Gas gauge not working, replace sending unit or gauge first?

[slam59]

My gas gauge on my 74 C10 is not working right, left me without gas over the weekend. Is it usually the sending unit that goes out more often than the gas gauge itself? I was wondering which should be replaced first? Is there a way to trouble shoot and tell which one is bad? My friend thats my mechanic on my ride says that the bed has to be removed to get to the tank to be able to remove the tank and get to the sending unit, says there are metal bands in between tank and bed that are in the way. Is this the only way to get to the tank?

[BLE 'BURBAN]

Disconnect the sending unit wire and ground it out with the key turned tto the "RUN" position. Your gauge should swing WAY past the full mark. If it does the gauge is good.
Your sending unit reads 90-0 Ohms resistance empty to full. You can pull the sender and check it with a multimeter by cliping the ground lead to the top of it and the positive lead to the senders terminal then raise/lower the swing arm float and watching your multimeter.

But yeah it usually is the sender that goes ten toes up on ya.

BTW if both check out ok, then you've got a break in the sending units wire that'll have to be repaired.

[slam59]

Thanks Ble Burban, we wil have to try that out.

[cfan10]

Never seen anyone remove the bed to get the tank. Usually you unbolt the straps and lower the tank with a jack. Sending unit is right on top, these trucks are known to have gas gauge issues.
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[tucsonjwt]

I have not found a fool proof way to test sending gauges. When I did the suggested tests they indicated bad sending units , but after replacing both sending uints I found out what I really needed was a new gas gauge. If you can get your hands on a known good gas gauge you can swap it out in about 20 minutes or less and not get dirty in the process. I would go that route first. A new gauge is about $50 + S&H at Classic Parts.

[slam59]

Yeah Tusconjwt, Id rather start with the gauge first. And yes, thats what I thought that you just undo the straps to take the tank down, unless the bolts to get to it are not accessible with the bed on?

[wilkin250r]

Originally Posted by BLE 'BURBAN

Disconnect the sending unit wire and ground it out with the key turned tto the "RUN" position. Your gauge should swing WAY past the full mark. If it does the gauge is good.
Your sending unit reads 90-0 Ohms resistance empty to full.

I'm fairly certain it's the other way around. 0 ohms is empty, 90 ohms is full. So if you disconnect the sending wire and ground it, your gauge should read stone cold empty, zero ohms. If you leave it hanging, basically creating a million ohms, your gauge should read full full full, past the full mark. If you're willing to spend a couple extra bucks and a few more minutes, you could go to Radio Shack and buy a few resistors in between 0 and 90 ohms and check those as well, if you wanted more definitive proof if your gauge is good or bad.

The bolts for the tanks should be easily accessible with the bed on, the hoses and electrical connections are more difficult, but not impossible. You just want to make sure you have the capability to lower the tank a little and remove the hoses and connections, don't drop it straight to the ground.

[tucsonjwt]

For sending units, I put some concrete blocks under the tank, then put pieces of 2X4 and 1x4s on top of the blocks until they were snug up against the bottom of the tank. Then I loosened the straps and pulled out the lumber one at a time until the tank was resting on the blocks and I could clear the truck. If you remove the bed you risk rounding out the square holes in the truck bed (unless you cut the bolts off and replace the bolts).
Or you could convert your bed to a dump bed and just lift the bed to access the tanks/sending units like I eventually did.