Dodge RAM TRUCK 1500 1995 Service Manual page 1174

14 - 92 COMPONENT DESCRIPTION/SYSTEM OPERATION—OIESEL ENGINE
The wait-to-start warning lamp is tied to this cir­
cuit. Lamp operation is also controlled by the PCM.
The wait-to-start warning lamp will not be illumi­
nated during the post-heat cycle.
The relays are not energized during engine crank­
ing. When initially energized, they will make a click­
ing noise.
PiEHEAT CYCLE
The PCM will supply a signal to the 2 relays when
the ignition (key) switch is initially turned to the ON
position. When this signal is supplied, electrical cur­
rent is passed through the relays for operation of the
2 heating elements.
If the intake manifold air temperature is 15°C
(59°F) or below, the air heater elements are ener­
gized and the wait-to-start warning lamp is illumi­
nated. The heater is energized for a specific amount
of time. Refer to the following Air Heater Cycle Chart
for a temperature/time comparison of relay engage­
ment.
Once the heater has cycled, the wait-to-start warn­
ing lamp goes out.
While the engine is cranked, the heater relays are
not energized.
POST-HEAT CYCLE
After the pre-heat cycle is completed, the PCM
must receive an engine crank signal (engine speed
between 32 and 475 rpm) followed by an engine run
signal (engine speed above 475 rpm). Intake manifold
air temperature must also be below 15°C (59°F). All
of these signals must be seen by the PCM before ini­
tiating the post-heat cycle.
Depending upon intake manifold air temperature,
engine rpm and predetermined PCM values, one or
both of the relays and one or both of the heating el­
ements may be activated. This may be observed as a
large needle swing on the vehicle voltmeter and is
due to the high-amperage draw of the heating ele­
ments. Each heating element will draw approxi­
mately 95 amps at 12 volts. This voltmeter
movement is a normal condition during the
post-heat cycle.
Refer to the following Air Heater Cycle Chart for a
temperature/time comparison of relay engagement.
The PCM is also programmed with battery saving
features. It will shut down the air heater relays if:
• the engine starter is operated during the pre-heat
cycle.
• the engine stalls during the post-heat cycle.
• the engine starter is operated for more than 10
seconds during the post-heat cycle.
• the vehicle speed is above 10 mph during the post-
heat cycle.
The post-heat cycle will continue for up to 3 1/2
minutes unless the PCM determines one or more of
these preceding features interrupts the cycle strat­
egy.
AIR HEATER CYCLE CHART
mmimmmB
mmmmm
-OfflONOtirBNONS
N O T I N N M N G
wmmmm,
•
Above 15° C
(59° F)
0 seconds No
-10°Cfo 15° C
(15° F to 59° F)
10 seconds Yes
-18° C to -10° F
(0° F to 15° F)
15 seconds Yes
Below-18° C
(0° F)
30 seconds Yes
J9414-133
GENERATOR F1ELD™PCM PIN #21—PCi OUTPUT
The powertrain control module (PCM) regulates the
charging system voltage within a range of 12.9 to
15.0 volts. Refer to Group 8A for charging system in­
formation.
INTAKE MANIFOLD AIR TEMPERATURE SENSOR—
PCM PIN #21—PCM INPUT
This pin provides an input signal to the powertrain
control module (PCM).
The intake manifold air temperature sensor is a
variable, thermistor type. It reacts to temperature
changes. At cold air temperatures, its resistance is
high. As temperatures increase, its resistance will de­
crease.
The air temperature sensor element extends into
the intake manifold air stream. It provides an input
voltage to the PCM indicating intake manifold air
temperature. The input from this sensor is used by
the PCM to determine if and how long to activate the
intake manifold air heater relays. When the relays
are activated, current will flow through the relays to
the intake manifold air heater.
As the temperature of the air-fuel stream in the
manifold varies, the sensor resistance will change.
This will result in a different input voltage to the
PCM.