erratic gas guage

[rebars1]

I am restoring a 1971 Mercury Trisonic, Ford 302 v8, pre-Alpha(?)outdrive. One gas tank is original and not in use. The other looks new, plastic. However, its gas gauge varies in its reading, showing sometimes 1/4 full, somtimes 1/2 full, even though the tank is full. Any ideas?

 

[rattana]

Re: erratic gas guage

Check the connections on the gauge and the sender on the tank, make sure they are tight. First check that you have +12VDC on the battery terminal on your gauge with the key on. If you ground out the "S" terminal on your gauge to ground, it should read full. If not your gauge is faulty. If your gauge checks out, ground out the terminal on your sender on the tank to ground, your gauge should read full, if it does then your sender/float is faulty. If not then you have a wire broken between the tank and gauge.

 

[rebars1]

Re: erratic gas guage

Thanks for the information! I picked up a multimeter today and will let you know how it went.

 

[boat doc]

Re: erratic gas guage

Also, I have seen guys install those "one size fits all" senders, where you cut the wire. Well, those senders have 5 holes drilled in them in the shape of a pentagon (bolt holes) beleive it or not, they will only fit correctly one way, meaning the float coult be hitting something in the tank, and not floating all the way to the top of the tank. You can clock those senders by unbolting the center (where the guage sender wire fits) and spining the whole assembly so it floats free in the tank. If I had to put money on it, I would say rattana is right on the money, those senders are notorious for going bad

 

[rebars1]

Re: erratic gas guage

I got 12v at the battery terminal at the guage, and the guage tested OK when shorted to ground. However, I could not get the the guage to change when I shorted the sender at the tank to ground. I removed the sender wire from the tank and when I turned on the key the guage still read 1/4 full. Sound like a short? I will try replacing the wire from the tank to the guage.

 

[Boatin Bob]

Re: erratic gas guage

Make sure you have a good ground at the tank itself, if you have an ohmmeter then meter the ground at the tank to engine ground if you can, should have zero or almost zero resistance, then if possible meter the sender wire from tank to guage (might need to make a jumper).

 

[rebars1]

Re: erratic gas guage

I checked the sender wire for resistance (unhooked from both guage and tank) and got a fairly high reading (5 ohms). I ran a new jumper wire from the guage to the tank but got no reading from the guage. Doesn't the sender wire just go straight from the tank to the guage? Sounds like the sender is bad.

 

[rattana]

Re: erratic gas guage

"Quote"I got 12v at the battery terminal at the guage, and the guage tested OK when shorted to ground. <br />Okay so you know your gauge is okay.<br />"Quote" However, I could not get the the guage to change when I shorted the sender at the tank to ground. <br />So either the wire is broken between the tank and the gauge, or the point you grounded the sender wire to is not grounded.<br />"Quote" I removed the sender wire from the tank and when I turned on the key the guage still read 1/4 full.<br />That reading sounds about right with nothing connected. You need to ground the wire at the tank, you should get a full reading on the gauge. It could be possible that your tank isn't grounded, maybe a wire got disconnected. Try running a jumper from a good known ground and connect it to the sender wire at the tank. Does the gauge read full now? See if the sender in the tank works with the temporary ground from the tank to a good known ground.

 

[rebars1]

Re: erratic gas guage

I finally found the problem! The ground wire at the tank was good and tight but some corrosion between the wire connector and the tank sender base was stopping current flow. After removing and making the connector shiney again it worked fine. Thanks to all for your input and clear methodologies for tracking down the problem. <br /><br />I'm finding that boats take a lot more maintenance than cars...but then, it's a lot easier to fish from a boat!

 

[rattana]

Re: erratic gas guage

Great, glad you found the problem.