Yet another fuel gauge issue

[broncobuddha]

Okay so here goes

Have a 73 that I have owned since 1996. It came from the factory with two tanks but it never had the auxiliary tank as the previous owner had dropped it due to leaking and I never got it so I have only ever had the main tank.

The fuel gauge has at best since I've owned it. Read 3/4 of a tank when full and then I would just have to guess. Many years ago it just stopped working all together and it reads below empty all the time. No movement whatsoever and I carry a Jerry can around in case I happen to run out.

After feeling like I was going to run out of gas last night, I finally just said screw it. I'm going to dig in this today so here's what I came up with.

To the best of my knowledge, my fuel tank is original. The sending unit is original, The gauge is original and the wire is original along with the voltage regulator for the gauges.

First up, I got behind and took off the wiring harness to the sending unit and I tested the ohms at the sending unit and it literally reads zero.

I then with the key on tested the ohms resistance at the harness that plugs onto the sending unit and I could see that it was pulsing like it should, but I have an analog gauge and it's really hard to tell what's what. But at least I can tell that the wire is probably good and the voltage regulator is at least working.

I'm not sure on the gauge end but I seem to be focusing now on the sending unit since I was getting no reading whatsoever on the sending unit at all. So I'm thinking you know for 50 or 60 bucks it's worth it to replace it. Hoping I can do this while it's still in the car without dropping the tank. I will see but I just wanted to get the kind of consensus that based on what I'm seeing if everyone else also thinks that it's a bad sending unit.

My only other question is would a bad sending unit show 3/4 full when it was completely full? Or is that also potentially a problem with the gauge?



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[67sport]

When you tested the sender under the truck, are you sure it read zero, and not open or infinite? If it read zero, your fuel gauge would read full, if all else was working.
If you short the sender wire going to the gauge, does your gauge go to full ? That is a free way to help figure out what parts may or may not be working. Other stuff that may help is having a close look at the tank selector switch, because often having corrroded contacts on it will prevent the signal from getting to the gauge. Low resistance should read as more fuel, but high resistance (from bad wiring, swtches, contacts or corrosion) will read as low or empty.
I will be following this, as I believe the general consensus is that the new senders Do not work very accurately, even if you 'tune` them by bending the arm when you install them. I have a new sender in the rear of one of my trucks, and a new sender in the front of my other truck, and both act more like a fuel switch than a gauge. I know i have some fuel, until it drops to jus above E, and then i better get some quick! Drives me crazy.

 

[broncobuddha]

Well it's an analog meter. There was zero movement when testing. I have not tried messing with the gauge.

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[DirtDonk]

Gauges fail, wires fail, switches fail, regulators fail and the sending units fail. Of all those far and away the sending unit is the most common culprit. Followed closely by the dash mounted switch.
And those two followed only by deteriorating grounds of course…

You’ve done some testing already, which is good. But you don’t usually use ohms settings for checking circuits with power in them. Maybe that can tell you something, but not sure.
Neither here nor there though, because the gauge reading 3/4 full is an absolutely classic sign of a misadjusted sending unit.
They probably came that way from the factory sometimes!

The change you had when the gauge stopped reading at all could be other things, but I’d still check the sending unit first. The float may have come off or started leaking.
Yes you can pull the sending in it out of the main tank without dropping the tank. And it’s good that yours sounds like it’s almost empty still? Bonus.

But first do it was suggested and go back to the electrical connection on the sending unit and remove it. Place a jumper wire securely into the two contact cavities, then go up and turn the key on while you’re watching the gauge.
If it goes to full now, you know it’s a problem with the sending unit.

 

[i3adme]

When I found out my float was sunk, I flipped the pigtail to the sender around. It doesn’t fit all the way on but it’s a enough to make a connection to the gauge. Since the connection was backwards my gauge read full. So that gave me confidence something was wrong with the sender.

 

[broncobuddha]

DirtDonk, you nailed it, brother. Just put a jumper wire on the harness, turned the key and the gas gauge went all the way up.

So, that's more evidence that it's my sending unit.

Only question is getting it calibrated. If the old unit only read 3/4 then bending the new arm to match isn't going to solve it. Any tips there?

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[DirtDonk]

Bending the arm is only part of the process. But in fact can do the whole job in some cases.
The other part of the job is to, if needed, tweak the stops. Or in your case just the upper stop to allow the arm to go a little higher.

First things first though. Either take the old sender out now and make sure it’s not just a float issue, or order a new one right off the bat and have it ready to test before you even remove the old one.
Whether new or old, move the thing through it’s full motion and watch the gauge to see what happens. If it still stops at 3/4, see if you can kink the stop a little bit to let the arm go farther.
And kink is the word there, since as far as I know they are all an enclosed loop rather than something that is easily bendable. That’s where bending the arm comes in…
You might have to very carefully and with strong needle nose pliers most likely, double bend the arm where it passes through the stop so that

 

[broncobuddha]

New one is already in hand. I figured $60 was worth it. (Plus I live like 1/2 a mile from Duff's.)



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[DirtDonk]

Excellent! Both in the parts acquisition, and the proximity to Duff's!
Definitely do that test then. Easy peasy...

Let us know how it works out.

Paul

 

[broncobuddha]

So I tested the new one tonight and out of the box it only reads at 1/2 a tank, lol.

So I started bending back the upper stop (tab). I had it bent practically all the way back and the best I could get was right in-between the 3/4 and Full markers.

Question. Does the upper stop even need to be there? In theory the arm will never go higher than the gas level in the tank.

Bending the arm seems like it would serve no purpose unless to kink it right at the spot where the stop is to allow just a tiny bit more travel.

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[DirtDonk]

That’s actually what I did. Works for me but mine didn’t start at half tank. I didn’t have to go that far.
And oh by the way, as much grief as I give imported parts, I had to do that to factory originals!

 

[DirtDonk]

But you’re probably right. You could probably remove the stop completely and not mess anything up.
Although keeping it in that track might also keep shaft from coming out of the center. I don’t know how else it’s anchored into the rheostat. You might double check that before cutting.

 

[broncobuddha]

So you just remove the upper stop completely?

Also a buddy of mine suggested taking the side cover off and seeing where the contact points are on the windings and that you can potentially clock those as well.

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[DirtDonk]

Good suggestion. I don’t think it would be easy to clock the thing, but since it’s been so long since I have had one apart I don’t really remember.
Might be worth a shot. You could start by taking your old one apart first.

By the way, what was wrong with your old one? Or is it still in the tank?

 

[DirtDonk]

So you just remove the upper stop completely?

I think that was your suggestion. Not mine!
But if you find that the float arm won't just pop out of the rheostat if left un-retained by the slot, then I would think you could lose the limiter.
Just want you to be diligent in your checking before you make such a permanent-ish mod. Not impossible to put it back maybe, but won't be easy.
Measure twice, cut once...

paul

 

[broncobuddha]

Old one still in tank.

I was just clarifying about the stop removal. I don't think that's the route I'm going. I'll see about taking it apart or bending the arm to get it to sweep just a little more towards full.


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[broncobuddha]

So I got the arm to read the way I want it.

I installed it. I got a smaller seal that came with the new unit and I ordered the Factory one which is much thicker.

Can't seem to get that thicker one in as it's already as high as the keyed notches in the tank neck.

I've installed the old seal and the new one. Both leak.

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[DirtDonk]

Well great news on the arm. Bad news on the sucky leak.

Are the seals round profile or square? I know they’re shaped in a circle, just wanna know what the cross-sectional shape is. The round ones tend to walk when they’re being tightened and the flat ones tend to seal better if I remember correctly.

 

[broncobuddha]

Paul,

The shape of the original seal that came out with the old unit was round.

The new smaller seal is thin but flat....thin about a rubber band sitting on it's ridge. Very flimsy.

The thicker seal is flat.

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[broncobuddha]

Bringing this one back to the top.

Sorry I didn't update everyone after I got the leak taken care of.

Fast forward to lately, where the gauge stopped reading altogether.

Worked fine for a couple of months and then seemed to not work suddenly.

Troubleshooting steps taken so far:

Tried to crimp the factory harness to get a tighter connection.

Pulled the factory plug off of the sending unit and put a jumper wire in between the two connections. Key on, and no movement. (when I did this test previously back when I had leak issues, the gauge would go all the way to full.}

I cut the factory rubber plug off and cut out the 90* connections out of the old rubber and butt spliced them back to the original ground and gauge wires.

Then I removed the gauge wire from the sending unit, left the ground connected and ran a long jumper wire with gator clips on both ends from the post on the sending unit and connected the other end directly to the back of the gauge. Key on and the gauge swept all the way past full.

I've yet to test light or volt meter the connect at the sending unit to see if I'm getting the 5.5/6 volt pulsing. That's next.

As I see it, it's either a problem with the orange wire or potentially an issue with the IVR.

The IVR is it a replaceable flasher type of component?