The Stovebolt Site Forums The Shop Area General Truck Talk gas gauge

On my 1951 GMC half ton pickup the gas gauge is a problem. When the ignition switch is turned on with the key the gas gauge needle swings all the way over to full then immediately falls back to empty. If I turn the key on and off repeatedly the needle will eventually stop at the correct place showing the amount of gas in the tank. Is there a fix I can do or does this indicate the gauge is shot and I need a new one?
Silver

I guess you've already checked your ground connection to the circuit. In my opinion,90 % of the electrical problems on these old trucks can be traced to one of two things;butchered wiring or weak grounds. If the wiring appears to be good,my first move would be to test for a bad ground. If it isn't the ground,I'd check the power wire for the gauge,it might have a connection that's nearly broken. If that isn't the problem,I think I'd change the sender. Just an idea...

Which resistor are you using? Using the voltage drop ceramic thing does not work as well as a runtz.

Chris

Check the ground at the tank.

silver,

Do you have a 12v or 6v electrical system (positive or negative ground)?

Speed is right - bad grounds are the usual culprits. The gauge cluster has to be grounded and, as Tab posted, the sender has to grounded. You could run temporary test ground wires from the cluster "cage" to ground and from a sender-unit screw to ground.

If you have a 12v system, it might not have been set up correctly with a (Runtz) voltage reducer (as mentioned by 4.3) and the gauge is ruined (usually, this is "all-or-nothing" - you would get no action from the gauge).

Since the gauge is sometimes indicating "Full", the gauge is most likely good.

One more thing to do before either giving up or removing the gauge cluster and gas tank sender for a bench test: run a new/test hot lead to the "In"/"Hot" post of the gauge.

If you do reach the point of buying a gauge, before you do, remove the cluster and sender and do a bench test (you will know that your grounds and power leads good). A bench test is a pain but you might as well do it if you are going to take the gauge out.

I bet fuel gauges (bad grounding or 12v burnout)and weak starters (insufficient ground cables)are the most common electrical problems experienced by people with 50+ year old trucks.

Good luck,
Tim

I always check ground on the sender and gauges first.

If you use a regular ceramic resistor, your fuel gauge will work, but will be really jumpy as the runtz resistor corrects for this, but I'm not sure how. I have runtz resistors on my 52 and 53 and a regular resistor was on the 39 when I bought it. The gauge in the 39 is jumpy, but settles out at steady state.

Chris

Got a resistor on the gauge, and the ground on the tank looks good, but do I need to run a seperate ground from the gauge cluster frame?

shooper,

The gauge cluster/housing is normally grounded by way of the screws/posts/nuts that hold it to the (grounded) dashboard. However, over the years (or after a paint job) the ground may not be good (metal-to-metal). By running a test ground-wire to the gauge cluster/housing, you can find out if the problem is the ground.

Tim

Thanks Tim..Think I'll just rewire everything and see what happens..Take care, Steve

Usually when a guage fails, it fails completely. If it still moves around and can still make a full sweep, I would look at the sendor and wiring side of things. My 63 had some serious varnish build up on it when I got it and a new sendor and a fresh ground strap fixed every guage problem I had,
Scott

Thanks everyone. I kinda thought gauges either worked or didn't work,so you have confirmed that. Checking the ground was my next step as I have experienced bad grounds with my '35 after a new paint job. I do have a ceramic resistor, so I guess that could be the culprit. I am not familiar with the Runtz resistor but can probably get one at NAPA or another parts store? The gas tank is new as is the sending unit and the tank has been newly painted so a bad ground could be there. I'll dive into the wiring tomorrow and let you all know what I find. Again, thanks for all your help.
Silver

Unfortunately you have to pay about $15 for the runtz resistor from somewhere like AC, Jim Carter, Patricks, NCA or the like, but it works much better than the ceramic resistor. I wish someone would tell me what is in a runtz and I would just make a bunch of them for the $2 in parts. I thinks is probably some kind of xener diode(sic?) or something.

Chris

Chris,

I think you are correct about the Runtz having a Zener diode in it. AutoWire has a gallery store on eBay and sells interesting auto electrical items, including two types of 12v to 6v voltage reducers. One looks like a simple solid state component with barrel connectors on each end.

Has anyone taken a Runbtz reducer apart to see what else might be inside?

AutoWire also shows inexpensive relays from Packard Electric. Maybe Horvath will comment on this component.

Tim

that gauge reducer is a zener diode with 2 butt connectors! costs less than a buck at Radio shack.