P1494 (DTC 184)

DTC 184 LDP SWITCH/MECHANICAL FAULT (General Scan Tool Code No. P1494)

Immediately after cold engine start-up (if all Leak Detection Test enabling conditions listed in this bulletin are met), the PCM detects the LDP switch status. Under normal operation, the LDP switch is initially on, and the PCM turns the LDP solenoid on and the LDP diaphragm is pulled up by engine vacuum, causing the LDP switch to turn off.

^ If the PCM detects that the LDP switch was initially off, it could be due to a failed LDP switch or an open circuit. If the system was sufficiently purged on the previous drive, the PCM then sets DTC 184. However, insufficient EVAP system purge on the previous drive can allow residual pressure to remain in the system and hold the LDP diaphragm up and hold the switch off. DTC 184 will not set if the system was not sufficiently purged.

^ If the PCM detects that the LDP switch was initially on and does not turn off within 2.1 seconds of the solenoid turning on, the PCM sets a temporary "fault." If this occurs in two consecutive cold start-ups when all Leak Detection Test enabling conditions are met, DTC 184 will set.





DTC 184 can be set under any of the following conditions:

- Insufficient vacuum supply to the LDP
- LDP internal mechanical malfunction
- LDP switch malfunction
- LDP switch circuit or PCM malfunction





DTC 184 Diagnosis

It is important to read and understand the EVAP system information in before attempting diagnosis.

Description

IMPORTANT
Before beginning this procedure, repair all vehicle conditions for DTCS that are not related to the EVAP system. If more than one EVAP system DTC is set, diagnose and repair in the following order:
(1) DTC 183 (P1495)
(2) DTC 184 (P1494)
(3) DTC 187 (P1486)
(4) DTC 160/161 (P0442/P0455)

1. Remove an install the gas cap. This will eliminate the possibility of misleading readings caused by residual pressure in the fuel tank or evaporative emissions system,

2. Connect the MUT-II scan tool and adapter harness MB991545. Turn the ignition key to ON.





3. Enter LDP Hold Mode to activate the LDP solenoid and observe the LDP switch, solenoid, and vent value status.





4. Check the LDP SW status as displayed on the MUT-II screen.

a. If the screen shows the LDP SW status as "ON:DN," go to Step 11.

b. If the screen shows the LDP SW status as "OFF:UP," go to Step 5 for further diagnosis.





5. To determine if the LDP switch has failed, disconnect LDP connector A-109. With the ignition key still on, connect a test light jumper between terminals 2 and 4, using a test light.
NOTE
A test light with a light bulb must be used in this step. To prevent misleading diagnosis, do not use a DVOM, analog meter, or LED test light.

a. If the test light comes on, there is a short to ground in the switch circuit or PCM input. Go to Step 6.

b. If test lights stay off, go to Step 8.

6. With the test light still connected, disconnect PCM connector A-107.

a. IF the light goes off, the PCM input is shorted to ground. Replace the PCM and confirm the test does not come on. Then also replace the LDP, since its switch was probably damaged by the high current during the short to ground condition. Then go to Readiness Test.

b. If the test light stays on, there is a short to ground between the LDP and the PCM.

7. Locate the short to ground between pin 4 of LDP connector A-109 and pin 72 of PCM connector A-107.

a. Inspect connectors A-109, B-76, B-63, and A-107 and the associated wiring. Repair or replace as needed.

b. After confirming the short to ground is corrected, replace the LDP, since its switch was damaged by the high current during the short to ground. Then go to the Readiness Test of this bulletin.





8. [From Step 5.b] With the test light still connected between pins 2 and 4. check the LDP SW status displayed on the MUT-II screen.

a. If the screen shows LDP SW status as "ON:DN", a problem exists with the LDP connector, Go to Step 9.

b. If the screen shows LOP SW status as "OFF:UP", a problem exists between the LDP connector and the PCM. Go to Step 10.





9. Inspect terminal 4 of LOP connector A-109 for pin damage.

a. It the pin is damaged, repair or replace as needed. Then go to the Readiness Test of this bulletin.

b. It no damage is found, replace the LDP. Then go to the Readiness Test of this bulletin.





10. Check the resistance between terminal 4 of LDP connector A-109 (red/blue wire) and terminal 72 of the lower PCM gray connector A-107. Resistance should be less than 5 ohms.

a. If resistance is less than 5 ohms, check the connector pins for damage.

If the pins are damaged, repair or replace as needed. Then go to the Readiness Test of this bulletin.

If no damage is found, replace the PCM. Then go to the Readiness Test of this bulletin.

b. If resistance is 5 ohms or more, inspect connectors A-107, B-76, B-63, and A-109 for damage. Repair or replace as needed. Then go to the Readiness Test of this bulletin.

11. Start the engine and allow it to idle.





12. With the MUT-II screen still displaying "2. Hold Mode", press "Y" (Yes) to activate the LDP solenoid. The screen should indicate that the solenoid changed from "VacBLC" (vacuum is blocked) to "VacAPP" (vacuum is applied to the diaphragm). Also, the LDP switch state should change from "ON:ON" to "OFF:UP" as the diaphragm moves up.

a. If the LDP switch does not change as described, go to Step 13.

NOTE:
If the PCM does not detect the switch change, the MUT-II screen will display: "NOW HOLDING, PLEASE APPLY THE VACUUM."

b. If the LDP solenoid and LDP switch change as described, the condition that set DTC 184 is not present at this time. Test for a possible intermittent electrical problem and inspect connectors for damage. Repair as needed. If no intermittent problem or connector damage is found, contact Tech Line for further instructions.

13. With the engine idling, disconnect the vacuum supply hose at the LDP solenoid fitting and connect it to a vacuum gauge.

a. If there is a noticeable idle flare immediately after the hose is disconnected, and the vacuum reading is strong, steady, and within one inch of intake manifold vacuum, go to Step 14.

b. If there is a no noticeable idle flare immediately after the hose is disconnected, and the vacuum reading is not strong, steady, and within one inch of intake manifold vacuum, check the following:





^ Check the service vacuum supply tee (SVST) fitting for proper installation as shown.

^ Remove the SVST and check for blockage by blowing air through it as shown.

^ Check for blockage or leaks in the vacuum supply line between the intake manifold and the LDP. Repair or replace as necessary.

^ Check for blockage in the intake manifold vacuum fitting.

After correcting vacuum supply malfunction(s), confirm that the LDP is now operating properly as follows:

(1) Start the engine and allow it to idle.

(2) With the MUT-II screen still showing the LDP in the Hold Mode, press "Y" (Yes) to activate the LDP solenoid. The screen should indicate that the solenoid changed from "VacBLC" (vacuum is blocked) to "VacAPP" (vacuum is applied to the diaphragm). Also, the LDP switch state should change from "ON:DN" to "OFF:UP".

(3) Then go to the Readiness Test of this bulletin.





14. Remove connector A-109 from the LDP. Check the LDP SW status displayed on the MUT-II screen.

a. If the switch status changes from "ON:DN" to "OFF:UP", this indicates that the wiring and PCM switch input are working properly, and that the LDP does not function correctly. Replace the LDP. Then go to Readiness Test of this bulletin.

b. If the switch status remains at "ON:DN", the PCM is faulty or there is a short to voltage in the wire between pin 4 of connector A-109 and pin 72 of connector A-107.

Using a DVOM, test for a short to voltage in the circuit. If there is no short, replace the PCM, then go to Readiness Test of this bulletin.