Jeep Cherokee 2000 Service Manual - page 1256
The duty–cycle is determined by the ECM from
inputs it receives from the fuel injector sensor and
engine speed sensor.
TACHOMETER—PCM OUTPUT
DESCRIPTION
The PCM recieves engine rpm values from the
ECM controller, and then supplies engine rpm values
to the Body Controller that then supplies the instru-
ment cluster mounted tachometer (if equipped). Refer
to Group 8E for tachometer information.
GLOW PLUG RELAY—ECM OUTPUT
DESCRIPTION
When the ignition (key) switch is placed in the ON
position, a signal is sent to the ECM relating current
engine coolant temperature. This signal is sent from
the engine coolant temperature sensor.
After receiving this signal, the ECM will determine
if, when and for how long a period the glow plug
relay should be activated. This is done before, during
and after the engine is started. Whenever the glow
plug relay is activated, it will control the 12V+ 100
amp circuit for the operation of the four glow plugs.
With a cold engine, the glow plug relay and glow
plugs may be activated for a maximum time of 200
seconds. Refer to the following Glow Plug Control
chart for a temperature/time comparison of glow plug
relay operation.
In this chart, Pre–Heat and Post–Heat times are
mentioned. Pre–heat is the amount of time the glow
plug relay circuit is activated when the ignition (key)
switch is ON, but the engine has yet to be started.
Post–heat is the amount of time the glow plug relay
circuit is activated after the engine is operating. The
Glow Plug lamp will not be illuminated during the
post–heat cycle.
GLOW PLUGS
DESCRIPTION
Glow plugs are used to help start a cold or cool
engine. The plug will heat up and glow to heat the
combustion chamber of each cylinder. An individual
plug is used for each cylinder. Each plug is threaded
into the cylinder head above the fuel injector (Fig.
14).
Each plug will momentarily draw approximately 25
amps of electrical current during the initial key–on
Fig. 13 Glow Plug Relay Location
1 – MOUNTING BOLT
2 – GLOW PLUG RELAY
3 – ELECTRICAL CONNECTOR
GLOW PLUG CONTROL
ENGINE
COOLANT
TEMP-
ERATURE
KEY ON
WAIT-TO-
START
LAMP ON
(SECONDS)
PRE-HEAT
CYCLE
(GLOW
PLUGS
ON)
(SECONDS)
POST-
HEAT
CYCLE
(SECONDS)
-30 C
15 SEC.
45 SEC.
200 SEC.
-10 C
8 SEC.
35 SEC.
180 SEC.
+10 C
6 SEC.
25 SEC.
118 SEC.
+30 C
5 SEC.
20 SEC.
70 SEC.
+40 C
4 SEC.
16 SEC.
60 SEC.
+70 C
3 SEC.
16 SEC.
20 SEC.
Fig. 14 Glow Plug
1 – GLOW PLUG
14 - 30
FUEL SYSTEM—2.5L DIESEL ENGINE
XJ
DESCRIPTION AND OPERATION (Continued)