Gas gauge issue

[truckeroy]

My 64 gas gauge reads 1/2 full when it is full, but once it gets to 1/2 it reads accurate down to empty. I took the sending unit out and it reads the same when I move it manually. When I ground the sending unit wire it goes to E. When is is just disconnected it goes to F so the gauge itself seems to be OK. I'm assuming the sending unit is defective, but strange that it reads only to 1/2. Any ideas on what is going on? Thanks.

[Palf70Step]

Measure the resistance of the sending unit in the various positions. I believe to register full it should be around 90 ohms out of the sending unit and zero for empty.

[chevy_mike]

If I remember correct, '64 should be 0-30 ohms. I think '66 was the first year with 0-90 ohms but I am sure I'll get corrected on that. LOL

[WOLFMANmike]

Originally Posted by chevy_mike

If I remember correct, '64 should be 0-30 ohms. I think '66 was the first year with 0-90 ohms but I am sure I'll get corrected on that. LOL

mike you are right ... mine is a 30 ohms and mine is a 1965

[ray_mcavoy]

Originally Posted by chevy_mike

If I remember correct, '64 should be 0-30 ohms. I think '66 was the first year with 0-90 ohms but I am sure I'll get corrected on that. LOL

You're close 1966 GM trucks were still using 0 - 30Ω senders/gauges even though most other GM cars had changed over to 0 - 90Ω that year.

So when measuring a sending unit in one of these '60 - '66 trucks, it should be close to 30Ω in the full position and close to zero empty.

Originally Posted by truckeroy

When I ground the sending unit wire it goes to E. When is is just disconnected it goes to F so the gauge itself seems to be OK. I'm assuming the sending unit is defective, but strange that it reads only to 1/2. Any ideas on what is going on?

The fact that your gauge reads "E" with the sending unit wire grounded and goes to "F" with the wire disconnected is usually a good indication that the gauge is okay.

However, there is one situation where the gauge can still pass that test but still not read correctly. That happens when the resistor on the back of the gauge fails (or isn't making a good connection).

On the newer 0 - 90Ω gauges, a failed resistor will prevent the gauge from reading much over 1/4 tank. I have never dealt with one of these older 0 - 30Ω gauges with a failed resistor to know exactly how high they'll read in that situation. But it would be worth checking before you replace the sending unit.

[Palf70Step]

Originally Posted by ray_mcavoy

You're close 1966 GM trucks were still using 0 - 30Ω senders/gauges even though most other GM cars had changed over to 0 - 90Ω that year.

So when measuring a sending unit in one of these '60 - '66 trucks, it should be close to 30Ω in the full position and close to zero empty.



The fact that your gauge reads "E" with the sending unit wire grounded and goes to "F" with the wire disconnected is usually a good indication that the gauge is okay.

However, there is one situation where the gauge can still pass that test but still not read correctly. That happens when the resistor on the back of the gauge fails (or isn't making a good connection).

On the newer 0 - 90Ω gauges, a failed resistor will prevent the gauge from reading much over 1/4 tank. I have never dealt with one of these older 0 - 30Ω gauges with a failed resistor to know exactly how high they'll read in that situation. But it would be worth checking before you replace the sending unit.

Good to know. I either didn't remember the resistance change or never knew it. Glad to have gained some new info, especially the gauge resistor part..

[truckeroy]

I just put the ohm meter on the sending unit. It reads just shy of 30 ohm at empty and 25 ohm full?

[chevy_mike]

Originally Posted by truckeroy

I just put the ohm meter on the sending unit. It reads just shy of 30 ohm at empty and 25 ohm full?

Bad sender then.

[truckeroy]

Thanks. I took it apart and everything looks OK. I bent the point contact out so it would make better contact with the windings, but no difference. I guess the small windings must wear out or break after 50 years of use. Guess it's time to buy my truck a Christmas present.

[65chevysub]

Hi everybody, and Happy New Year.
Sorry for breaking in on this thread but I have an issue with my gas gauge too. When my tank is full, it is steady, but when it gets to about half, the needle starts going back and forth like crazy. I have removed the tank a few times, and the tube is a little loose and I've had to bolt the wire on to the senser. I'm afraid all the fluctuating is going to damage my gauge. Any ideas of why it's doing this? The ground is good. The one positive about it is when my guage reads 1/4 I still have almost a half tank.
Thanks in advance for any help and advice.

[Palf70Step]

The swaying gas gauge is fairly normal. These tanks have no baffles and the gas moves around a good bit, thus giving the moving gauge syndrome. You kinda lkearn how the swing is compared to how much fuel is left.

[65chevysub]

palf70step
Thanks for the reply. The needle osnt judt swaying, it's really fluttering back and forth. It never did this until I removed the sender a few times. Even on my original sender after zip-tying the housing together, it didnt do the fluttering. Could it be because the intake tube is loose? I've JB weld it to the flange but it broke loose again.

[Palf70Step]

Originally Posted by 65chevysub

palf70step
Thanks for the reply. The needle osnt judt swaying, it's really fluttering back and forth. It never did this until I removed the sender a few times. Even on my original sender after zip-tying the housing together, it didnt do the fluttering. Could it be because the intake tube is loose? I've JB weld it to the flange but it broke loose again.

Not sure. The only other thing that comes to mind is does the gauge have a good electrical ground. My truck has done sooooo many weird things because of grounding issues. I added a grounding cable to the gauge cluster and it help with a lot of my weird gauge issues.

[nsb29]

Originally Posted by Palf70Step

The swaying gas gauge is fairly normal. These tanks have no baffles and the gas moves around a good bit, thus giving the moving gauge syndrome. You kinda lkearn how the swing is compared to how much fuel is left.

I had a 65 f100 with a 4 camer in it the pomp was in the tank the gas gage did not work so I got to know how much gas I had by how fast I could turn the corner