Operation - Checking For Diagnostic Codes; Diagnosis And Testing - Road Test - Chrysler PT Cruiser Service & Repair Manual

PT
SPEED CONTROL (Continued)
train) and can maintain the set speed at a reduced
throttle position, the transmission will upshift, as
appropriate, until the set speed can be maintained in
Overdrive.
GRADE HUNTING
DESCRIPTION
All vehicles equipped with a four speed automatic
transmission have a grade hunting feature for the
2nd to 3rd gear upshift and the 3rd to Overdrive
upshift.
OPERATION
The TCM (on SBEC vehicles) (PCM on NGC vehi-
cles) identifies the powertrain loading conditions and
selects the proper gear to maintain the current vehi-
cle speed. Under moderate loading conditions the
transaxle will stay in 3rd gear until the top of the
grade is reached or the powertrain loading is
reduced.
If powertrain loading is severe, the transaxle may
shift into 2nd gear and remain there until power-
train loading is reduced, then a 2nd to 3rd gear
upshift will be scheduled. Grade hunting features
always operate regardless of whether or not the
interactive speed control is engaged. If the interac-
tive speed control is not engaged and power-
train loading is not reduced, the driver may
have to completely lift off of the throttle before
an upshift will occur. If the driver does lift off the
throttle to induce an upshift under these conditions,
vehicle speed will reduce and the Overdrive to 3rd
and 3rd to 2nd gear downshifts will reoccur when the
throttle is reapplied. If grade hunting is repeatedly
induced by the driver, transaxle damage may result.
AUTOMATIC SPEED CONTROL OVERSPEED
REDUCTION
DESCRIPTION
Transmission control software includes an auto-
matic speed control overspeed reduction feature. This
maintains vehicle speed at the selected set point
when descending a grade.
OPERATION
The TCM (on SBEC vehicles) (PCM on NGC vehi-
cles) first senses that the speed control is set. If the
set speed is exceeded by more than 4 mph (6.5
km/hr) and the throttle is closed, the TCM (on SBEC
vehicles) (PCM on NGC vehicles) causes the trans-
axle to downshift to THIRD gear. After downshifting,
the automatic speed control resumes normal opera-
tion. To ensure that an upshift is appropriate after
the set speed is reached, the TCM (on SBEC vehi-
cles) (PCM on NGC vehicles) waits until the speed
control system opens the throttle at least 6 degrees
before upshifting to OVERDRIVE again.
If the driver applies the brakes, canceling auto-
matic speed control operation with the transaxle still
in THIRD gear, the TCM (on SBEC vehicles) (PCM
on NGC vehicles) maintains this gear until the driver
opens the throttle at least 6 degrees to avoid an inap-
propriate upshift. The upshift is also delayed for 2.5
seconds after reaching the 6 degrees throttle opening
in anticipation that the driver might open the throt-
tle enough to require THIRD gear. This will avoid
unnecessary and disturbing transmission cycling. If
the automatic speed control RESUME feature is used
after braking, the upshift is delayed until the set
speed is achieved to reduce cycling and provide bet-
ter response.
OPERATION - CHECKING FOR DIAGNOSTIC
CODES
When trying to verify a speed control system elec-
tronic malfunction: Connect a DRB scan tool if avail-
able to the data link connector. The connector is
located near the steering column, and at lower edge
of the dash panel.
A speed control malfunction may occur without a
diagnostic code being indicated. For further informa-
tion and usage of the DRB scan tool and a more com-
plete list of Diagnostic Trouble Code and No Trouble
Codes, refer to the Powertrain Diagnostic Manual.

DIAGNOSIS AND TESTING - ROAD TEST

Perform a vehicle road test to verify reports of
speed control system malfunction. The road test
should include attention to the speedometer. Speed-
ometer operation should be smooth and without flut-
ter at all speeds.
Flutter in the speedometer indicates a problem
which might cause surging in the speed control sys-
tem. The cause of any speedometer problems should
be corrected before proceeding. Refer to the Instru-
ment Cluster for speedometer diagnosis.
If a road test verifies an inoperative system, and
the speedometer operates properly, check for:
• A Diagnostic Trouble Code (DTC). If a DTC
exists, conduct tests per the Powertrain Diagnostic
Procedures manual.
• A misadjusted brake (stop) lamp switch. This
could also cause an intermittent problem.
• Loose or corroded electrical connections at the
servo. Corrosion should be removed from electrical
terminals and a light coating of Mopar Multipurpose
Grease, or equivalent, applied.
• Leaking vacuum reservoir.
• Loose or leaking vacuum hoses or connections.
• Defective one-way vacuum check valve.
SPEED CONTROL 8P - 3