The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel Gauge problem '53 gmc 1/2 ton 12v

Sorry but could not find an answer or conclusive test on other posts. Fuel gauge worked before restoration, now cannot get it to move more than a qtr inch beyond empty when key is on. New wiring harness, ground from sending unit bolt to batty neg terminal. Checked sending unit, float appears good. 12v present at sending unit with wire disconnected. When connected only about 5v. When connect sending unit wire to ground no change in gauge. I'm not sure but looks like gauge cluster may not be grounded, is that necessary if I'm getting 12 volts to the sending unit an have the ground from tank to batty neg? I'm thinking its a new sending unit but before I purchase wanted to check with the experts.

John

The sending unit supplies ground to the gauge through resistance controlled by the float. If you disconnect the sending unit wire from the gauge and turn on the ignition, the gauge should show full. If you ground the terminal to which the sending unit is normally connected, the guage should show empty. If the gauge doen't work this way, it is at fault, not the sending unit.

If you have an Ohm meter you can check the tank unit.Remove the tank unit from the tank.Connect the Ohm meter to the tank unit terminal and the other lead to the tank unit housing.It should read 0-90 Ohms as you move the arm empty to full or full to empty.I have cleaned the dead spots up with good results.Tooth paste works well.*(but makes your breath smell like gas lol)

John,

Your 12v conversion most likely has a negative ground (just checking - you do describe the ground as negative)?

Do you have the original 6v gauge or a 12 volt gauge or a voltage reducer on the power side of the gauge?

Tim

Tim,

Yes ground as negative. The gauge has two terminals on it one has a red label against the gauge back and assume that was the IGN connection the other I connected the wire to the sending unit. I assume since the system is 12 volt (I did not convert it) that this gauge is also 12 volt. Bad assumption? It all worked before restoration.

John

John

If your sending unit is good it'll never read 90 ohms resistance. Your gauge will show full at 30 ohms and empty at about 1-2 ohms. Checking voltage at the sending unit won't tell you much.

what brand? reg. or with whitener?

If your truck has the original gauge, it would have been for a positive ground system, and in that case the wires would have to be reversed on the gauge in order for it to work, assuming that it is not now burnt out.

Baldy,

Reversed the wires on the gauge and same result, so I guess the gauge is blown. Anyone know where you can get a new OE fuel gauge assuming that's the issue?

John

John Clark,

You have measured 12 volts at the sending unit wire (disconnected), so this proves the integrity of the wire. I saw some of this mentioned above, but might clarify it. With the key ON and the wire disconnected from the sending unit, the gauge should read FULL. With the key still ON, ground that same wire to a good ground and the gauge should swing to empty.

If this doesn't happen, your gauge is defective.

You said the gauge worked before the restoration, but that the 12 volt conversion WAS NOT part of that restoration. What was the part of that restoration that could have affected this system? Somehow the actual gauge in the dash has to be grounded. This is usually through its case.

Did any part of the restoration involve any painting or such that might have insulated some component that was not insulated before.

If you do the test outlined above and the gauge doesn't respond, it is the gauge....unless it isn't grounded in the dash assembly (maybe?).

Stuart

Stuart,

You are correct in your first paragraph. As for the restoration, to my knowledge the system was fully 12v except for the starter which was converted during the restoration. There is a metal strap that goes from the gauge to the dash (not painted) that is broken which I think occured when the mechanic installed the oil pressure line to its gauge. I think someone told me I was to ground the sender which I did from one of its mounting bolts to the battery neg terminal(should be a good ground?)Assume that's not a good enough ground and that the gauge also needs a ground. What about reversing the wires on the gauge, should I put them back? The gauge does move when the ignition is on so it's trying to move which makes me think its ok.

John

John,

Are you indicating that your gauge does respond properly when the wire is removed and then when the wire is grounded. It swings full scale from FULL to EMPTY.

Stuart

Stuart,

The gauge slightly moves between empty and a quarter tank when IGN is on. This same thing happens when sending unit wire is disconnected and IGN is turned on. I know the tank is almost full so it should be registering close to full if everything was operating.

John

John,

I'm stumped. The diagrams I have seen of this type of gauge show two coils in series...powered at one end and grounded at the other end. The sending unit is a tap between the two coils. The coils act as electromagnets on the little needle, swinging it one way or the other based on the variable ground..the sending unit.

If the gauge isn't grounded properly through its case, maybe the whole deal doesn't work.

I agree with you. Some gauge movement would indicate the gauge is good and another problem might exist. A 6 volt gauge that had been cooked with 12 volts would be dead..no movement.

I don't know how a positive ground gauge would work if connected backward. I would suspect it wouldn't work at all, but would not be destroyed.

This system passes such a tiny amount of current I think it would take a real hit to kill the gauge.

If the gauge truly did have a strap or something that looked like a ground, and it is broken, it wouldn't take much to repair it and see if that solved the problem. Good luck and keep us posted.

Stuart

Stuart,

Thanks. I will try to somehow ground the gauge cluster, even temporarily to see if that makes it work. I'll alsoput back the wires the way they were as this seemed tomake no differnece. I'm trying to avoid removal of the gauge cluster since all the wiring and other stuff would need to be disconnected.

John

The gauge ground is through the variable resistor in the tank. The only thing that would require the gauge case to be grounded would be for a dash light.

The fuel gauge is nothing more than an ammeter and works the same as an ammeter when wires are reversed, except it can't go below zero when reversed like the ammeter can.

It sounds as though you have not tried to disconnect the wire to the sending unit at the gauge and grounding that terminal yet. It is necessary to do this for diagnosis. The five volts you read at the tank would probably indicate about a third of a tank, and could mean that the tank unit is not working correctly, but check the gauge first.

John,

If you are interested, and for the sake of accuracy, the enclosed link provides the blow by blow operation of the fuel gauge.

It doesn't appear to function at all like a ammeter, and does require a case ground to function. The sending unit path to ground is one of two. The difference between two ground paths is what causes the needle to move in either direction. This is how I think it works.

http://chevy.tocmp.com/1955truck2shop/55ctsm1208.htm

Stuart

Thanks Stuart.

I tried to push the metal strap from the cluster to underneath the dash temporarily to see if I could get the gauge to move but no luck. I guess I'm going to need to pull the cluster out and some how permannetly attach that strap to the underside of the dash.

John