Dodge RAM TRUCK 1500 1995 Service Manual page 457

B R VEHICLE SPEED CONTROL SYSTEM 8H - 3
D I A G N O S I S
ROAD TEST
Before beginning diagnosis, perform a vehicle road
test to verify reports of speed control system mal­
function. The road test should include attention to
the speedometer. Speedometer operation should be
smooth and without flutter 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 Group 8E -
Instrument Panel and Gauges for speedometer diag­
nosis.
INSPECTION
If a road test verifies a system problem and the
speedometer operates properly, check for:
(1) 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.
(2) Correct installation of the vacuum check valve
in the hose from servo to vacuum source. The word
VAC on the valve must point toward the vacuum
source.
(3) Loose or leaking vacuum hoses or connections.
(4) Secure attachment of both ends of the speed
control servo cable.
C A U T I O N : When test probing for voltage or conti­
nuity at electrical connectors, care must be taken
not to damage connector, terminals, or seals. If
these components are damaged, intermittent or
complete system failure may occur.
USING ON-BOARD DIAGNOSTIC SYSTEM
The Powertrain Control Module (PCM) monitors
critical input and output circuits of the speed control
system, making sure they are operational. A Diagnos­
tic Trouble Code (DTC) is assigned to each input and
output circuit monitored by the OBD system. Some
circuits are checked continuously and some are
checked only under certain conditions.
If the OBD system senses that a monitored circuit
is bad, it will put a DTC into electronic memory. The
DTC will stay in electronic memory as long as the
circuit continues to be bad. The PCM is programmed
to clear the memory after 50 engine starts, if the
problem does not occur again.
DIAGNOSTIC TROUBLE CODES
Diagnostic Trouble Codes (DTC) are two-digit num­
bers flashed on the malfunction indicator (Check En­
gine) lamp that identify which circuit is bad. A DTC
description can also be read using the DRB scan tool.
Refer to Group 14 - Fuel Systems for more informa­
tion.
A DTC does not identify which component in a cir­
cuit is bad. Thus, a DTC should be treated as a
symptom, not as the cause for the problem. In some
cases, because of the design of the diagnostic test
procedure, a DTC can be the reason for another DTC
to be set. Therefore, it is important that the test pro­
cedures be followed in sequence, to understand what
caused a DTC to be set.
See Speed Control Diagnostic Trouble Code chart
for DTC's which apply to the speed control system.
Refer to the Powertrain Diagnostic Procedures man­
ual to diagnose an on-board diagnostic system trou­
ble code.
RETRIEVING DIAGNOSTIC TROUBLE CODES
To start this function, cycle the ignition switch ON-
OFF-ON-OFF-ON within 5 seconds. This will cause
any DTC stored in the PCM memory to be displayed.
The malfunction indicator (Check Engine) lamp will
display a DTC by flashing on and off. There is a
short pause between flashes and a longer pause be­
tween digits. All DTC's displayed are two-digit num­
bers, with a four-second pause between codes.
An example of a DTC is as follows:
(1) Lamp on for 2 seconds, then turns off.
(2) Lamp flashes 1 time pauses and then flashes 5
times.
(3) Lamp pauses for 4 seconds, flashes 3 times,
pauses, then flashes 4 times.
The two DTC's are 15 and 34. Any number of
DTC's can be displayed, as long as they are in mem­
ory. The lamp will flash until all stored DTC's are
displayed (55 = end of test).
If a DTC 15 is observed, see diagnosis for Vehicle
Speed Sensor in this group. If a DTC 34 is observed,
see diagnosis for Speed Control Servo and Power-
train Control Module in this group. If a DTC 11 is
observed (diesel engine only), refer to the Powertrain
Diagnostic Procedures manual and perform the test
for a faulty engine speed sensor. Correct any prob­
lems found in your diagnosis, then recheck for DTC
after corrections are completed.
VEHICLE SPEED SENSOR
For diagnosis of the VSS, refer to the Powertrain
Diagnostic Procedures manual.