on the voltage, when it could in fact have been some other reason.

Don't know, but either way I would not hesitate to use an OE sender at least initially, if that's what I had. Only change it out if it's necessary.
I would still test the old sending unit however, as it could be one of the reasons your gauge is reading wonky. Test, test, test... That's what we do with our trucks more than drive them!
Old stuff fails. And new stuff fails more!

Paul

 

Thanks for all your help 1Ton. I am off to check/clean grounds, re-wire, and test...test....test! I will let you guys know how I fare.

 

Sounds good. Let us know what you find.
And don't forget to just check the gauge input voltage right off the bat. If it's the lower voltage, that could be the major part of the problem. If it's a full 12v, then you'll have to keep digging up the line until you find the fault.

Paul

 

Turns out the sending unit is faulty, easy enough fix I thought, but after an hour on the phone/online it appears the sending units for the in-cab tanks are simply unavailable. No one has one nor can they get one. Searching multiple online vendors came up empty as well, they don't even list it in their catalogs.

I can't be the first to experience this, what have other guys done in this situation? I am going to make a junkyard run tomorrow but I don't have my hopes up on finding much. Does anyone have one they would be willing to sale? (working of course lol).

I was going to attempt a repair as per the tech article I posted above but not only is my metal wipe broken off the shaft (easy repair with a soldering gun) but my resistor wire is broken in two (not repairable). You can see the damage in the pics below.



metal wipe broke off shaft


resistor wire broke in two

 

Sending unit repair: https://www.bobsspeedometer.com/1/120/services.asp

 

Thanks for the link! I may end up going that route if the price isn't too ridiculous. I am going to try the junkyard first though and see what I come up with.

The metal wipe would of been an easy repair, I believe I could have also repaired the resistor wire were not the plastic piece it's wrapped around broken too. The plastic piece keeps wanting to "rebound" to its broken position and obstructing the movement of the wipe.

 

Anytime! Have a Ford day.