Jaguar XJ (X350). Manual — part 765

C (104° F) with a 12 Volt supply.

The glow plug control module (GPCM) is located in the engine bay on the right-hand side. It
acts as a switch for the current to the glow plugs and is controlled via the engine control
module (ECM). Above a given coolant temperature, glow plugs will not be switched on
before, during or after starting. Within a certain coolant temperature range, the glow plugs
will be active before cranking to increase combustion chamber temperature, during cranking
to assist engine starting, and after the engine has started in order to reduce the emissions of
white and possibility black smoke from the tail pipe immediately following a cold start.

The ECM is connected to the GPCM via a reference control line and a diagnostic line. The
former will be pulled down by the ECM causing the GPCM to switch on and to remain
switched on while the reference voltage is low. The ECM will store the error information
generated by the GPCM in the case of glow plug failure.

Diagnosis and testing

Glow Plug System

Overview

There are changes to the engine management system (EMS) for 2006 my, the most obvious of which

will be the change to 7-digit diagnostic trouble codes (DTCs) from the familiar 5-digit.

Refer to the DTC index in this section for guidance on how to use these codes with the Jaguar

approved diagnostic system or a scan tool.

For information on the operation of the system,

Glow Plug System

Inspection and Verification

1 . Verify the customer concern.

2 . Visually inspect for obvious signs of electrical damage.

Electrical

Glow plug lamp

Fuses (100 Amp midifuse and fuse 9 of the front power distribution box)

Glow plug control module (GPCM)

Engine management control relay

Wiring harness(es)

Electrical connector(s)

Glow plug(s)

Engine control module (ECM)

3 . If an obvious cause for an observed or reported concern is found, correct the cause (if possible)

and test the system for normal operation before proceeding to the next step.

4 . If the cause is not visually evident use the Jaguar approved diagnostic system or a scan tool to

retrieve the fault codes before proceeding to the DTC index, or the symptom chart if no DTCs are set.

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Symptom chart

Symptom

Possible cause

Action

Poor starting

(extreme

weather

conditions)

Glow plugs

inoperative/ineffici
ent

Fuel temperature too

low

Check the glow plug harnesses at the glow plugs and at

the connection to the main harness. For additional

information, refer to the wiring diagrams. Check for

DTCs indicating a control module or circuit fault. The

fuel system recycles fuel until operating temperature is

reached to reduce this possibility.

High cold-

engine

emissions

After-glow phase

inoperative

Check the glow plug harnesses at the glow plugs and at

the connection to the main harness. For additional

information, refer to the wiring diagrams. Check for

DTCs indicating a control module or circuit fault. After-

glow is designed to function at engine temperatures

below 50° C (122° F), and below 2500 rpm.

High cold-

engine noise,

vibration or

harshness

After-glow phase

inoperative

Check the glow plug harnesses at the glow plugs and at

the connection to the main harness. For additional

information, refer to the wiring diagrams. Check for

DTCs indicating a control module or circuit fault. After-

glow is designed to function at engine temperatures

below 50° C (122° F), and below 2500 rpm.

DTC index

NOTE:

Generic scan tools may not read the codes listed, or may read only 5-digit codes. Match the 5

digits from the scan tool to the first 5 digits of the 7-digit code listed to identify the fault (the last

2 digits give extra information read by the manufacturer-approved diagnostic system).

NOTE:

For a full list of powertrain DTCs,

Electronic Engine Controls

DTC

Description

Possible source

Action

P038000

Glow plug control

module (GPCM)

The GPCM is driven, but the

glow plugs are not
powered

For GPCM circuit tests, GO to

Pinpoint Test G552799p2.

P038200

Glow plug control

module (GPCM)

The GPCM is not driven, but

the glow plugs are
powered

For GPCM circuit tests, GO to

Pinpoint Test G552799p2.

P038300

Glow plug control

module (GPCM) circuit

low

GPCM control circuit: short

circuit to ground

For GPCM control circuit tests,

GO to Pinpoint Test

G552799p1.

P038400

Glow plug control

module (GPCM) circuit

high

GPCM control circuit: short

circuit to power

For GPCM control circuit tests,

GO to Pinpoint Test

G552799p1.

P067000

Glow plug control

module (GPCM) circuit

open

GPCM control circuit: high

resistance

For GPCM control circuit tests,

GO to Pinpoint Test

G552799p1.

Pinpoint Tests

CAUTION: When probing connectors to take measurements in the course of the pinpoint

tests, use the adaptor kit, part number 3548-1358-00. Failure to follow this instruction may result

in damage to the vehicle.

NOTE:

When performing voltage or resistance tests, always use a digital multimeter (DMM) accurate to

3 decimal places, and with an up-to-date calibration certificate. When testing resistance, always

take the resistance of the DMM leads into account.

NOTE:

Check and rectify basic faults before beginning diagnostic routines involving pinpoint tests.

NOTE:

If DTCs are recorded and a fault is not present when performing the pinpoint tests, an

intermittent concern may be the cause. Always check for loose connections and corroded

terminals.

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