Low voltage at fuel sender

yeesh

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Jul 20, 2012
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Trying to figure out problem with fuel gauge/ sending unit components which have been inoperable from r some time. On the back of the gauge, with the key on, I get 12 volts at the gauge ignition terminal and about 5 volts at the gauge sender terminal. At the fuel sender end, the wires are disconnected. When they were connected, it still didn’t send to the gauge. Shouldn’t I have 12 volts at the gauge sender terminal if I have it at the gauge ignition terminal? Gauges are by Honda and came with a new outboard less than 3 years ago. Appreciate any advice on this. Thanks.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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you should not have any voltage at the fuel sender. if you apply voltage, the resistor gets hot, ignites the fuel in the tank and you join the ranks of those that have blown up their boat and killed themselves from a lack of understanding of boat systems

for the gauge sending unit in the tank, there is a ground wire to the common bonding ground (green) and a signal wire to the gauge (pink)

the gauge itself will have 4 terminals. Ground (black), ignition (purple), signal (pink) and the light (blue)

the variable ground from the sender to the gauge is what makes the gauge read the fuel level.

if you are getting 5 volts on the sender terminal, you either have a grounding issue or you have a gauge issue.

ground the pink wire and the gauge should go full scale one way
disconnect the pink wire and the gauge should go full scale the other way.

Honda doesnt make gauges, they buy them. they are either private-branded faria or veethree (depending on year)
 

yeesh

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Yeah, I probably didn’t make it clear. I was only measuring the voltage at the gauge terminals (“sender” terminal & “ignition” terminal- not at the tank). Don’t want to join that “blow up your boat” club! So, There should be no voltage at all on the sender terminal on the gauge? And if mine is showing 5 volts, there’s probably a ground issue, right. I didn’t try grounding the pink sender wire- thanks for that advice. I’m assuming if the gauge works according to the test( full scale each way), then the gauge is probably ok. Thanks for the response.
 

dingbat

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Yeah, I probably didn’t make it clear. I was only measuring the voltage at the gauge terminals (“sender” terminal & “ignition” terminal- not at the tank). Don’t want to join that “blow up your boat” club! So, There should be no voltage at all on the sender terminal on the gauge? And if mine is showing 5 volts, there’s probably a ground issue, right. I didn’t try grounding the pink sender wire- thanks for that advice. I’m assuming if the gauge works according to the test( full scale each way), then the gauge is probably ok. Thanks for the response.
Gauges are rarely the issue.

Sending units are a simple rheostat. Put a meter between the wire come from the sending unit (pink) and ground. Move the float up and down to simulate full and empty tank. Your readings should vary between 33 - 240 ohms. If not, your sending unit is bad
 

sam am I

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Jun 26, 2013
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you should not have any voltage at the fuel sender.

LMAO......Okay, I couldn't let this one slide. I have to ask then, how do they work without "any voltage at the fuel sender"? Hmmmm....

Okay so OP,

As Ding says, the sender's resistance (Rs) varies from around 33 ohms full (Rsf) of gas to 240 ohms empty (Rse) of gas.....The gauge's "S" terminal has an internal resistor (Ri) that is tied to the Ig, terminal, when the key is on, 12V (Vs) is applied to this internal resistor and divided (a voltage divider for the techie's) between this internal resistor and the sender's resistor, this sender resistor is wired as a rheostat, again, as Ding says.

The internal resistor is fixed at about 140 ohms....

On full the sender's pink (or "S") wire(at the gauge or sender, same wire) should read around....

Vs/(Ri + Rsf)*Rsf = 12V/(140+33)*33 = 2.29V

On empty the sender's pink (or "S") wire(at the gauge or sender, same wire) should read around....

Vs/(Ri + Rse)*Rse = 12V/(140+240)*240 = 7.57V


Sounds like your tank, by your voltage reading of 5V that SHOULD be present at the sender, your tank is around 1/4 full.

Guessing needle on gauge is stuck/bad meter movement.........
 
Last edited:

yeesh

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Joined
Jul 20, 2012
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19
Final note: going to change the 20 year corroded sending unit regardless. So, on back of fuel guage:

pink wire- removed (leaving bare terminal)
ground- bypassed with jumper to battery
ignition terminal- tests at 12v with key on
sender terminal (bare)- still tests at 5v with key on

bad guage?
 
Last edited:

yeesh

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Jul 20, 2012
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Sorry, Sam m i. Wrote my post before seeing yours. No one will ever mistake me for a techie, so I got lost early in your excellent explanation. But it sounds like the 5 volts on the guage sender terminal might be normal. I’ll change the sender, hook it up to the guage and see what I get. Worst case- change the guage too. Thanks.
 
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