Gas Gauge Sending Unit

[Toolmaker Tim]

My gas gage sending unit doesn't appear to be working. First thing I did was replace the connectors because the stock one was broken. Since that was the most obvious thing I hoped it would work, It didn't. Is there any reading I should be getting from a multimeter if I hook one up to tell if the thing is still good?

[3Phase]

Here you go, I'm in the middle of rebuilding mine, too:

Full 6 to 10 Ohms

1/2 28.5 to 36.5 Ohms

Empty 75 to 85 Ohms


Regards,

Scott

[3Phase]

If the Fuel Sender has the right (or close) resistance, check for Voltage.


Set the multimeter to DC Volts.

Unplug the Sender from the harness.

Turn on the ignition.

Test for 7 Volts between the Green wire and the Black wire at the harness connector.

If there is no voltage at the harness connector, turn off the ignition.

Set the multimeter to Ohms.

Check for an open circuit or high resistance between the Black wire and ground.

If the Black wire is providing a good ground, set the multimeter back to DC Volts.

Turn on the ignition.

Check for 7 Volts between the Green wire and the Black wire at the tachometer connector in the headlight shell.

If you get 7 Volts at the tachometer connector, there is a broken wire or the connector is dirty between the tach and the tank sender.

If the Green wire does not have 7 Volts, check for 12 Volts between the Brown wire and the Black wire at the tachometer connector.

If there is 12 Volts on the Brown wire you need a new 7 Volt regulator.

Turn off the ignition.


Regards,

Scott

[Toolmaker Tim]

Thank you Scott. Very nice instructions. I'll give it check tomorrow.

[3Phase]

Tim, was the gauge not working before it was knocked over? I hope you find the guy!

It's possible the fuel gauge itself could be damaged. If everything else checks out okay it's easy to take out the tach and check the gauge.

If the movement isn't broken, the gauge resistance is ~50 Ohms.

I hope it's just a loose wire somewhere!


Regards,

Scott

[dbeardslee]

Tim - When I first got my XS, I took the sending unit out of the gas tank and cleaned it. It has a little potentiometer in it, and mine was absolutely loaded with crud. It's inside the little metal box that's attached to the cover. If it's gummed up bad enough to keep the float from moving it could be the cause of the guage not working. My $.02.

[Toolmaker Tim]

Gas Gage

I unintenionally may have given bad info. It's a low fuel indicator lite, not a gage. I wish it was a gage, sorry. No, 3Phase, it wasn't working before the tipover, but since I'm waiting on parts, I'm trying to get other things that were going to wait until winter taken care of now. Dbeardslee, I haven't looked at it closely yet, but gunk from old fuel does seem to cause most of the problems with our bikes, doesn't it? Thanks for the heads up. 3Phase,I saw your thread on the 7volt regulator, going by your handle and what you written, you sound like an electrical tech, or at least a very knowledgable hobbist. Thanks for the help. I won't get a chance to check today, but I'll let everyone know what I find out.

[3Phase]

Yes, I used to be a maint. tech. Now it's just an hobby!

I thought you'd put a Standard tank and gauge set on your '79SF. You don't have a 7 Volt regulator; not a problem. The Low Fuel Warning light is just a a little different test and setup.


At the tach harness connection you should have:
Brown (into the warning lamp) to ground = 12 Volts
Green (from the warning lamp) to ground = 12 Volts

Down at the tank sender/harness connection you should have 12 Volts from the Green wire to the Black wire/ground.

The thermistor resistance at 25 C (77 F) is
1500 Ohms +- 100 Ohms.

It should be a NTC thermistor.

As the temperature goes up (less fuel covering and cooling the thermistor) the resistance goes down until, eventually, there is enough current flowing in the circuit to light the Fuel lamp.



Regards,

Scott

[Toolmaker Tim]

Fuel Sender

Thank you Scott. You are an electrical guru

[3Phase]

You're welcome, Tim.

I forgot to mention:

PLEASE!

Turn off the ignition and disconnect the harness before you test the thermistor!

If you accidentally short 12 Volts to ground while you're working around the fuel tank... it's not something you want to test.


Regards,

Scott

[Toolmaker Tim]

Yah mean" puff, no eyebrows?"

[3Phase]

Yep, no eyebrows at best. You know what happens next, right, after checking the thermistor?

"Okay, the resistance is good but how much gas is in the tank and how much voltage does it take before the light comes on...."

Check the resistance, preferably outside of the garage, in a well-ventilated area.



Regards,

Scott