Hello and welcome to Yachting Monthly's How To series brought to you in association with GJW
Direct. I'm Matthew Sheahan and over the course of 13 episodes we're going to be dealing with some of
the key issues when it comes to maintenance, and talking to some of the experts to get their tips.
So every boat has a number of gauges on it for your fuel tanks, for your water tanks. It can
even be for your waste tanks and from time to time you're going to find that one of those gauges is
not giving you the reading. It's either ceased to function completely or it's giving you a reading
that's blatantly not correct. On our set of gauges here we can see that our forward fuel tank gauge
is giving us a completely meaningless reading. It's off the top of the gauge for full which is a
sure sign – the tank is not that full – that that gauge has stopped working. Now that can seem like
a complex or kind of mysterious problem to solve but actually, with a very little bit of knowledge,
you can pretty quickly diagnose what the problem is and quite possibly fix it.
We only have the gauge itself, the wires from the gauge and then what's called the sender in the
tank, in this instance the fuel tank. One of those three things will be faulty and what I'm going to
do is show you how to find out which one it is. So before you do anything to your electrical system,
I would always just recommend you switch it off. You can just switch off the circuit that
relates to your gauges but it's very easy just to drop a nut or a wire on one part of it and short
everything out so just switch off your electrical system first which I'm going to do right now.
And all the gauges are off
Now you'll usually have four wires going into your gauges and these are really very simple,
but just look at the instruction manual for your gauges. So on this one we have a positive
DC feed in; this powers the gauge. We have a negative the other side of the DC circuit and
there are two wires on this post because we daisy chain the gauges one to the other so that's just
joining the negatives together. So, a positive and negative and finally a sender and this is
the wire that transmits the information from the sender in the tank to this gauge
and, as we're going to see, all the sender circuit does is chop into the negative part of the circuit
and increase or decrease resistance and that will affect how much voltage goes into the needle
and therefore affect how much that needle moves from empty to full. You will often find there's
also another positive wire which this gauge is missing which is just the positive for the light
in there to light up the gauge. Our one seems to have gone missing on this so expect to see
four different posts: positive, negative, power to the light and the sender wire
and these will usually be marked. You can see the I for the positive, it actually says here. GND
for the ground for the negative and it has, if you can see it there, an S
for the sender. It is very important you never touch a positive wire to the sender
or you will certainly break the sender in the tank and likely also break the gauge.
So the first thing we want to do is check that the gauge itself
is getting voltage because if there's no voltage it's obviously not going to show you anything.
So take your multimeter and switch it on to DC voltage which is the V with the straight lines
next to it. Your multimeter might be set up slightly differently but just make sure
it's on the voltage. There. And then we're going to take our black and our red prongs
and it doesn't actually matter which way around you do this. Put one on the positive,
one on the negative and we're looking to see that we've got a good voltage. This is a 12 volt
system. We've got our battery charger on and if I could just hold it steadily there you'll see we've
got around 13 volts which is a normal voltage for a 12 volt system having lots of charge. So we know
that this gauge has got voltage going to it. One of our problems is ruled out. Now it can only be
the sender wire from the gauge to the tank or the sender in the tank itself.
Here's our fuel tank and here is a very standard type of sender you'll see very quickly where the
sender is. Our one happens to be our inspection hatch but it could be anywhere on your tank.
This wire here don't worry about this – this is our bonding wire we
should have seen from our previous bonding videos.
Now from this sender you have two wires - they'll usually be different colour - blue and black,
it doesn't actually matter which one is which. One will go to the sender post on the gauge
and the other one will go to the negative post or the ground post
on the gauge it doesn't matter which way is which for this system check the instructions
on yours now let's see what a sender looks like. To do that i'm going to pull these out of their
connections like so and then all we need to do is just unscrew our sender
like so, careful not to spill the diesel and this is a very typical sender.
This is the doughnut model which in effect just puts a magnetic signal up the inside
post. Your one might have an a lever arm but all this does is record the level of fuel
in the sender. Now we need to make sure that this sender works properly and as we've talked about
this is just about changing the resistance in the circuit. So we're going to get our multimeter.
Put it on to resistance mode and then move this gauge up and down and we'll see the
resistance change if this sender works with our multimeter. Now we're going to move it to
the resistance setting with the ohms and we can lay that down somewhere near our gauge.
We'll always just put our two probes together to check that it's working.
Whoops, and now we're just going to put one gauge into each of our wires like so.
Now as we move the doughnut up and down we can see the resistance change
in the multimeter, so we know this sender is working nicely. It doesn't matter what the
resistance is, we can just see that as the fuel level moves the resistance is changing
from the sender. All we've done is resistance setting on the multimeter and one probe
onto each of the wires. Now we saw from looking at the sender in the tank that all we're doing is
getting a different level of resistance in the circuit through to the gauge and that affects
where the needle shows from empty to full. By default therefore if we remove all the resistance,
in effect ground out the sender, we'll quickly see if this gauge is working. To ground out the
sender we just take a test piece of wire put one on the sender post and one on the negative post,
and as we do that we can very quickly see that the gauge is working. We've removed
all the resistance by in effect touching the battery negative. This gauge has got power
and it's working fine. So now we know the problem with this gauge or the with this fuel system,
the recording system here, is either the wire to the centre in the tank or the sender itself.
So we're going to put one end of our wire on the sender post
and be careful not to touch the positive we're going to touch the other end
to the ground post to the negative post and you should very quickly see the gauge
flicking 100 from one side to the other. It doesn't matter which way it goes. Each gauge
is different. This is one of the wires that comes from the sender in the tank
and this is the other wire that's come from the sender in the tank. We've done a continuity
check – there's no breaks in the wire. What we actually found with this gauge is that
the sender wire wasn't firmly attached – the nut had come loose so we're going put that back on.
Put the washer on
and the nut.
We'll get that tightened up nicely first finger tight
and then with a little spanner. Don't over tighten these – you're just tightening metal
into plastic and it's quite easy to over tighten and break in them
like so and now we can be absolutely sure –
I'll tighten that in afterwards then when we switch the power back on, the gauge is working.
So you only have a very few things to check if a gauge isn't working. Check you have power to
your gauge, check the resistance in the sender and check you have continuity in the wires between the
sender and the gauge. One of those three things won't be working; the gauge won't be working,
the sender won't be working, or there's a break in the wire. It can't be anything else.
Well thanks for watching. I hope that was a help. Make sure that you like us,
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