C3 Vette Dude

My gas guage on my 79 has issues. It was registering 1/4 even with full tank, but when I used the gas lower than the 1/4 tank it would read as I used the fuel from 1/4 to empty. I read where it is most likley the ohms tab on the back of the guage. It said it might be as minor as cleaning it and putting it back on. I cleaned it put it back on and now it reads pass full with a 1/2 tank of gas in it. I figured I need to put a full tank in it so I can drive it and reset the trip odometer so I could keep track of it how much gas I had. I went to the gas tank to check the connections I took them off cleaned the connections over the tank and now it reads a 1/2 tank. It has a full tank!

Korevette

Sounds like it may be a 'connection' problem on the wiring as it gets better as you take off and re-attatch. A connection sitting for a long time can build up corrosion and will need a service.

I think of it like the battery posts on the battery. They look good but are not making any connections. Service and clean and it works just fine.

nz corvette

Hi,have you checked both leads on top of the tank?the earth one is a little hard to get to but was the one that fixed my problem
cheers Dave

scottw

Sounds like the connection. If you cleaned it and it went up to 1/2 from 1/4, you have an issue reading the ohms. You can trace the wire back from the sender unit back and make sure there is no fraying or anything wrong with the wire.

C3 Vette Dude

I believe you are right. I un hooked the wire from the sending unit over the gas tank (ohm wire) and it went to full tank. then I grounded the same (ohm) wire and it went to Empty. So I then cleaned the connection of the wire and the post. Reconnected the wire and it went to full tank. Which right now I have a full tank in it, so I think it was the wire connection to the sending unit. I did this once before and it didn't solve the issue. I will drive the car for a couple of days to see if the hand goes down as I drive. If it doesn't work it's got to be the connector on the end of the wire or the sending unit itself! Thank you!

0Willcox Corvette

Ok.. You tested the dash unit properly so the problem is at the tank.. What is the ohms reading at the tank when full? 0 is empty 90 is full. If you get 45 then you have a sender issue. Also, make sure the sending unit ground wire is a good ground...

C3 Vette Dude

I have driven the car for 84 miles and the gauge is still on full. I took the ohms wire off the tank and grounded it again and it went to empty. I now think I have a bad sending unit. Is that what you would think?

0Willcox Corvette

If.. with the ohms wire pulled and not grounded you have a needle pointing to the top of the rivet... and when grounded you go to empty.. you have verified the gauge in the dash is working fine.


So.. I would test the ohm's output of the sender and see what it is putting out. I would then verify the ground for the sender is a good ground.

If the sender is putting out a wrong ohms signal then yes... you have found your problem...


Willcox

C3 Vette Dude

Thank you. I will test that later today and post my results

0Willcox Corvette

C3 Vette Dude

I Bought a multimeter today and checked the ohms wire. It has 54 ohms. I put a full tank in the car 128 miles ago, so I think it's reading correct. The only thing is the tank is still on full. If I ground the ohms wire it goes to empty and the when its not connected it goes back to full. I think it's the ohms tab on the back of the gauge. I'm not looking forward to getting behind the gauge ,but it looks like thats my only choice. What do you think? I call RadioShack today to see if they had a 90 ohms wire and all they had was a 100 ohms wire. Would that work?

Steve COSD

if the tank is the same as on an 82 with a plastic inner liner than most probably the liner has collasped slightly where it interferes with the swing of the float. This is why it will only read from a quarter tank to empty. I did a post on this years ago and how to go about an easy repair.
steve

0Willcox Corvette

I think everything from the tank to the dash unit is working perfectly. The sender should be putting out 90 ohms when full! So now you need to find out what is wrong and don't get that wire or resistor, it's not your problem.

As mentioned above, the sender float could be hitting the bladder inside the tank, or the sending unit itself has gone bad. The only way you will know is to remove the sending unit from the tank and test it on the bench. If you bench test it and it goes from 0 to 90 ohms then you need to look in side the tank and see if the bladder has pulled away from the tank and is interfering with the float. I've been out sick the last two days but just saw your other emails. I will be back in my office on Friday and take a look at the antenna email.


Willcox

C3 Vette Dude

The sending unit is probably right. I tested it again today with the multimeter and it was giving a crazy reading around 1200. I think you are right. The sending unit must be hung up in the tank. Can I take the sending unit out through the hole over it or do I HAVE TO DROP THE TANK?

0Willcox Corvette

Before you go further.. check this page out, make sure you have the multi-meter set properly. http://willcoxcorvette.com/repairand...lp.php?hID=277

Can it come out from the top? I don't think so.. but to be honest, I don't remember. I can ask my shop on Friday if I go in.. (still got the funk). Willcox

Steve COSD

[QUOTE=C3 Vette Dude;1583118322]The sending unit is probably right. I tested it again today with the multimeter and it was giving a crazy reading around 1200. I think you are right. The sending unit must be hung up in the tank. Can I take the sending unit out through the hole over it or do I HAVE TO DROP THE TANK?

If the tank is like an 82 the sending unit will come out of the top.
Here's the contents of two posts I did years ago
It's been a while since I did mine but this should get you there. This process is used to protect the paint while doing the repair.
1. Disconnect battery
2. Remove fuel door
3. Use masking tape around area of fuel door
4. Use duct tape on top of masking tape
5. Use sheet plastic to cover back portion of car
6. Cut hole in plastic for the fuel door
7. Use duct tape to seal plastic to lower layer of duct tape

8. Disconnect wire harness
9. Mark the location of the fuel lines and disconnect
10. Remove the bolts that mount the unit to the tank
11. Remove the unit by lifting upward and may have to lean it
from side to side to get it out. Go slow because it is a tight
fit.


I have your solution. The inside of your fuel tank is lined with a plastic type bladder. The bladder inside the fuel tank will distort in the area where the float travels up and down. This is why your gauge will only read to a half tank empty. The float is hanging up on the collapsed area of the bladder.

Here's how to get by this problem without inventing the wheel. The float arm needs to be shortened just slightly. If you look at the float arm it has a jog or a bend in the center. Measure the float arm from the sending unit where it pivots to the very end of the float. You will now need to bend the float arm in the area of the jog so that the arm is about an 1/8 to 1/4 inch shorter in length.

To test to see if this will work. Make sure you have little or no gas in the tank. Get some string or fishing line that is about four feet in length. Tie the string or line to the end where the float is. bring the string through the filler hole. Install the sender unit in the tank an tighten about three of the bolts, and connect the wires and hoses. Turn the ignition key to the on position. While standing behind the car look through the rear window at the gas gauge. While pulling on the string slowly see if the gas gauge will work from empty to the full mark. If it does remove the string and your on your way. If not you will have to exagerate the bend in the arm a little more. Let us know how this works for you.
Good Luck

0Willcox Corvette

It very well could be the bladder and the only way to know is to pull the sender. You should be able to see the liner bowing outward from the wall of the tank once the sender is removed.

I hesitated on leaning toward the bladder because of the odd readings from the sending unit. The page I posted above shows exactly how to test the output of the sender so my thought was for you to read it and double check the reading, and test the ground on the unit.

I like Steve's thinking but I'm not so sure I'd bend he arm as a solution. Maybe for a short term fix but if the bladder is pulling away from the tank it will usually continue to do so.

C3 Vette Dude

I am trying this out tonight. I will let you know what I find.
Thanks Very Much! Thanks for the help on the antenna. Worked out great!! Willcox is the best!!!

C3 Vette Dude

Steve as I read your explanation of how to fix the problem with the sender it made me day dream about fixing that thing tonight! I appreciate the information and I am going (fishing) tonight!
I will let you know what I find. Thank you very much.

C3 Vette Dude

Ok, so here it is. I did everthing you suggested Steve, Willcox. Everything was going well until I got to the point of pulling the sending unit through the hole of the body of the car. It would not come through no matter how I turned it so I got under the rear and loosen up the tank straps and loosened the bracket holding the tank in ( without taken the nuts all the way off I didn't want the tank to fall on me) and it droped the tank about 2inchs. The sending unit came out easy.(HAD TO TURN IT A LITTLE TO GET IT OUT). After that I started on the bench test. At first the sending unit was giving out high readings. I took the cover off the ohms reader (on the sending unit) and cleaned it with carburator cleaner and it was reading 12 at empty and 114 at fuel. I adjusted it (ohms reader) and now it reads 1.9 at empty and 93 at fuel. Great feeling! After that I tied the fishing line to the round float leaving it loose around the float. I reinstalled the sending unit putting the (NEW)gas hoses on the tank first.( It would be harder if the sending unit was in the way) Then I hooked up the hoses to the sending unit. So far so good.
Tightned everything down.( including the tank bracket and tank straps. Tested it by going fishing. I pulled the fishing line up and it went to 99 ohms let it go down and it went to 9 ohms. (Tank was low on fuel) I removed the fishing wire buy sliding it off the float with a long piece of plastic. It worked fine! The gas was low so I put gas in the tank.9 gallons just to see what it would read and it reads just over empty on the guage! (%$#*)! I took the mulitimeter and tested the ohms reading on the tank and it was at 29ohms.It's not the sending unit. It's the ohms reading on the guage. I grabbed that long piece of plastic I told you about earlier (to get the fishing line out). Well I heated the end of the plastic curled it around and let it cool with cool water. Slid it down into the tank. moved it around in the tank until it grabbed the float. I pulled the float up and the guage still read a little over empty. I hooked up my multimeter to the sending unit and pulled the plastic wire up. Guess what? the ohms went up to 99 (all the way up). I pushed the plastic in and I watched the ohms go down. So It is a combination of issues. The sending unit needed to be cleaned and now the ohms resistor on the guage needs to be replaced. I had more issued than one. Just for your information. The box that covers the ohms reader is brittle. I snapped off two of the pieces that you have to bend to open the cover. I went to advanced auto and there is a product called quick steel. Great stuff. It's puddy that you form with you fingers and apply to the broken area. Within minutes it's stronger than the steel you broke (Metal)(It says ok to use in gas tanks). It worked fine to hold the cover on! I will let you know what happens next. (Question)Can I use a 100 ohms wire on the guage. That's all radio shack has and it's local or does it have to be 90 ohms??? Thank you very much. I took pictures of this job I just don't know how to up load them.
:w oohoo: Even though the project is not finished I have enjoyed learning how to fix it! Thanks to your help!!!!!