Air Tractor, Inc.
Maintenance Page 13
AT-502A/502B/504
March 14, 2018
Fuel Tank Senders
The L/H fuel tank sender has a shorter float arm than the R/H sender and is positioned differently in
the tank. For these reasons the cork at the end of the float arm is still about 3" from the bottom of the fuel
tank when the L/H sender is at the end of its stroke and the receiver on the instrument panel shows “E”. This
results in about 3 to 4 gallons of fuel still remaining in the
L/H tank when the receiver shows “E”. The fuel
tanks are inter-connected and if coordinated flight has been maintained, the R/H tank will show about 1/8 full
at the same moment the L/H tank shows empty.
The L/H tank sender is Rochester p/n C7740-87 and the R/H sender is p/n C7740-43. The L/H sender
is attached with five 1032 x 1 1/4 stainless screws, 20194-2 bushings, and AN363C1032 nuts. The R/H
sender is attached with five 1032 x 1 3/4 stainless screws, 20194-1 bushings, and AN363C1032 nuts. Both
sides have p/n 20173-1 teflon washers under the screw heads for sealing purposes.
Both senders have a 60008-1 wire attached with a banana plug on one end which is inserted into a
banana jack mounted on the inboard fuel tank wall. If a sender is not working properly, the procedure is to
remove the inspection cover from the fuel tank top just forward of the five sender attach screws and before
removing the sender, first push the banana plug firmly into the jack to be sure proper connection has
previously been made. If this has no effect, pull out the banana plug and connect to an Ohmmeter with the
other lead grounded. With the float arm all the way down the Ohmmeter should read zero and with the arm
all the way up the Ohmmeter should read 29 to 30 Ohms. Cycle the arm and the needle should not have an
erratic movement. If all is OK, the sender does not have to be replaced. If there is still indication of a faulty
sender, remove the five screws attaching it to the tank top, being careful not to damage the teflon washers
under the head of each screw. The fuel tank should be drained before the sender is replaced. The five
bushings will probably be held in place by the tank sealing compound if care is taken when removing the
sender. Pull the banana plug from the jack and remove the sender.
If the R/H sender is being replaced the cork at the end of the float arm should just touch the bottom of
the tank when the arm is at the end of its stroke. It should touch the top of the tank at the upper end of the
stroke. If it will not quite touch the tank bottom or touches too soon, bend the arm gently by hand to achieve
the desired position. The L/H sender will not touch the tank bottom, but will touch the tank top.
Before re-installing the inspection plate on the fuel tank top, move the float arm through the full stroke
with the battery switch “On” and the receiver switch on the proper tank to see that the sender is working
properly. Seal the inspection plate per instruction in “Fuel Tank” and re-install.
Fuel Tank Receiver
The fu
el tank receiver (gauge) has an adjustment screw for the “E” side of the needle range on the
lower side of the face of the instrument. The trimmer screws for the “F” side of the needle range is on a small
circuit board mounted to the back of the instrument panel below the gauges. It is possible to reach under the
panel and with a small screwdriver reach the “F” trimmer screws.
To check the receiver settings, drain all the fuel from the aircraft, park it on flat level surface out of the
wind and jack the tail wheel up onto a platform until the bottom of the wing tanks are level. Be sure parking
brake is “On” as the tail is quite high in this position. Use a carpenter’s level to level the bottom of the wings.
Level the wing tips by adjusting the air in the tires and measuring the wing tips from the level floor. It is ok for
a tire to look pretty low for this procedure. Be sure to air the tires back up to normal pressure when you are
finished.
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