The Stovebolt Site Forums The Shop Area The Electrical Bay Gas gauge woes

Ok so the gas gauge is not working. I do not have a great understanding of how these things work, but here is what I know so far:

There is a black and yellow wire coming out of the sending unit (which was replaced recently by the PO). The wires go down behind the tank and I can see the bigger black one under the cab, so I assume the yellow one is grounded in the cab. When I measured the volts on the black wire with my digital multimeter on the 20 setting, I got an average reading of .06 from the wire to a clean ground on the frame.

Any ideas on what is going on? The PO said bad ground, but I would like to see if the problem is under the truck before I try to get under the tank...

You need to explain what you are working on. Stock sending units did not have wires coming out of them. There was a stud for the sender wire and it was grounded with a mounting screw.

If you are using an aftermarket sender you need to be sure it reads from 0-30 ohms.

It is aftermarket. Do I measure the resistance across the points on the sender?

measure the resistance while moving the float arm over it's full range of travel - should be about 0-30 ohms for an original gauge

the voltage goes to the gauge, not the sender, the sender provides a variable resistance to ground for the gauge

Bill

Bill,
so the gauge picks up the ohms from the ground on the gauge?

If so, the sender is grounded to the gauge?

the sender, which is a variable resistor, provides the ground for the gauge, the change in resistance to ground is what moves the needle

Bill

how does that move the gauge???

this is new to me, but also very compelling

how does your ohmmeter move? or the multimeter you used to check the ".06 volts"? > electrons
current flows from the power source thru the gauge to the ground, the amount of current flowing being controlled by the resistance of the sender .... the gauge has small coils inside that act like electro-magnets to pull the needle across

Bill

I going to jump in and try to help Bill a little. Dalyman72, think of current flow like water to your house. The water starts in a big tank on a hill somewhere (your truck battery). The water flows through pipes (your truck wiring) to the water meter outside your house (your gas gauge). The water then flows through your house pipes to the faucets (truck gas tank sender), which control how much water comes out. The water then goes drown the sink drain (ground).

Thanks guys. On the sending unit, there is the power coming in from the gauge, but the "ground" is attached to a mounting screw. Isn't the ground in the bolts holding the tank into the cab? Is a redundant ground a bad thing?

Ok, i have ran new wires from the gauge to the sending unit, and made a new ground from one of the mounting screws on the sending unit straight to the frame. On the gauge end, I connected the power directly from the accessory point on the ignition switch, which as of now successfully powers the fuse board, so it is a good power source.

But, the gas gauge still doesn't work. its registering .8 ohms of resistance across the points on the back of it. is the gauge bad? i can think of nothing else that could be causing this. it, besides maybe the sending unit not supposed to be grounded with a mounting screw... but that is common from what i have seen.

Ideas?

the sender should get a ground from it's mounting screws to the tank and the tank to the cab, and the factory setup is a ground from the battery to the cab, but it never hurts to have extra grounds .... sounds like all is as it should be, so when you turn on the key, the needle should move up, unless you have a totally empty tank .... have you tried clipping a wire on the sender terminal of the gauge and touching it to ground? if there's no action I'd guess the gauge is the problem .... the other question is whether you've checked the action of the sender and it's resistance value, you don't say but moving the float thru it's travel should give a value of 0-30 ohms [not 0-90]

Bill

What Brand after market gauge is it?
Ive been using a autometer gauge while attempting to get my stock looking gauge to work.

The aftermarket gauge has it own ground.
3 wires comming off the back, Power, sender, its own seperate ground

Have you tested your sender?
here is a video of a test I did on my sender
...video...

Its 0-30 ohm

I got it! The PO gave me an extra instrument panel from a 55, and I swapped everything except the temp gauge (because the 55 was a manual thermometer looking apparatus). But after all that, the problem was with the gauge. But, now I know the wires a well run and not about to catch on fire.