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Thread: fuel gauge or fuel sender problem?

  1. #1

    fuel gauge or fuel sender problem?

    Hi all.

    I have a fuel gauge or fuel sender problem. I’m not sure which is it, so I am looking for advice.

    I did all the ground checks, sub panel and the ground strap at the wing/sender location. The problem is my right tank indicator shows ½ full all the time. Today I drained the right tank and when empty it still reads ½ full.

    So now I am basically trying to figure out if I have a bad sender unit, or a bad gauge.

    Can anybody tell me how I can tell which is bad? I figure I can ohm out the empty tank sender unit to see what the resistance is, but I don’t know what the resistance should be on an empty tank.

    Does anybody know what the resistance should be on the sender unit when the tank is empty?

    Thanks

    Barry
    Sierra, N1959L

  2. #2

    Fuel gauge or fuel sender problem?

    Barry, there is a lot of info about this on BAC, and you didn't mention your year, model, and serial number. How the sensing works is 180 different between SW and Rochester systems. If you look very closely at the front of the gauges, it will probably say one or the other in very small print. Rochester systems have about zero to 30 ohms resistance in the sender, over full travel. If I remember right, a grounded (Rochester) circuit will show an empty tank, while an open circuit (disconnected or broken wire) will show a past-full tank (pegged high needle). This tells you how to check your system, regardless of brand. Watch the gauge with the Master on. It should peg high or peg low with the sensor wire disconnected or shorted.

    My bet is that you will find a bad gauge. Most of them have never been replaced or overhauled, and are badly out of tolerance (or have sticking movements). The gauges that tend to fail are the oil temp, left fuel, and right fuel. They are electric with meter movements, and operate on very small signal strengths. Airparts Of Lockhaven does overhauls. Not cheap, but new ones, if you could find them, would be three to four times the price. Much more about this in a BAC search, and an MML archive search on BAC.

    ----- Original Message -----
    From: Barry
    To: musketeermail@yahoogroups.com
    Sent: Friday, November 25, 2005 10:45 PM
    Subject: [musketeermail] Fuel gauge or fuel sender problem?


    Hi all.

    I have a fuel gauge or fuel sender problem. I'm not sure which is it,
    so I am looking for advice.

    I did all the ground checks, sub panel and the ground strap at the
    wing/sender location. The problem is my right tank indicator shows ½
    full all the time. Today I drained the right tank and when empty it
    still reads ½ full.

    So now I am basically trying to figure out if I have a bad sender
    unit, or a bad gauge.

    Can anybody tell me how I can tell which is bad? I figure I can ohm
    out the empty tank sender unit to see what the resistance is, but I
    don't know what the resistance should be on an empty tank.

    Does anybody know what the resistance should be on the sender unit
    when the tank is empty?

    Thanks

    Barry
    Sierra, N1959L







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  3. #3

    Fuel gauge or fuel sender problem?

    >So now I am basically trying to figure out if I have a bad sender
    >unit, or a bad gauge.

    The fastest and easiest way would be to remove the wire from the sender and ground it to see if the gauge reads FULL. Removing the wire should cause the gauge to read empty.

    No changes = bad gauge or wiring fault. If it reads both full and empty with the wire grounded and off respectively, then you have a sender problem.

    In ether case, I'd recommend Air Parts of Lock Haven PA as a good vendor to repair the defective part.

    www.airpartsoflockhaven.com

    --Bob Steward A&P IA
    Birmingham, AL


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  4. #4

    Fuel gauge or fuel sender problem?

    Barry, I'm chiming in because I'm not sure Bob gave you the very best
    advice (quite uncharacteristically for him!).

    I've become something of an unwilling authority on fuel gauges and
    senders lately, having yanked out my gauge cluster 4x in the past few
    months to send in for repair, and looking down the barrel of a fifth
    time. I have a 1977 Sierra, MC472, so my system should be virtually
    identical to yours. Mine has five Rochester gauges in a common
    housing in the extreme lower left of the panel.

    Despite what Bob says, the very fastest and easiest way to isolate
    the problem would be to switch the sender wires on the RH and LH
    fuel gauges and see what the LH gauge says when thus connected to the
    RH sender, and vice versa. If the LH gauge then erroneously reads
    1/2 tank, you know the problem is with the sender. If the RH gauge
    still reads in error, the problem is the gauge.

    Keep in mind that the gauge is supposed to read "full" anytime the
    tank is filled at or above the slot in the tab.

    It's way easy to get confused about which wire is on which post -
    don't dive under the panel without a voltmeter. The RH sender wire
    is the starboardmost terminal post (not counting the cluster mounting
    screw) on the gauge cluster - the LH sender is on the second-to-
    portmost post.

    The Rochester fuel gauge should read "empty" when the sender terminal
    post is shorted to ground. In theory it reads "full" when there's a
    ~30 ohm resistance between the sender and ground, but I'm told that
    sometimes there's a bit of mix'n'match of senders and gauges done
    when one or the other is repaired or replaced.

    Access to the gauges is a million times easier than access to the
    sender as Bob recommends - ask me how I know.

    Craig MacCallum
    Sierra N525SB
    Montclair, NJ

    --- In musketeermail@yahoogroups.com, "Barry" <bpalmatier@x...> wrote:
    >
    > Thanks Bob, also thanks to you Mike for the good info.
    >
    > My Sierra is a B24R, 1976, sn mc424
    >
    > I will go remove the wire and see what happens. I am kind of hoping
    > the problem is with the sender as that seems to be the least
    expensive
    > and fastest repair. Any chance the float could be stuck ?
    >
    > Barry
    >
    > PS. I did a bac search but coulden't find anything on the resistance
    > of the sender full vs empty.
    >
    > --- In musketeermail@yahoogroups.com, n76lima@m... wrote:
    > >
    > > >So now I am basically trying to figure out if I have a bad sender
    > > >unit, or a bad gauge.
    > >
    > > The fastest and easiest way would be to remove the wire from the
    > sender and ground it to see if the gauge reads FULL. Removing the
    > wire should cause the gauge to read empty.
    > >
    > > No changes = bad gauge or wiring fault. If it reads both full and
    > empty with the wire grounded and off respectively, then you have a
    > sender problem.
    > >
    > > In ether case, I'd recommend Air Parts of Lock Haven PA as a good
    > vendor to repair the defective part.
    > >
    > > www.airpartsoflockhaven.com
    > >
    > > --Bob Steward A&P IA
    > > Birmingham, AL
    > >
    >






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