Faulty Fuel gauge

[Shox Dr]

The fuel gauge on the pickup is not reading correctly. before I describe the fault a little background info.

It's on a 1950 GMC pickup that's been converted to 12v Neg from 6v +. there is a resistor inline, I have yet to find the value for.​

So the fault

It never drops below 5/8 full while the ignition is on. Ignition off it drops to Zero. I've had the sender to pieces and all looks ok.

Help please, suggestions and draws on the back of a pack of Capstan Full strength please​

 

[rtbcomp]

What's the basic circuit? Presumably live to the resistor, resistor to gauge, gauge to sender, sender earthed in the block tank.

Do you know if it's a magnetic or hot-wire gauge?

 

[Shox Dr]

Just in the process off getting the gauge out.

 

[rtbcomp]

https://www.fillingstation.com/articles/fuelgaugetesting.htm

From the above article

"Check to make sure that the proper 6 or 12 volt power is present at the ignition side of the dash gauge. If changing from 6 volt to 12 volt, be sure to install a special fuel gauge resistor inline before hooking up to 12 volts. Make sure to have a good ground at the sending unit, fuel tank and also the dash gauge."

 

[Shox Dr]

I've got the gauge out of the pod, by the looks of it they have swapped over the wires leading to the coils, so it's now Neg earth.

The resistor might well be a Diode: Black with silver ring at the feed end?

 

[rtbcomp]

You shouldn't have to reverse the connections if it's an electromagnetic moving iron or a hot-wire type. If the resistor was a diode it wouldn't work at all unless they reversed that as well. Did you check the link in my last post? It mentions changing the resistance if you change to 12 volts.

Your description sounds like a diode, have got an ohm-meter to test it with?

 

[Shox Dr]

No Flittin from house to workshop to truck all the time. Will do when I get a min. Cheers

 

[AndyStobbs]

I would remove the sender, and check that out to be certain its still functioning across its whole travel

 

[rtbcomp]

It only happened after he converted to 12volts, and you need to change the resistor, so until that's done there's no point in looking for faults that may not be there.

 

[AndyStobbs]

It only happened after he converted to 12volts,

Not how I interpreted it. I got the impression this is a new fault but the conversion is as old as the hills?

 

[rtbcomp]

We need an answer on that one then!

 

[Shox Dr]

Correct. I bought the truck converted, it has alway had this problem, and as I'm in the middle of rewiring it...

 

[rtbcomp]

We're both right then!

 

[Shox Dr]

Quick update

All working, rewired the dash inc the tank and gauge, so looks like the 64yr old wiring was at fault.

Tis a double edged sword, I can now see that last half of the tank disappear