Faulty Fuel Gauge

[daviddiscenza]

I just took possession of a 1993 Catalina 34. During the seat trial, the fuel gauge was working. Now that I own the boat, it's not. I've done some preliminary troubleshooting with these results:
- I checked the voltage with my multimeter at the instrument and it read 12.5 volts, same as the battery meter.
- I disconnected the sending wire and used a screwdriver blade to short across the sender input post and the ground post. The needled jumped to the 1/2 full position on the gauge.
- I checked the resistance of the sending wire at the instrument. It read zero ohms at all settings of my multimeter
- I checked the resistance of the sending wire at the fuel tank. It read zero ohms at all settings of my multimeter

The needle of the gauge is all the way to the left, off the scale of the gauge. When I turn the ignition key, the needle jumps a millimeter or two and goes right back to all the way to the left, off the scale of the gauge.

I want to see if the repair shop at my marina has a fuel gauge that's known to be good. If they do, I'll temporarily connect it and see if it works. If it does, then it's definitely the gauge, but could it also be the sender? If it does, then it's definitely the gauge. If it doesn't could it be the sender as well as the gauge? I'd appreciate the thoughts of others on this problem. Thanks to all who reply.

[SteveSails]

I'll take a crack at it...

I'll assume you have a float type sender that is the standard 240 Ohm empty 33 Ohm full. They are known to be prone to failure.
Disconnect the two sender wires at the gauge and measure the resistance across the wires, depending on the fuel level should be between the above. You said you measured 0 Ohms, did you mean 0 or is it an open circuit which might say OL on the meter for overload? If it is an open circuit check the connections at the sender and measure the sender itself. If still an open circuit the sender has failed and needs to be replaced. I doubt the gauge itself is bad.

Let us know what you found.

[daviddiscenza]

Originally Posted by SteveSails

I'll take a crack at it...

I'll assume you have a float type sender that is the standard 240 Ohm empty 33 Ohm full. They are known to be prone to failure.
Disconnect the two sender wires at the gauge and measure the resistance across the wires, depending on the fuel level should be between the above. You said you measured 0 Ohms, did you mean 0 or is it an open circuit which might say OL on the meter for overload? If it is an open circuit check the connections at the sender and measure the sender itself. If still an open circuit the sender has failed and needs to be replaced. I doubt the gauge itself is bad.

Let us know what you found.

Steve,
I meant 0, as in zero, null, nada, and zip. I did measure at the sender as best I could (tight quarters on a Catalina) and got the same measurement.

I'm unclear as to what you mean by "Disconnect the two sender wires at the gauge". There's a sender wire, a hot wire , and a ground on the back of the gauge.

[SteveSails]

The two sender wires would be the sender wire and ground. In your case just disconnect the sender wire and measure resistance from that wire to ground. Zero Ohms would be a short circuit, same as holding the two leads from the meter together. If that's what you have then there is a short in the sensor wire to ground.

[daviddiscenza]

Originally Posted by SteveSails

The two sender wires would be the sender wire and ground. In your case just disconnect the sender wire and measure resistance from that wire to ground. Zero Ohms would be a short circuit, same as holding the two leads from the meter together. If that's what you have then there is a short in the sensor wire to ground.

Steve,
Thanks for clarifying. That's what I did, BUT (there's always a big but) I'm positive I set up the multimeter incorrectly, at least according to a YouTube video I just watched.

So, let's say I retest the wire with the correct setting on the multimeter and I get a reading somewhere between 30 and 300 ohms. What's that telling me?

[SteveSails]

If you get between 30 and 300 ohms then the sender in tank is OK. A good practice when using the Ohms function on a multi meter is to short the leads together before taking a measurement, the meter will read about 0.1 to 0.3 if you are on the lowest scale. With the leads separated the meter might say "OL" or "OPEN" or just flash something, means the resistance is higher than the meter can read.

[daviddiscenza]

Steve,
I'll retry this weekend. If the sender is okay, then it must be the gauge. I hope that's the case cuz it's a heck of a lot easier to change the guage than the sender.

Thanks for your help.

[coopec43]

I had a problem with a car fuel gauge and it turned out to be a resistor at the fuel gauge. It had to be a certain value (for the particular gauge/vehicle) and when I replaced that it was OK

[tkeithlu]

Let's make sure everything is clear here.

Ohms across the sender prongs with the gauge wiring removed -33 to 240.

Voltage across the leads from the sender to the gauge with the switch on and the leads disconnected from the sender - battery voltage.

With everything connected and the switch on, voltage across the leads at the gauge, somewhere between battery voltage and zero. Ditto ascross the leads at the sender.

Conceptually, a fuel gauge (and bunches of other gauges) is a small voltmeter. The circuit is battery-switch-one side of gauge-variable resistance (sender) -gauge - ground. Very simple. Not accurate if your batteries are down.

Don't forget and connect the ohmmeter across a voltage. That tends to result in a small puff of smoke. Everyone does it at least once.

Most digital multimeters today indicate infinite resistance with a "l".

[GordMay]

Do I understand you correctly?
Your gauge reads Zero scale (all the way Left)?
Your sender reads Zero Ohms?
Shorting sender wire to block results in gauge deflecting to half scale?

These results don’t make sense, to me, because:

Disconnecting Sender Wire from Gauge, and turn power ON. The Fuel Gauge should read ZERO Scale (< Empty)

Short the Gauge Sender Terminal to Ground. The Gauge should read FULL Scale (> Full).

If a Sender fails “Open” (Infinite Resistance), one or both (if Dual Station) Gauges will read Zero Scale.
If a Sender Fails “Closed” (shorted = Zero Resistance), one or both Gauges will read Full Scale.

Other confusing Ohmmeter results, such as readings lower than expected, could indicate a poor Ground Connection at the Sender Body (Sender to Engine Block). Check for pipe dope or tape on Sender Threads.

➥ http://www.cruisersforum.com/forums/...sting-645.html