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Old 06-19-2009, 06:47 PM
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Default How do you test fuel gauge vs. sending unit?

My in dash OMC fuel gauge either reads full or empty. You can hear it click when it swings from one extreme to the other. It does not bounce around but stays on say full for 20 min and then goes to empty etc.

How do you determine if it is the gauge or the sending unit? I have already cleaned all contacts.

Thanks!
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Old 06-19-2009, 09:53 PM
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The fuel tank sending unit sends a signal in ohms to the gauge depending on the float position. Not sure on the empty/ full values. Get your self a multimeter
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Old 06-19-2009, 10:29 PM
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Faria has a gauge diagnostic guide. You need to figure out whether you have Std /USA ohms range sender and gauge or Metric/ Euro. Both are common on US boats and should never be mis matched.

http://faria-instruments.com/site_manuals/IS0085E.pdf

http://faria-instruments.com/site_ma...100_ApxVII.pdf

http://faria-instruments.com/site_ma...MiscGauges.pdf

Did you clean the tank sender from-gauge & neg ground contacts at both ends from the tank to engine ground or other confirmed good ground , or just the contacts on the gauge? Have you confirmed steady 12 V or higher getting to the gauge from 12v + source such as daisy-chained through other gauges from the ignition I terminal?
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Old 06-20-2009, 07:44 AM
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Originally Posted by mobjack22 View Post
My in dash OMC fuel gauge either reads full or empty. You can hear it click when it swings from one extreme to the other. It does not bounce around but stays on say full for 20 min and then goes to empty etc.

How do you determine if it is the gauge or the sending unit? I have already cleaned all contacts.

Thanks!
American senders are about 230 ohms at low and 30 ohms at high (full).

If you disconnect the wire from the sender to the gauge (it may be pink) the gauge should read empty. If you now take the wire from the gauge and ground it, the gauge should read full. If nothing happens or the readings are intermittent, you need to find where that wire connects at the gauge and do the same tests, no connection to the gauge and grounded. If the gauge doesn't react properly it is bad OR the battery connection is bad/intermittent. To check that you need to measure the voltage to the meter and wiggle things (connections) to see if it fluctuates.
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Old 06-20-2009, 08:12 AM
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Its the sender. The accuracy of the gauge depends on the sensitivity of the sender.
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Old 06-20-2009, 11:22 AM
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Its the sender. The accuracy of the gauge depends on the sensitivity of the sender.
Not true. The accuracy depends entirely on the geometrics of the tank. At full up ( either arm/float or ring, the sender will be at 30 ohms. At full down, the sender will be at 230 ohms. At halfway it will be at 130 ohms. At halfway, the actual display will say half but there may be more or less fuel depending on the bottom position of the float relative to the actual tank bottom, the geometry of the tank and the angle of the tank.

If the tank in question is full or close to full of fuel and the battery supply to the gauge is intermittent, the gauge will read full or empty depending on the battery connection. If the battery connection is loose and the sender is at half position, the gauge will read half or empth depending on the battery connection,
If the sender/tank ground is loose, the guage will read either empty or the level of the fuel. Same thing if the lead between the gauge and the sender is intermittent. These conditions apply to gauges that respond rapidly to level changes. These are the electromechanical type. There are some thermal types of gauge mechanisms and they act differently and slower to input changes.

The sender could be bad but we don't have solid evidence yet. If I knew what the estimated fuel level is, I may be able to eliminate some possibilities. In other words if the tank is relatively empty ( or half empty), it is unlikely that the gauge would swing between full and empty. Regardles, you still wouldn't know if the problem is the sender or the gauge without testing.
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Old 06-20-2009, 11:51 AM
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Not true. The accuracy depends entirely on the geometrics of the tank. At full up ( either arm/float or ring, the sender will be at 30 ohms. At full down, the sender will be at 230 ohms. At halfway it will be at 130 ohms. At halfway, the actual display will say half but there may be more or less fuel depending on the bottom position of the float relative to the actual tank bottom, the geometry of the tank and the angle of the tank.

If the tank in question is full or close to full of fuel and the battery supply to the gauge is intermittent, the gauge will read full or empty depending on the battery connection. If the battery connection is loose and the sender is at half position, the gauge will read half or empth depending on the battery connection,
If the sender/tank ground is loose, the guage will read either empty or the level of the fuel. Same thing if the lead between the gauge and the sender is intermittent. These conditions apply to gauges that respond rapidly to level changes. These are the electromechanical type. There are some thermal types of gauge mechanisms and they act differently and slower to input changes.

The sender could be bad but we don't have solid evidence yet. If I knew what the estimated fuel level is, I may be able to eliminate some possibilities. In other words if the tank is relatively empty ( or half empty), it is unlikely that the gauge would swing between full and empty. Regardles, you still wouldn't know if the problem is the sender or the gauge without testing.
Yes it is true... but maybe I should have used the word precise instead of accurate. I didnt mean the gauge accurately represented the amount of fuel that's in the tank, I just meant it precisely represented the current position of the senders arm. Senders can get dead spots in them and they will go from 30 to 230 ohms with no inbetween which causes the gauge to read accordingly. Generally analog fuel gauges work or they don't unless the needle is getting stuck. Easy enough to test with a VOM.
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Old 06-20-2009, 12:17 PM
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Yes it is true... but maybe I should have used the word precise instead of accurate. I didnt mean the gauge accurately represented the amount of fuel that's in the tank, I just meant it precisely represented the current position of the senders arm. Senders can get dead spots in them and they will go from 30 to 230 ohms with no inbetween which causes the gauge to read accordingly. Generally analog fuel gauges work or they don't unless the needle is getting stuck. Easy enough to test with a VOM.
You are 100% correct. I didn't mean to jump on your post but I am sitting here at my computer instead of on the boat. The weather in NY has been just plain awful.

Note that even with an ohmeter it may be hard to se a probelm with the sender since whatever position the arm or float is currently at might be a good position. Unless you pull the sender so that you can manually move the arm/float, you can't completely test.

Regards
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Old 06-20-2009, 12:28 PM
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You are 100% correct. I didn't mean to jump on your post but I am sitting here at my computer instead of on the boat. The weather in NY has been just plain awful.

Note that even with an ohmeter it may be hard to se a probelm with the sender since whatever position the arm or float is currently at might be a good position. Unless you pull the sender so that you can manually move the arm/float, you can't completely test.

Regards
It's all good. That's what I get for typing a one sentence post and expecting people to understand what the hell I'm talking about. Hope the weather clears up for you.... It's hotter than hell in Florida.


OP... time to pull the sender out.
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Old 06-20-2009, 05:40 PM
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Thanks guys, I messed with this for a few hours to no avail. The O meter reads about 84 (on the wires at the tank) which is about right given the fuel level. I will try a new sending unit.
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Old 06-20-2009, 10:21 PM
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In the setup you describe, the ohmmeter was temporarily replacing the gauge. If it is reading correctly connected to the sender then the sender was ok at the time of the reading.

How did the gauge test out with the Faria diagnostic directions?

If you do replace the sender , consider the calibratable, no-moving parts Centroid Products type that will give steadier-needle gauge readings and more linear readings of levels than a swing arm sender.
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