Fuel sending unit installation question

[Gsanti61]

Good morning to everyone in the forum. I will try to be brief and hopefully i will have a reply on this post.

I recently replaced the fuel sending unit in my 1997 Sailfish 198 CC. The problem is that my fuel gauge does not work (always reads empty) or so I thought. I try to maintain my fuel tank topped off just to be on the safe side.

I have not fueled up my last two times out on the water and noticed that my gas gauge started creeping from Empty to Full as I used up more gas. Somehow when I am completely FULL it reads EMPTY.

Is it possible that when I installed the sending unit I put it in upside down? Is this even possible?

The manufacturer of the sending unit is Mallory and it reads universal. Thanks in advance for your replies.

 

[Bondo]

Re: Fuel sending unit installation question

Ayuh,... While the sender can't be mounted upside-down, it can be wired backwards...

 

[Silvertip]

Re: Fuel sending unit installation question

The center (threaded) terminal on the sender goes to the "S" (send) terminal on the gauge. The shell of the sender must also be grounded. If you search this forum for fuel gauge wiring you will find well over 50 posts -- most of them with the diagram included.

 

[EricJRW]

Re: Fuel sending unit installation question

If a sending unit is basically a rheostat, is there really a backwards? I would have thought resistance is resistance, but it would certainly be worth a try to swap the wires. I'm very curious about this as I continue to build my boat knowledge database.

 

[Silvertip]

Re: Fuel sending unit installation question

You are correct that the sender is a rheostat. But one end of the rheostat goes to the S (send terminal) on the gauge and the other end goes to ground. Flipping the sender wiring therefore puts ground (or near ground) on the "S" terminal. The gauge registers full when the resistance to ground at the sender is about 33 ohms. 1/2 full = 109 ohms and empty - 240 ohms. So wiring the send terminal on the gauge to ground sends the gauge to full because of the low resistance and it wouldn't matter how much fuel is in the tank. If the resister were a "series" element rather having one end connected to ground, your assumption would be correct.

 

[Gsanti61]

Re: Fuel sending unit installation question

Thank all of you for your quick replies. You all were definitely correct. I had three wire coming from the sending unit. 1 black terminal grounded to the sending shell, 1 red wire to the center post and 1 black spade terminal also attached to the shell. Checked the gauge and it had a black wire with a yellow collar to a post with an "S" on it. went back to the sending unit and swapped the red wire attached to the center post and "Voila" all works fine. Thank you gentlemen.

 

[Gsanti61]

Re: Fuel sending unit installation question

Thank you for your response. You were correct Silvertip I had the wires reversed.

 

[Gsanti61]

Re: Fuel sending unit installation question

I appreciate the input EricJRW I had the wires reversed. Thank You

 

[Gsanti61]

Re: Fuel sending unit installation question

Thanks Bond-o you got me on the right track. Learning quickly.

 

[Fed]

Re: Fuel sending unit installation question

my gas gauge started creeping from Empty to Full as I used up more gas. Somehow when I am completely FULL it reads EMPTY.

No amount of 'wire swapping' can fix this, you either have the wrong gauge or the wrong sender so choose your poison.

 

[Gsanti61]

Re: Fuel sending unit installation question

Swapped wires, it started reading correctly. Thank You for your reply.