Fixing a C4 Sport Seat Lumbar Support

A very common problem with
the C4 sport seat is the failure of the pump that inflates the lumbar
bladders internal to the back of the seat. The most prevalent failure is
not the pump assembly, but simply the diaphragm within the pump
sub-assembly. To gain access to the pump, look under the front of the seat
and there will be a wire clip that holds the bottom seat cushion in place.
Push this clip down and back and then lift the bottom cushion and position
it so you can see underneath. Next, follow the air tubes from the lumbar
support switch and bleed console (right side bolster) to the pump. Remove
the three small bolts that hold the pump to the seat frame. Note, it is
easier to remove if you use the seat adjustment on the console to lift the
front of the seat to get your hand in. These screws must be removed from
the bottom as the nuts visible from the top are set into a hex shaped
depression in the plastic. Once the pump is free, disconnect the two power
connections (disconnect the battery first) and then disconnect the three
tubes from the pump. The pump should now be free to remove from the car
and take into a work area where you won\’t lose all the small parts that
you will have during disassembly.

To access the diaphragm,
remove the two small Phillips head screws at the pump end of the motor and
remove the access plate. Make note of the orientation of the three tube
nipples in relation to the pump housing and then slide the pump
sub-assembly out of the housing while carefully making sure that the
plunger slides off the crankshaft of the motor at the same time. Once the
pump sub-assembly is free of the housing, you will quickly find out if the
diaphragm is torn as is usually the problem with these pumps. Remove the
four Phillips screws that hold the pump sub-assembly halves together and
then remove the diaphragm. This is a good time to take the motor back out
to the car without the pump sub-assembly, plug in the two connectors,
re-connect the battery and ensure that the motor runs. If not, then you
will need to replace the entire pump. Disconnect the battery again prior
to removal.

There are two ways to fix the diaphragm, by making
a replacement using household items and a more expensive way using
replacement parts. GM would like you to buy the entire pump assembly at
more then $130 rather then fix the diaphragm. However, you can buy the
pump subassembly from Jasco Products (original supplier to GM) in Sun
Valley, CA for around $33. The second more creative fix, it to use a dish
washing glove to make a new diaphragm (make sure you wife isn\’t looking
when you take the glove). Trace the outside diameter of the original
diaphragm onto the flat portion of the glove and then cut out the circle
(approximately a 19/32 inch radius). Next, remove the screw and the
sealing washer just under the screw head from the plunger. You will find
the center of the old diaphragm in the recess of the plunger, leave this
in place to fill the recess. Next, punch a hole in the center of your new
diaphragm with a pin and then push the screw with the sealing washer
through the diaphragm and then attach to the plunger. The final assembly
order should be: screw, sealing washer, new diaphragm, old diaphragm
center piece, sealing washer, plunger. Finally, put the diaphragm into the
pump sub-assembly halves. To ensure the correct alignment of the plunger,
put a drill bit through the hole in the plunger where it slides onto the
crankshaft. The hole should line up with the previously noted position of
the tube nipples, i.e., the drill bit should be perpendicular to the tube
nipples. (Failure to get this hole in the correct orientation will cause
the pump to bind.) Now screw the two halves back together with the four
screws previously removed. Put a small dab of grease on the crankshaft
(the grease used should be compatible with rubber) and then slide the pump
sub-assembly back into the housing while making sure that the plunger
slides onto the pin of the crankshaft. Once again, take the pump out to
the car and plug in the connectors to make sure the pump operates
correctly. Re-install the cover plate and then re-install the pump into
the seat by reversing the removal operation.