36 2dr crazy gas gauge reading

[dean333]

With the ignition on, 6V at sending unit and gauge seems to read normal. With the engine running, the gauge pegs past full out of sight. With the wire disconnected at the sending unit and ignition off, the wire at the sender shows continuity to ground. Isn’t that the same 6V with the ignition switch on whether the engine is running or not??

[Mickd]

Mate, you've got a short to ground on the wire from the gauge to the sender unit when the engine's running. Don't know why it's only happening when the motor's running - that's strange. Might be a vibration somewhere causing the sender wire to short to earth somewhere. There are a few articles on the forum here about the Ford fuel gauges. Have a search. You'll find them. If you're technically minded it might make sense to you, but if not, then you need to know this:


These gauges (King Seeley gauges) have 6V on one terminal of the gauge when the ignition's on, Doesn't matter which side!) that goes through a winding on a bimetal strip inside the gauge then comes out on the other terminal of the gauge.

The wire on this other terminal then goes back to the tank unit (to that single terminal on the top of the sender unit). Inside the sender that terminal goes through another winding around a bimetal strip and the other end of the winding is physically bonded (connected/soldered?) to the actual bimetal strip itself. That strip has a contact (like the contact points in your distributor if you like, but a lot smaller).

When the bimetal strip has no current going through it, the contact is closed - which connects the strip (and consequently, the far end of the wire) to EARTH.
So the 6V from the ignition has gone through two windings (around two bimetal strips) to earth.
That has the effect of heating the two bimetal strips. As the one in the gauge begins to heat up, being bimetal it'll start to bend - pulling the needle on the gauge with it.
At the same time the bimetal strip in the sender unit starts bending also. But as this one bends it physically opens the contact.

So the circuit goes open - current flow ceases -strips cool down and straighten out again - fuel gauge needle begins to go back down again - the strip in the sender unit straightens back out again - causes contacts to close - which causes current to flow again and the whole cycle happens again, and again, and again.

If you watch any Ford fuel gauge for a while with the ignition on, you'll notice that the needle is constantly moving, but the bending and straightening of the needle is so dampened, that you don't normally notice that tiny movement happening all the time.


Back to the sender unit.

Depending where the float level is, there is more or less pressure applied to the bimetal strip (Via a bellows and a little steel rod, or arm), so with more pressure applied to the strip - that is, more gas in the tank, so the float is higher, it takes more bending of the strip before the contacts open, so both strips have current through them for longer, which means the strip inside the gauge consequently pulls the needle further up, giving a higher reading on the gauge.



Now. Getting back to your problem.
Remember the 6 volts from the ignition is flowing through TWO WINDINGS IN SERIES. Which means that it shouldn't go past , or very much past FULL. If the needle goes way hard past FULL your sender wire isn't going through that second winding - the one inside the sender unit. It's finding EARTH somewhere before it gets there ! Now that could be a break in the insulation of the wire somewhere along it's route to the tank, or a short inside the sender unit.
Actually, don't let it sit hard past full for very long, because it will overheat the winding inside the gauge and you'll end up with a blown gauge. Remember that winding is in series with the sender unit winding, so they would normally share the current. If the sender wire gets to earth instead of through the sender winding, then the gauge winding cops all the current instead of half of it!


So try this:- Disconnect the sender wire from the tank unit. With the ignition on the gauge should then read empty. If you touch that disconnected with to earth you'll see the needle pretty quickly go up past full. Best not do that, or as I mentioned earlier you'll risk frying the gauge!


If it stays on empty, wiggle/jerk the sender wire (stick a piece of tape or something over the exposed terminal end in case it dangles against earth somewhere. If the needle moves at all when you do this, then the intermittent short is somewhere in that wire - from the gauge to the tank.
If that's all good, connect it back onto the sender then tap the sender with the back of a screwdriver or something. If the needle shoots past full, you've got a crook sender unit.
One other thing to check is make sure the sender wire isn't earthing on the back of the gauge. I guess that's a possibility.


So there ya go. It took me bloody near an hour to write this spiel, so I hope it helps.


good luck. Let's know how ya go !


mick

[Mickd]

Oh... and you WILL measure 6v on that sender wire as you found, because you're reading that voltage through the gauge. If you reconnect the wire back on the sender unit, you'll get a lesser voltage reading. Dunno what what, but it'll be less than 6v.

You see, what's happening is there's 6v there as you found out, but because you've lifted the wire off the sender unit, then there's no load presented to the 6v supply, so you'll see the full 6v.

Funny stuff electrickery.

[Mickd]

Dean333... How did you go with your fuel gauge problem?

Cheers

Mick

[dean333]

Today I connected a ground wire from one of the sending unit screws to the trunk floor and the gauge seems to be reading correctly.

[Mickd]

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Ahha young Tim !
Crook earth by the sound of that. Make that fix permanent.

Cheers
Mick

[dean333]

Mick, the ground wire isn’t going anywhere. It’s installed under the lid that covers access to the sender, same screw.

[Mickd]

So your piece of wire goes from one of the screws holding the sender in, to the floor of the boot - mechanically connected at both ends?
If so, then that's telling us that the tank isn't earthed like it should be.
The tank is held up by steel straps, but between those straps and the tank metal are rubber strips. They're there I guess to prevent the the thing rattling I guess. The only problem with that is, it's insulating the tank. If there's any rust or paint at the top of the tank or down the filler tube then the tank is battling to find an earth. When the engine's running the car, tank etc are obviously vibrating enough for the tank to contact earth somewhere.
As I say, that wire you installed is providing that earth. Leave it in.

Bad earths on cars can throw up some strange faults. I remember an old Falcon a mate had, the front indicators (in the bumper) wouldn't work intermittently.
I found that the front bumper wasn't earthed because the big mounting bolts were rusted. Removed them one at a time, hit the subframe where they mountedwith a wire brush, same with the bolts and threads, put them back in and VIOLA! Good indicators.
90% of electrical problems with old cars could be put down to bad earths. (The other 10% are christ knows what!)

Cheers
Mick

[dean333]

Problem NOT fixed. The gauge appeared to read correctly as the car sat and idled. It showed 3/4 full which I felt was correct. As I drove today it slowly creeped up past full and out of sight. Back to square one.

[petehoovie]

Is yours a King Seeley sending unit???

[dean333]

How would I know?

[Mickd]

Sorry Tim. I went off on a bit if a tangent there. Yeah. Yours is going past full. Like I said earlier, that's not a bad earth problem. That's a FULL earth on the sender wire. You've either got that wire shorting down to earth somewhere along it's route from the gauge to the sender, or the sender's faulty inasmuch as it's shorting the wire to earth INSIDE the sender unit itself before it gets to the winding around the sender.

Petehoovie, I hope it's a KS mate. If it's not then I'm guilty of sending Tim on a wild goose chase. According to a "Motors Auto Repair Manual" 1935 - 1949, the King Seeley gauges were used from '36 onwards. '35 was a King Seeley "Liquid" unit. And of them.... I know nothin'!

Do me a favor..... disconnect the wire from the sender and insulate the end of it with tape or something, then start the car, take it for a spin and tell us what happens then. (It SHOULD stay south of empty as if the ignition is off).

[petehoovie]

Originally Posted by dean333

How would I know?

Looks like this >

[petehoovie]

Originally Posted by dean333

How would I know?

See this thread > https://www.fordbarn.com/forum/showt...eely&showall=1

[JayChicago]

I like Mickd's idea to separate the wire from the sender and drive a while, to isolate the source of problem. If needle still climbs, the intermittent short is somewhere in the wire. If needle stays dead but then after re-connecting to sender problem reappears, is in the sender.

[Mickd]

I found that same picture Petehoovie but when I went to put it up on my Photobucket storage, which I haven't used for yonks I got a message that I need to pay these days Being a tightarse, (or a tightass as you say in American) I didn't bother. Don't use it much. But yes. That's the baby

[Steve in Denver]

What about completely isolating the gauge and sending unit by just running a wire from the gauge, to the tank. Should not even need to dress it out, just run the wire outside the car to the trunk (boot for those down under). If that solves the problem, then it is definitely the wire.

[terry]

And the float hasn't been mentioned yet. If it no longer floats, then the gauge will show a tank full of gas. In 36 and I don't know how much longer, float was cork and had some varnish type of stuff on it. Later on the float was changed to metal of some sort. terry

[terry]

Oops! My mistake. If float no longer floats, gauge will show EMPTY! It certainly isn't a pbm with the float. terry

[dean333]

Yeah, very weird. Sat idling in the garage 35 minutes, stayed at 3/4. Went around the block still ok. I’ll check more later, it really sounds like the sender once the gas gets moving in the tank.