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Fuel Gauge output voltage? New sending unit not working!

Mav2u

Mav2u

1/2 ton status
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Here is the run down: 89 Jimmy, TBI, 31 gallon tank. Original sending unit worked fine, it had a nicked up cable where it rubbed on the frame at one time and I just wanted to replace it instead of repairing the wiring.

Fuel Gauge and fuel pump worked fine before I replaced the sending unit with the brand new one.

I put the brand new one in and now the gauge is stuck at 1/4 or 1/2 or empty. Depending which time I drop the tank again and put it back in the tank.

When I test the sending unit outside the tank it seems to function correctly with the gauge on the dash. I checked with an ohm meter and it runs from 0 to 90 ohms range which is the correct range. 90 ohms for empty and 0 for full.

I checked at the dash and the voltage going into the gauge is 12 volts.

When I check at the connector for the sending unit coming from the dash I only get 8.75 volts. I don't know if that makes a difference as the wiring diagram doesn't show that info as this is the voltage passed from the fuel gauge.

I've tied it to another ground, directly to the battery ground and still nothing.

They even sent me another sending unit to make sure that the first wasn't some how defective. Still the same thing.

I'm tired of dropping the tank. I'm at my last straw with the stupid sending unit problem. I've dropped this tank like 7 times now.... this is the last time.

I need some help people! The sending unit I ordered from Classic Chevy Parts. It does have a slightly different resistance type swiper board but the ohm meter on all the sending units reads correct ohm output. So the reading should be correct on the dash but it is fubar!!!
 
Right, I see that test setup. But if you read my post you'll see that the gauge works!

It does not work when the new sending unit is locked into the tank. It work perfectly with the old sending unit. Neither of the two new sending units make it work. If I test them on the ground outside the tank they move the gauge correctly.

Which is why it is a mystery. Which the only thought I have is the the 8.75 volts may be an issue.

So to recap, the gauge works if I manually run the fuel arm on the sending unit when outside the tank up and down. Both the new and the old sending unit work the arm.

Tomorrow I'm going to stick the old one back in the tank just to verify it still functions as it did before I started this headache!

I will be building the tester tomorrow just to rule out any other possibilities and report back here what else I discover.
 
If the gauge works with the sending unit OUT of the tank with 8.7V, that is not the problem. I would look more to a bad ground. The voltage drop seems a little high, so I would look for a bad or corroded connection in the bulkhead connector or somewhere along the wire.
When you install the sending unit, it doesn't hit the side of the tank?
 
Well I plugged in the make your own tool gauge tester as per the link to make a 90 ohm tester. The fuel gauge read right on the full mark. So the gauge is confirmed as still working.

The only thing I can think is that the new wiper arm on the new sending unit is some how not exactly the same as the original. Of course the lower voltage makes me wonder also if that is why the newer sending unit is not working correctly.

The grounds are good, I manually ran one straight to the battery just to check. No change. The unit is grounded to the tank as the lock ring that holds the unit in place is metal. Then the straps that hold the tank in place are also metal.

I've looked inside the tank, the baffle is 100% still in place and does not move. The wiper arm has free movement.

I place the sending unit in a bucket of water while I ohm checked the arm and it read the correct values for full to empty as I dipped it in and out raising the arm up and down.

I will show a picture of the arm of the new sending unit. I will go take a picture of the units so you all can see. It is just baffling. And extremely irritating I can figure this out!
 
The sender MUST be grounded to the FRAME (this is how they are from the factory) as there are isolator strips on the fuel tank straps that could cause a bad or no ground situation. Also remember there is a rubber gasket between the tank and sender and the lock collar might not make a good enough ground and even if it did you still have the isolator between the straps and tank.
 
What difference would it make if the gas tank was touching the metal straps on the tank?

Besides, It reads the same with just the tank sitting on the ground as it does strapped up to the vehicle. The isolator strips are merely for rubbing of the metal against metal. True the rubber seal is under the sender but the metal top lock ring smashes against the send and the fuel tank so they are grounded there also.
 
Mav2u said:
What difference would it make if the gas tank was touching the metal straps on the tank?

Besides, It reads the same with just the tank sitting on the ground as it does strapped up to the vehicle. The isolator strips are merely for rubbing of the metal against metal. True the rubber seal is under the sender but the metal top lock ring smashes against the send and the fuel tank so they are grounded there also.
Click to expand...

You answered your first question yourself, BUT if the sender was only grounded to the tank and there were no isolator between the straps and the tank then the sender would work as the ground would then be made between the tank and straps to the frame.

When you tested the sender with the tank on the ground did you hook a ground wire from the tank to the battery? If not then it will either not read at all or not correctly.

If you've held the sender in your hand while hooked up to an Ohm meter and then moved the float from empty to full and were able to get a 0-90 ohm reading then the sender is good. If you've hooked the sender wiring up and grounded the sender and then moved the float from empty to full and your fuel gauge reads correctly you have a bad ground at the sender.
 
Yes, it is grounded to the frame. The ground at the frame was tested and it is good. I've also ran another wire clamped to the sender top itself to another ground.

The unit is fully grounded to the frame so that is a check.

Yes, It remained hooked to the same ground in the tank an outside the tank when tested.

I just tested it again, it seems to be working on the ground now with the new sender in the the tank. I tipped the tank side to side and it went from empty to 1/4 tank which seems about right since I tipped the tank so the sender would not read any fuel.

I'm going to try an hoist it back up into place again for the 10,000 time and see if it still keeps reading, and reading accurately.

The only thing not answered yet is what should the actual voltage be at the connector for the sending unit? I read 8.75 volts coming from the wiring of the backside of the gauge. I know on the ignition side of the gauge I checked the voltage with ignition on it read the full 12 volts.

Perhaps the lower voltage is causing this issue with the new sender? I check the wiring all the way to the firewall and it is still top notch condition the hole way.

The fuel pump side is getting it's full 12 volts. Just the sending unit voltage is 8.75 volts. So if someone could answer that question it would help. As that is the only thing I see that is a little different. Everything else checks and looks good.
 
No ground spade like the oem, it is actually wired to the ground inside the sending unit in the tank. So, it is grounded on the sender.

I hate this tank!!!!! I'm gonna light this bitch on fire here shortly!!!!

So, with tank wedged up, one side of the tank at a 70 degrees and the other on the ground it thinks it is sitting a 1/4. If I tilt it up to about 85 degrees it says it is empty. So it does actually move now. But, if I lift the whole tank flat horizontal it only moves from a needle below 1/4 to a needle above 1/4.... It really likes 1/4 tank reading for some reason.

I took the other two sending units, hooked them up outside the tank, ran the arms and the gauge worked perfect. So, it is not the gauge itself..

Something about the damned thing in the tank. Which shouldn't matter at all. As when I hook the sending units outside of the tank, everything works perfect! Stick it in the tank, it gets stuck reading no higher than around 1/4 or zero. Which just pisses me off, cause there is no reason it should do that!
 
British Slave said:
Where did you check the voltage at the gauge? I've never had a reason to check on the back of the gauge, but remember you have a 42.5 ohm resistor across the terminals, plus voltage drop per foot of wire. It may be the 3.25 you are losing. It may be normal.
Click to expand...

I checked the connection on the ignition side of the gauge that supplies the 12 volts. Yeah, as far as normal I have no idea. The GM manual has no voltage specifications in it for what it should be.

So if someone else can go read their voltage on their truck then I guess we won't know. Ugh, how frustrating!
 
Okay, I installed the new gauge after much deliberation. The new sending unit they sent me to replace the other actually works. Even though the other one tested fine. It reads the tank settings correctly again.

Yay! Guess the made in Taiwan sticker on the box should have tipped me off. But for $54 bucks I figured it was worth the gamble. I am now an official tank dropping fool!

Thanks for the help everyone. Now on to the next fix project on the truck. Got to get her back to fully working status again. One project at a time!
 
I would still like another person to still do the voltage check if they could. I would like to know if it is normal or just a sign of something I need to fix.

So if someone could get the voltage that would be great.
 
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