WORKHORSE CUSTOM CHASSIS
Service Manual
Automatic Transmission – Allison
2007 W Series
© 2006 Workhorse Custom Chassis — All Rights Reserved
SECTION
7.2
PAGE
277
2007 W Series Chassis
WSM082106
DTC P0843
Circuit Description
The pressure switch manifold (PSM) is a multiple-
switch assembly made up of 3 normally open (N/O)
pressure switches and 1 normally closed (N/C) pressure
switch. Pressure switch 1 (PS1) monitors shift valve
1 positioning and relays it to the transmission control
module (TCM).
DTC P0843 is a type A DTC.
DTC Descriptor
This diagnostic procedure supports the following DTC:
• DTC P0843 Transmission Pressure Switch 1 Circuit High
Conditions for Running the DTC
• The components are powered and ignition voltage is
greater than 9.0 volts and less than 18 volts.
• Engine speed is greater than 200 RPM and less than
7,500 RPM for 5 seconds.
• Shift valve 1 is commanded to the ON, stroked, position.
Conditions for Setting the DTC
DTC P0843 sets during steady state operation when shift solenoid
1 (SS1) is commanded ON and PS1 indicates OFF for 0.1
second.
Action Taken When the DTC Sets
• The TCM illuminates the malfunction indicator lamp
(MIL).
• When in a forward range and shifting is completed, the
transmission will fail to another forward range, unless
the transmission is compromised by overspeeding or
direction change, then the transmission will shift to
NEUTRAL.
• When in a forward range and the shift is in process, the
transmission will return to a previous range, except for
1-2 shifts and 2-1 shifts which will be completed.
• When in NEUTRAL or REVERSE, the transmission will
fail to NEUTRAL.
• While diagnostic response is active, the shift selector
is moved from a forward range to NEUTRAL or from a
forward range to REVERSE, then the transmission will
shift to NEUTRAL.
• DTC P0843 is stored in TCM history.
• The TCM freezes shift adapts.
• The TCM inhibits torque converter clutch (TCC)
engagement.
Conditions for Clearing the DTC
A scan tool can clear the code from TCM history. The
TCM automatically clears the DTC from TCM history if
the vehicle completes 40 warm-up cycles without failure.
Diagnostic Aids
• Inspect the wiring for poor electrical connections at the
TCM. Inspect for the following conditions:
– A bent terminal
– A backed-out terminal
– A damaged terminal
– Poor terminal tension