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Fuel Gauge or Sender

A

abl1111

Regular Contributor
2003 Mercruiser Flagship Gauges -

Best way to confirm if the fuel sender is shot or the gauge ? The fuel gauge temporarily reads accurately and then pins to FULL and stays there.

With out a spare gauge, what's the easiest way to test which is bad ?

FYI - the tach gauge has been jumpy too lately - a few months prior to the fuel gauge issue. Might just be I'm nearing the life expectancy of these gauges ?!
 
I'd say disconnect the sender lead and turn the key on....should go full to the E side....shunt the sender terminal to ground and it should go to F....depending upon the gauge and the sender configuration, the readings may be reversed...
 
More than one funky meter in a panel spells a common flaky ground or 12 v connection. since most panels are wired with grnd or 12 wired in one end and then bussed to others look for flaky grnd or 12 volt connection between last good meter and first bad one as well.
 
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Took hot off of the fuel sender - it went to E, then re-attached and went to FULL. Might be the sender. All wires look darn clean and tight on all gauges and feeds.

Settings on Tach - I don't see any - just all the posts where wires hook up.
 
Older post I know but I finally got around to pulling the fuel tank hatch to replace the sending unit - and... it's not the sending unit… I'm going to replace it anyway. I hate to just buy parts to troubleshoot so… based on posts above - and my limited electrical testing prowess, how can I test for good ground, and + in my instrument panel ?

My gauges are all (12) years old - Mercruiser Flagship, however, the Tach has acted sporadically, the Temp gauge shows hotter than it actually is ( could be the sender too ) and the Fuel gauge ( not the sender ) is pinned at FULL ...
 
12 yo gauges are not all that old. I know of working Mercruiser gauges on circa 1976 boats. If you look at the wiring for the gauges, they have a feed to +12V and a direct connection to ground ( via the mounting screws o some gauges). They also have a single wire to the sensor which is grounded by virtue of its connection to the engine block or the gas tank as the case may be.

A good start, given you don't have test tools would be a good visual inspection of the connections ( contortionist time!!!) at the gauge. Next, a gentle wiggle of ring terminal wire connection end to make sure its TIGHT to the mounting screw and also of the wire to ground ( both ends into ring lugs) to make sure they are secure. Also make sure that the gauge's wire connection to the sensor ( "S") is not rubbing up against and shorted to the ground screw ( brass bolt).

On many boat instrument panel wiring harness, the +12V and ground connections are "daisy chained" which saves on wire but can cause funny things to happen. On any boat with instrument problems I've repaired, I've rewired the panel +12V and ground connections to bus bars.
 
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You have a bad ground connection

Double check the ground wire to the sending unit. It has to connect to the sending unit. if it is to a mounting screw loosen and re-tighten and see if you have a good reading.

In essence, the sending unit is a stand alone component. Therefor the ground must be connected to it for it to work. If there is a second ground connected to the tank itself that is a safety thing

I have seen the "rivets" that connect the top plate to the bottom plate of the sending unit not making a good connection. (brand new boat under warranty)

A simple punch and hammer solved the issue vs replacement.
 
Thx Capt Bob and KGhost.

I do have a test light and a really nice digital Fluke meter ( I am not great with the meter ) that I can test for continuity, resistance, voltage etc.
All seems to be tight and right in terms of connections at the panel.

I will check the ground - now that sender is out - I will run temp, new wires to gauge. I'll report back to you.
 
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All wire connections are tight. When the new sender or the old one is connected to the gauge, and the float is moved - the gauge needle either stays pinned at Full, or will move past Full.

What I tested:

I tested resistance between all the grounds of the gauges. There was .2 -.3 Ohms across any given ground. No gauge grounds are daisy chained - they all 'go' to their own ground ( I did not follow where ).

Resistance from battery ground to each gauge with ground attached - .1-.2 ( the fuel gauge was .2-.3 )

Black wire ( ground ) on sender to ground on gauge - .2 Ohms
Red wire on sender to Red wire on gauge - .2 Ohms

Only other ground connected to the fuel gauge is from the Trim indicator ( I did all tests with this on and off the ground gauge post ) And, the other wire from the Trim gauge ( a purple wire ) goes to the "1" on the gauge..

The BUS board ground to gauge ground is .3 Ohms

I did not run new wires as a test as I said I would, because I thought these resistance tests proved I was getting a good connection from gauge to sender and ground.

What do you guys think ?

Note - I installed the new sender anyway ( I'm there and pulling the floor up is not an everyday occurrence.

One thing I don't remember if I checked is - did I do these testes with the ignition on ? Hmmm. Don't know if that would matter.
 
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Put your ohm meter across the sending unit (new gas sender),

Red lead to center post, black lead to ground spade or how ever the ground connects to the mounting flange.

No other wires connected to sender.

You may need some alligator clips to hold connections to sender.

Now set meter to 1k or 2k ohms, which ever you have.

Slowly move float arm up and down and watch meter.

it should be smooth ohms changes from ~60 to ~240 +/- a few ohms.

if it jumps like from 90 ohms to say 150 ohms then sender is bad.

What I would do if you can is to remove the gauge from dash, rewire with a red to terminal I

Black to ground

any other color to S

Connect S to center post of sender,
connect black to battery ground
connect red to battery +

now move float arm and see if gauge works properly with clean and known good wires. If it does then wiring is issue if not then either gauge or sender.

If this works correctly and you feel gauge and sender are good then you have a bad ground in my opinion.

Note: Do this away from gas tank!!!!!!!!!!! if possible out side of boat!!
 
Geez that s*cked.

A lot of testing and still it was hard to confirm if it was the gauge or not with the meter. I wired everything straight from the battery and both senders, new and old, made the gauge do the same thing. Gut told me it was the gauge so I went to Waste Marine and bought a gauge and sure enough - it was the gauge.

Going to buy an OEM mercruiser gauge ASAP.

Thanks for your help guys. This process has me wanting to change out my + and - Buss bars, which I'll do next.
 
Thank you for the reply and advise. I restored this boat (12) years ago including all new wiring, etc. I have (2) bars under my dash; a (-, ground ) and (+) buss bar, that are fed directly from the batteries. All (+) are bridged with glass fuses. Off the top of my head, I'm not sure how many posts there are on the (+) bar - approximately (8). The ground has about (12). These buss bar also feed my boats hard top buss bar via thick gauge wires ( just a VHF and a horn ).

Switching over to the newer style that you spec'd would be nice but what I have has worked great - still does. However, time and a briny environment makes brass oxidize and connections suffer. I'd like to copy what I have; it's easy/simple, I understand it and replacement would be straight forward. When I first did the project I covered everything in a waxy electrical protectant - that wears and is a PITA to work with nice you do need to get in there !
 
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