HELP!!!!Gas Guage driving me crazy

[hotrodder]

I have a '65 GMC the original guage was complety useless so I bought a gas guage and sending unit from Speedway Motors. Now that I have it installed the needle goes straight to full.As soon as I put power to the guage wham! it hits full with a loud click. I have grounded both the sending unit and the guage. This should be a simple install three wires to hook up.! What is going on?

[LostMy65]

Is it a replacement fuel gauge for the original fuel gauge in the gauge cluster?

[Mack B]

Originally Posted by hotrodder

I have a '65 GMC the original guage was complety useless so I bought a gas guage and sending unit from Speedway Motors. Now that I have it installed the needle goes straight to full.As soon as I put power to the guage wham! it hits full with a loud click. I have grounded both the sending unit and the guage. This should be a simple install three wires to hook up.! What is going on?

Is it a 30 ohm gauge to match the sender? I know being mismatched will cause it to read wrong, but I don't know if it will cause the issues you're having.

Edit: missed you said it was a complete kit

[luvbowties]

When hooking up wires, remember the sending unit only varies the amount of ground that the power is seeking. Also, so long as the unit and gauge are properly matched, the pair can run on 30 ohms, 60 ohms, or whatever combo--but they must be matched.

Sam

[hotrodder]

The gauge and the sending unit was purchased as a kit, came in the same box so I figure they are matched to each other. I think I will pull the unit out of the tank and check it out, start over and see what happens. Thanks for the responses.

[Lil Loafer]

I hope you find the answer. My gas gauge does the same thing, just haven't had the time to find out why.

[hotrodder]

Hey Lil Loafer keep me posted on what you find out. I am going to be out of town until next Monday before I will have a chance to look at my truck. Thanks for the interest.

[Captainfab]

Sounds like either a ground issue or a bad sender or gauge. I would check your ground to the gas tank first.

Originally Posted by Lil Loafer

I hope you find the answer. My gas gauge does the same thing, just haven't had the time to find out why.

[nail pounder]

Originally Posted by hotrodder

I have a '65 GMC the original guage was complety useless so I bought a gas guage and sending unit from Speedway Motors. Now that I have it installed the needle goes straight to full.As soon as I put power to the guage wham! it hits full with a loud click. I have grounded both the sending unit and the guage. This should be a simple install three wires to hook up.! What is going on?

I had the same issue on my 58 a couple years ago, it's a .10¢ fix. If I remember correctly, you need an insulated (plastic) washer before you attach your hot wire on the sender stud so it doesn't ground out to any metal. I think I bent the eye up a little to be sure it wouldn't touch the top of the sender unit. The ground wire should be attached to one of the perimeter screws and grounded. I may have a pic somewhere.

[Cris]

I think nail pounder may have hit this one on the head.

I don't know if you have an electrical multimeter in your tool box. If you do, disconnect the gauge and measure the resistance from ground to the sender of your tank. If it measures zero ohms (and your tank is not completely full), you have a short in the sender connection and nail pounder's suggestion is probably the issue.

You can even remove the sender from the tank, connect the multimeter, and move the float around. You should see the resistance change with the float level.

[luvbowties]

Originally Posted by hotrodder

I have a '65 GMC the original guage was complety useless so I bought a gas guage and sending unit from Speedway Motors. Now that I have it installed the needle goes straight to full.As soon as I put power to the guage wham! it hits full with a loud click. I have grounded both the sending unit and the guage. This should be a simple install three wires to hook up.! What is going on?

You said, "I have grounded both the sending unit and the guage.". If the gauge/needle circuit is grounded to anything other than sending unit, it will provide FULL GROUND all the time and 'wham' straight to FULL. This assumes your kit gives less ground when EMPTY and more when FULL. And based on your description, this is precisely what you have. Gauge-needle circuit needs a varying ground which the sender provides--sender varies amount of ground depending on where fuel level is. Unhook your gauge-ground & see what happens. If gauge just stays on EMPTY, **Make sure your sender wire is connected to correct post: you probably have the 2 ground wires reversed, which again would provide full ground to the E/F circuit all the time, and give the "straight to FULL", which is what you described. The hot wire and sender's varying neg. wire must be tied to the ends of the E/F circuit--NOT the hot wire and a full ground.

HTH,
Sam