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Author Topic: Fuel sending unit read full all the time  (Read 577 times)

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schupaul

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Fuel sending unit read full all the time
« on: June 27, 2019, 09:52:01 AM »

Just bought my C34 a few weeks ago and am knocking out the punch list that I created.  1st on the list are the tachometer and fuel gauge not reading.  Ordered a new 12" fuel sensing unit (old was float type) and checked against the gauge before I inserted into the tank after wiring it.  Worked great.  Read fuel for first 2hr journey to new home.  However, I noticed that the gauge still reads full even after motoring for 4hrs to new home (bad headwinds).  Any ideas?  is the unit faulty, or something making it stick?
« Last Edit: June 27, 2019, 09:52:36 AM by schupaul »
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Paul Schultz
1987 Hull 47#, Tall Rig, Wing Keel, MXP25, Cedar Mills Marina, Texas

Jon W

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Re: Fuel sending unit read full all the time
« Reply #1 on: June 27, 2019, 02:45:34 PM »

If you have a Universal M25XP, it will burn ~ 1/2 to 3/4 of a gallon diesel per hour. Four hours motoring is ~2-3 gallons. How many gallons of fuel did you start with?

FWIW - I've just finished an involved fuel tank project. As part of the project I defined what the fuel gauge reads per gallon. If your fuel gauge reads like mine (sender  in tank is reed switch type), it won't show any change until there's less than 22 gallons in the tank. In the below photos, the number taped over the voltmeter is the number of gallons in the tank for that gauge reading.
« Last Edit: June 27, 2019, 02:47:53 PM by Jon W »
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Jon W.
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Hull #493, 1987 MK 1, M25XP, Manson Supreme 35
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Ron Hill

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Re: Fuel sending unit read full all the time
« Reply #2 on: June 27, 2019, 04:40:39 PM »

Paul : You answered your own question.  "The gage is NOT working"!!  You should be down about 2.5 Gal for 4 hours of motoring from full so the tank is only down slightly!! However, The gage should at least wiggle in a medium seas!!   :cry4`

A thought
« Last Edit: June 29, 2019, 12:30:21 PM by Ron Hill »
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Jon W

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Re: Fuel sending unit read full all the time
« Reply #3 on: June 27, 2019, 05:12:10 PM »

Could be a failed gauge, but there isn’t enough information to make that decision. He said the gauge responded to the new sender. This isn’t an urgent issue, why send someone off to spend money without a little more troubleshooting. If he put 23-24 gallons in his tank, after 4 hours motoring the gauge will still read as full.

I recommend waiting until you’ve motored another 4-5 hours. Keep track of engine hours using 1/2 or 3/4 gph. If it still reads full, you know something failed. Before buying a new gauge, I would check your electrical connection at the gauge. There’s plenty on the boat to spend money on, why rush it?
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Jon W.
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Hull #493, 1987 MK 1, M25XP, Manson Supreme 35
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Craig Illman

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Re: Fuel sending unit read full all the time
« Reply #4 on: June 28, 2019, 06:02:12 AM »

Also make sure you have a good ground at both the tank and panel. You might have fried the new sender as power was trying to find the best path to ground.

Craig
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Noah

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Re: Fuel sending unit read full all the time
« Reply #5 on: June 28, 2019, 08:58:43 AM »

Not enough info, or I missed it. Did you install the new 12 in. reed switch/sender with the tank empty, then fill up tank and watch the gauge move up to full?  If yes, try running engine another 5 hours or pump out 5 gl. of fuel from tank and see if gauge moves down—before declaring gauge is “bad”. Also if wiring is reversed gauge MAY peg full. On the Catalina direct website they have a pdf. document on how to test for a bad gauge.
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britinusa

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Re: Fuel sending unit read full all the time
« Reply #6 on: June 28, 2019, 09:59:30 AM »

Our fuel gauge sender has 1/8 incremental reads.

With our 23 Gallon Tank, 1/8 would be about 3 gallons, but the tank is not symmetrical so the volume in the top 1/8th is a lot more than bottom 1/8th.

I would guestimate that the 1st 1/8 from full is probably nearer to 4 gallons or higher.

And as stated by others, at 1/2gph to 3/4gph, that would be somewhere between 8 and  5 hours running.

Paul

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Paul & Peggy
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Dave Spencer

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Re: Fuel sending unit read full all the time
« Reply #7 on: June 28, 2019, 10:03:05 AM »

Good advice from all.  Don’t give up on your gauge yet. As Jon pointed out, you have used very little fuel and it wouldn’t be surprising if the gauge stayed on full assuming you started with a full tank. Also, keep in mind gauges / senders aren’t necessarily linear. Over time, you will develop a sense for your fuel consumption based on engine hours. I used about the same amount of fuel as Jon indicated in his earlier post. For planning purposes, I assume a burn rate of 3 litres/hr which is 3/4 USG / hr (approximately). I usually burn less but if powering hard in rough seas, 3 l/hr is about right. I assume a max run time of 25 hours per tank  based on a 25 USG tank (95 litres), assume 20 litres is unusable leaving me with 75 litres to burn at 3 l/hr. All of this is very conservative and I rarely run more than 15 hrs without filling up. 
Bottom line - no hurry to condemn the gauge or sender. If, in time, you determine the gauge or sender is bad, you have lots of great advice on this post and elsewhere on this site to correct.
Let us know how you make out.
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Dave Spencer
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KWKloeber

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Re: Fuel sending unit read full all the time
« Reply #8 on: June 28, 2019, 10:47:36 AM »

Paul
If it’s of any help to you I posted an engine panel gauge troubleshooting guide on the TechWiki. Under
Engine > Electrical.

Ken
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Twenty years from now you'll be more disappointed by the things you didn't do, than by the ones you did.
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Explore.  Dream.  Discover.   -Mark Twain
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