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14 Apr, 2003 By: Bill Voelker imageSource

Technical Tools

technicians run into an error that we have trouble figuring out. Other times, we
seem to be able to fix a problem without knowing exactly why; we changed a part
and the machine was fixed. This has happened to many technicians, including
myself. In this article, we'll talk about one of those errors-what it is, what
it does, and how to fix it. For this problem, fixing it is the easy part. It's
understanding what it is and what it does that's difficult.

Complex Coupling On
the Lexmark C710, a 990 Error is a coupling operation error and the 991 Error is
a coupling sensor error. To understand these errors it is necessary to first
understand what a coupling is and what it does.

The coupling is a
mechanical device that engages the OPC drum in each toner cartridge with the
drive block once the carriage has properly positioned the toner cartridge with
the transfer belt. This position is determined by the carriage detect ribs that
pass through a home position photo sensor. All these ribs are the same size
except for one. This longer detect rib is home position and is located near the
magenta cartridge slot on the carriage. When this longer rib passes through the
home position sensor, (Figure
) the cyan cartridge is then engaged by the coupling to begin its share of
the writing and primary transfer process.

As each color comes
into position, the coupling motor rotates a cam that locks the carriage in place
and engages each OPC drum. When the carriage is ready to rotate to the next
color, the coupling motor reverses to disengage the coupling. The coupling
position is detected by the "coupling on" sensor, or the
"coupling off" sensor (Figure

If either one of
these sensors does not see the sensor flag when it is supposed to, a 990 Error
will appear on the display. If either of the sensors is bad, a 991 Error will be
displayed. For troubleshooting these two errors, the first thing that should be
done is to gain access to the area (as explained below) and perform a Coupling
Unit Service Check (page 2-42 in the service manual).

Below are two things
to look for that might cause the flag not to move and result in a 990 Error. The
third scenario offers steps to take should you encounter a 991 Error.

a. If the coupling
motor itself does not turn at all, rotate the green knob to manually reset the
coupling. By resetting the coupling, the technician can observe the in and out
action of the coupling itself and check for binding. If there is no binding,
then disconnect the motor cable at its inline connector and measure the
resistance of the motor. It should measure between 9 and 15 ohms. If the ohm
reading is correct and the motor still shows no evidence of moving the coupling,
first check for a bad connection, then check for a bad cable (check for
continuity). If everything else checks out, the engine controller could be at

b. If the coupling
motor is turning but the coupling does not move, turn the green manual knob and
feel for binding. If necessary, check the entire assembly for broken parts,
especially the gear on the motor shaft (Figure
). To do this, you will need to remove the coupling drive block itself (see
steps listed below).

c. A 991 Error
indicates one or both of the "on" "off" sensors is bad. If
this is the case, and the sensors are good or have been recently replaced, check
the cables to the sensors for continuity. If these are ok, then the engine
control board would be suspect. Unfortunately, this printer does not use the
diagnostic sensor tests that other printers do, so there is no way to test the

To access the
coupling and drive block remove the rear panel by loosening the four captive
screws (Figure

Remove the right
side panel by removing the four screws (Figure

To remove the
coupling and drive block you should remove the fan and fan shroud-two screws
each (Figure

Remove the fuser
drive block by disconnecting the cable harness (Figure
) and opening all ties that hold the cable in place. Then remove the six
mount screws (Figure

Remove the coupling
drive block (Figure
) by removing the e clip and washer (green arrow). Slide the belt off the
pulley (yellow arrow) and then remove the seven screws (red arrows).

Inspect the drive
block (Figure
) to determine any possible causes. If the motor gear is bad, the entire
motor has to be purchased (part # 12G1620). Due to the complexity of
disassembling the drive block, it would be a time-saver to replace the entire
coupling drive block (part # 12G122).

So far, every
incidence involving this problem that I am aware of, has involved the motor gear
itself, internal hub stripping or the gear breaking. Because of this, we have
chosen to concentrate on the coupling. If it becomes necessary to change the
engine controller, you will need to refer to your C710 service manual (page

This problem also
appears to be specific to the C710. As this printer ages and more calls for this
error come in, our goal will be to help with the troubleshooting and repair in
order to save time for technicians in the field.


Voelker is a Printer Trainer for PARTS NOW!, a major printer parts wholesaler
for HP, Canon, Lexmark and Okidata. To contact PARTS NOW!, call 800/886-6688 or
visit www.partsnowinc.com.

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