Diagnostics and testing of glow plugs (Discovery 3)

Inspection and verification

1. Ensure that the customer's complaint is justified.

2. Visually inspect for obvious electrical problems.

Electrical factors

Glow plug indicator light Fuses and fuses Glow Plug Relay Engine Management System Relay Wiring harness(es) Electrical connector(s) Glow plug(s) Engine Control Module (ECM)

3. If an obvious cause for the problem you identified or the customer described is found, resolve it before moving on to further action. (if possible).

4. Use the approved diagnostic system or scan tool to retrieve all diagnostic trouble codes (DTCs) before referring to the symptom chart or DTC index.

Clear all DTCs after troubleshooting.

Table of signs of malfunction

Sign of malfunction	Possible reason	Actions
Bad start (extreme weather conditions)	Glow plugs not working/not effective enough Fuel temperature too low	Check the wiring harnesses at the glow plugs and where they connect to the main wiring harness. Refer to the electrical manual. Check for glow plug related DTCs. The fuel system cycles the fuel until operating temperature is reached to reduce this possibility.
High emissions when engine is cold	Post-heating phase does not work	Check the wiring harnesses at the glow plugs and where they connect to the main wiring harness. Refer to the electrical manual. Check for glow plug related DTCs. The afterglow phase should operate at engine temperatures below 50 degrees C and engine speeds below 2,500 rpm.
High NVH levels when engine is cold	Post-heating phase does not work	Check the wiring harnesses at the glow plugs and where they connect to the main wiring harness. Refer to the electrical manual. Check for glow plug related DTCs. The afterglow phase should operate at engine temperatures below 50 degrees C and engine speeds below 2,500 rpm.

DTC Index

NOTE: Generic scan tools cannot read the codes listed or can only read 5-digit codes. Match the 5 digits from the scan tool with the first 5 digits of the listed 7-digit code to identify the fault. (the last 2 digits provide additional information readable by a manufacturer approved diagnostic system).

NOTE: For a complete listing of ECM DTCs, refer to Section 303-14, Electronic Engine Control System, in the service manual.

DTC codes	Description	Possible reasons	Actions
P038300	Low Voltage in Glow Plug Control Module Control Circuit	Glow Plug Relay Control Circuit High Resistance Glow Plug Relay Control Circuit Short to Ground Glow plug relay malfunction	Refer to the electrical manual and check the electrical circuit. Check the operation of the relay. Repair/replace as necessary.
P038400	High Voltage in Glow Plug Control Module Control Circuit	Glow Plug Relay Control Circuit: Short to Battery + Glow plug relay malfunction	Refer to the electrical manual and check the electrical circuit. Check the operation of the relay. Repair/replace as necessary.
P067000	Glow Plug Control Module - Control Circuit Open	Glow Plug Relay Control Circuit High Resistance Glow Plug Relay Control Circuit Short to Ground Glow plug relay malfunction	Check glow plug relay circuits. Refer to electrical manual. Check relay operation. Activate relay and listen for "click". Repair/replace as necessary. Clear DTCs and check for proper operation.