Technical Wiring help for SW fuel gauge

I bought an old SW fuel gauge off eBay and I’m trying to wire it up to my 32 Ford. There’s no markings on the back except some wrote 40 on the side. I think the sender goes to the smaller terminal on top, how do I know which one the positive wire goes to from ignition on?
thanks in advance, just a young mechanic learning to work on early fords

 

I suggest editing the thread title to something like:

Wiring help for SW fuel gauge

 

You also need to make sure you have a compatible sending unit.

 

The top stud is the ground, per this instruction sheet. Also, you need the correct sending unit, SW uses 240 ohms empty and 33.5 full.

Stewart-Warner offers different sending units, right or left hand and also different depth tanks.

 

Do you know if is 6 or 12 volt? Your hold down bracket will need insulating bushings in the stud holes too.

 

Thank you! This is perfect

 

not sure if it’s 6 or 12 and that’s a great suggestion on insulating the bracket

 

Most fuel gauges are the "balanced" type [2 electromagnet coils inside] and can work successfully with either 6v or 12v.
Once you know the correct wiring connections you can "spark it up" [bench test it] with an AC adaptor.

For this purpose I use an old 12v 1a cordless phone charger AC adaptor.[with alligator clips]
Then use a cheap potentiometer as the sender unit and you can dial it up /down on the gauge and use a multimeter to measure the resistance across the potentiometer.

With these resistance values you can go searching for a tank sender unit.

Even if you have a total brain-fart and connect it up wrong it will not damage the gauge [not at 12v 1a] The only major issues I've ever had were seized needles and broken wires [which is challenging for my eyesight to solder]
I have never seen one with a meltdown inside.

They are stupid simple in principle! With 2 electromagnets fighting each other [one has variable resistance via the sender unit]

Here is a simple schematic [compliments of the interwebs] explaining how a Temp / Fuel gauge works. The only difference is the sender [and the resistance values]
But there is not much that can go wrong with them.


Don't be scared of them [Even a fool like me can fix them]

 

I don't want to hijack this thread, but is it possible for people who know what they're doing to recalibrate fuel/temp gauges to match the resistance range of a sending unit it wasn't made for?

 

Maybe; but Tanks makes a "black" box that will make things match.

 

Firstly you try and buy the sender unit that is nearest match to the gauge. [This is half the battle won!]

Now if math isn't your Forte you can use the "applied research and development approach" [aka: let's try it and see]

With OEM gauges like 57 Chevys there is no increments in the sweep


I like the needle to be at the 11 o'clock position with normal driving [cruising] but some gauges read too cold when at running temperature. and some gauges read too hot.
So get a potentiometer with alligator clips and connect this in-series [between the sender and sender wire] and try and "tune" it to read normal.
Read the resistance of the potentiometer and solder a resister of the same value into the sender wire.

This ^^^^ usually works but sometimes you need to go the opposite direction.

If this is the case, make sure the sender is connected and then connect the potentiometer parallel [from the sender to ground] and "tune" the gauge.
Then measure the resistance of the potentiometer and make up a wire with a resistor and splice it into the sender wire [to ground]
You can hide these anywhere along the sender wire

This is also what you do if you change the thermostat in your engine and the gauge reads too hot or too cold.


Edit: with fuel gauges which are also "Balance" type gauges ,GM and Ford gauges cannot be swapped over/mixed with the sender units
GM fuel senders pull the needle down, and when disconnected read full. Ford's are the opposite.

 

If you don't know if it's 6 volt or 12 volt I sure wouldn't start by testing it on 12 volts! If it went up in smoke it's just wall art after the 12 volt test.
Get the correct sending unit and test it all wired 6 volt on a bench to see if it works. Then if it's way off you can repeat the test with 12 volts to see if it's correct.
But my question is why even bother if you don't have the correct sending unit? A sending unit for that gauge is going to be very expensive, and it might be cheaper to use a more common gas gauge and sending unit.

 

This is great tech, thank you.

 

@Kerrynzl , thanks for the sketch and explanation. Reminded me of an issue I had with old Chev pickup I drove. Just got it ready and started driving it. Read 1/4 tank, thought I'd fill it up. Only went to half-on the gauge. Mechanic friend told me of the 2 magnets in the gauge. Swapped it for one out of my parts stash, good to go. Never got in to the gauge to repair, but knowing me, I probably kept it to repair later.

 

I'm sure I read somewhere that there is a way to make a SW fuel gauge 240-33 ohm work with a GM sender 0-90 ohm. I think it involves using a certain resistance resister in line. But, of course, now that I am looking for it, I can't find it. Anyone know??? Thanks, Gene.

 

Not going to work correctly.
SW fuel gauge goes from high resistance to low resistance.
GM sender goes from low resistance to high resistance.
First, you need to add a 33 Ohm series resister to the circuit in order to prevent a overcurrent situation in the SW fuel gauge.
Second, the SW fuel gauge will read backward.
Third, the SW fuel gauge will only read about 43% of full scale.

Other than that, if you understand all this, it is much better than not knowing how much fuel is in your tank.

May I suggest that you Google: "fuel gauge 0-90 Ohm"

Russ

 

I did Google after I asked this. I found one on the Autozone web site and ordered it. Thanks for the info... VERY interestinnnk!!

 

Has anybody tried one of these gizmos? They're all over Ebay
https://www.ebay.com/itm/2345906839...8T+IUZ3KVb4jNGVmHZoksQ82Q=|tkp:Bk9SR_7igofEZA

 

Have not... but I saw Tanks offers one too