Expedition 4WD V8-4.6L SOHC VIN W (1999)
^
Audible clunking or grinding noises, and/or Amber low range light illuminated
a.
If the Generic Electronic Module (GEM) receives a false Mode Switch input during or shortly after the Digital Transmission Range Sensor
(DTR) indicates a Neutral range pass-thru, an uncommanded shift is possible if remaining 4L pre-conditions are met. The pre-conditions
besides transmission in Neutral range include: service brake depressed, and vehicle speed less than 5 km/h (3 mph). A false switch input
may also set a P1812 or P1815 DTC in the GEM.
b.
Check mode switch circuits 682 (dark blue) for short to power and 780 (dark blue) for ground short, loose connections at inline connectors,
and chafes.
c.
Monitor Mode Switch Parameter Identification Display (PID) 4WD_SW for false readings while slightly pushing in and wiggling the mode
switch knob. Refer to Workshop Manual Section 308-07A for mode switch resistance tests for all switch positions.
d.
Replace Mode Switch if it fails testing or repair wiring on circuits 682 or 780 if fault indicated. If tests pass, inspect build date stamped on
GEM and replace if GEM built prior to 9/98.
e.
Test drive vehicle for proper 4X4 operation in all modes. Include parking lot maneuvers and transmission gear range lever transitions while
testing. Pull DTCs from GEM (even if there are no DTCs available). Clear any codes present and cycle ignition.
2.
Road Speed: Reported symptoms for a higher speed uncommanded shift to 4L event include:
^
Rapid deceleration
^
High engine revving possibly accompanied by a P1270 DTC in the PCM
^
Clunk/grind noise
^
Speedometer may spike higher than actual speed.
After the initial event the vehicle may exhibit restricted vehicle top speed without 4X4 indicators illuminated but possible MIL (Malfunction Indicator
Lamp) on.
a.
Monitor contact plate A, B, C, D PIDS. With Mode Switch in;
^
A4WD, plate PIDS should read "0C00"
^
4H PIDS should read "C00C"
^
4L PIDS should read "C0C0" (sequentially read starting with plate A then B, then C, finally D).
If contact plate PIDS DO NOT correspond to the set 4WD position, check for continuity/shorts/moisture/corrosion in the vehicle side of the transfer case
shift motor connector (toward the GEM). Visually inspect all terminals, pins, crimps, and connectors closely. Repair any wiring conditions in the contact
plate circuits as necessary.
b.
Road test at speeds above 16 km/h (10 mph) with Mode Switch in both A4WD and 4H to see if the condition returns. Clear DTCs from
GEM (even if no DTCs exist) and cycle ignition. Clear DTCs from PCM if a P1270 (engine RPM or vehicle speed limiter reached) code
was initially present.
UNCOMMANDED SHIFT TO 4H
1.
Autolock Strategy (Expedition/Navigator Vehicles Only): Uncommanded 4H shifts with red 4X4 light on "solid" while driving in Automatic Mode
may be a vehicle characteristic if driven off-road or under slippery conditions. The GEM Autolock strategy commands transfer case clutch to
minimum duty cycle and engages 4H (with mode switch still set to A4WD) to prevent continuous cycling from prematurely wearing the clutch.
This is design intent. Excessive tire circumference variations or axles with unmatched gear ratio's can result in unexpected Autolock function as
well, for more detail check the speed sensor section below. Once the condition that caused Autolock strategy to activate is no longer present, the
system stays in 4H until operator cycles the ignition key or sets the mode switch to 4H then back to A4WD.
2.
Transfer Case Speed Sensors
(Expedition/Navigator Vehicles Only): If uncommanded shift to 4H occurs on hi-traction surfaces, check TRA_FSP and TRA_RSP PIDS for
excessive speed sensor variation between the front and rear transfer case speed sensors.
a.
If PIDS show sensor inputs not within 2-3 km/h (1-2 mph) of each other during steady-state driving with possible DTCs P1836/P1837, first
verify tire pressures, sizes, circumference within 1/4 " among all four tires, and front and rear axle gear ratio's for matching.
NOTE
