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04 Emissions wiring question....

990 Views 6 Replies 3 Participants Last post by  Blackwidow TJ
Long story short, my highlift shorted out on my emissions wires and melted the body harness. The wires running to the evap canister are orange, White/blue seam and white/green seam. What do they do and which is suppose to have power? Do they get ignition power or does the jeep have to be running? The jeep runs and everything works as before, all except the emissions. I'm hoping it is just the body harness that is damaged and not the rest that runs through the firewall. Any help or info is greatly appreciated. Thanks in advance- Steve
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Best I can tell from my iPhone edition 04 FSM :laugh: The orange (OR), white with green (WT/DG) & white with blue (WT/DB) all deal with the leak detection pump.

The white with green & the orange connect the PCM to the leak detection pump. White & green being the "leak detection pump solenoid control" in the PCM. The orange goes to the "leak detection pump switch sense" in the PCM. And the white with blue goes to what I am assuming is the power source? Not sure on that one. Hope this helps, I hate wiring! Let me know if you need anything else and maybe I can pull it for you.

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Thanks alot! You happen to know if there should be constant power or maybe if its only when it does a leak test? Yeah I hate this too, again thank you!
That is what I cannot find a clear answer on reading in the FSM. I've read that section twice now and it seems unclear to me. Best I can tell (may or may not be totally accurate) is that upon start up the PCM checks the leak detection pump (LDP) for any issues. Once passed it then uses the LDP to pressurize the EVAP system to determain any leaks and if leaks are present then what size of leak. Obviously it is getting power from the PCM this whole time. But once all this is finished I cannot acertain rather the LDP maintains power or is shut off.
Again thank you! Seems as of now my problem "may" be beyond the body harness and into the engine compartment. If thats the case I wont be fixing this problem. I need te see if I can get a signal from the LDP at the computer and go from there. Do you think the wires will still be the same color at the computer plug in or will the be diffrent than at the plug for the LDP? Hope that made sence. thanks.
The wires are the same color at the pcm. The switch signal should have 12 volts when the switch is closed and 0 volts when it is open. This is the orange wire. The w/db is the generator source, should be 12 volts. The w/dg is the ground control for the solenoid.
Here is the operation.
The main purpose of the LDP is to pressurize the fuel system for leak checking. It closes the EVAP system vent to atmospheric pressure so the system can be pressurized for leak testing. The diaphragm is powered by engine vacuum. It pumps air into the EVAP system to develop a pressure of about 7.5" H2O (1/4) psi. A reed switch in the LDP allows the PCM to monitor the position of the LDP diaphragm. The PCM uses the reed switch input to monitor how fast the LDP is pumping air into the EVAP system. This allows detection of leaks and blockage. The LDP assembly consists of several parts (EVAP LEAK DETECTION SYSTEM COMPONENTS). The solenoid is controlled by the PCM, and it connects the upper pump cavity to either engine vacuum or atmospheric pressure. A vent valve closes the EVAP system to atmosphere, sealing the system during leak testing. The pump section of the LDP consists of a diaphragm that moves up and down to bring air in through the air filter and inlet check valve, and pump it out through an outlet check valve into the EVAP system. The diaphragm is pulled up by engine vacuum, and pushed down by spring pressure, as the LDP solenoid turns on and off. The LDP also has a magnetic reed switch to signal diaphragm position to the PCM. When the diaphragm is down, the switch is closed, which sends a 12 V (system voltage) signal to the PCM. When the diaphragm is up, the switch is open, and there is no voltage sent to the PCM. This allows the PCM to monitor LDP pumping action as it turns the LDP solenoid on and off.

When the LDP is at rest (no electrical/vacuum) the diaphragm is allowed to drop down if the internal (EVAP system) pressure is not greater than the return spring. The LDP solenoid blocks the engine vacuum port and opens the atmospheric pressure port connected through the EVAP system air filter. The vent valve is held open by the diaphragm. This allows the canister to see atmospheric pressure (LDP AT REST).

When the PCM energizes the LDP solenoid, the solenoid blocks the atmospheric port leading through the EVAP air filter and at the same time opens the engine vacuum port to the pump cavity above the diaphragm. The diaphragm moves upward when vacuum above the diaphragm exceeds spring force. This upward movement closes the vent valve. It also causes low pressure below the diaphragm, unseating the inlet check valve and allowing air in from the EVAP air filter. When the diaphragm completes its upward movement, the LDP reed switch turns from closed to open (DIAPHRAGM UPWARD MOVEMENT).

Based on reed switch input, the PCM de-energizes the LDP solenoid, causing it to block the vacuum port, and open the atmospheric port. This connects the upper pump cavity to atmosphere through the EVAP air filter. The spring is now able to push the diaphragm down. The downward movement of the diaphragm closes the inlet check valve and opens the outlet check valve pumping air into the evaporative system. The LDP reed switch turns from open to closed, allowing the PCM to monitor LDP pumping (diaphragm up/down) activity (DIAPHRAGM DOWNWARD MOVEMENT). During the pumping mode, the diaphragm will not move down far enough to open the vent valve. The pumping cycle is repeated as the solenoid is turned on and off. When the evaporative system begins to pressurize, the pressure on the bottom of the diaphragm will begin to oppose the spring pressure, slowing the pumping action. The PCM watches the time from when the solenoid is de-energized, until the diaphragm drops down far enough for the reed switch to change from opened to closed. If the reed switch changes too quickly, a leak may be indicated. The longer it takes the reed switch to change state, the tighter the evaporative system is sealed. If the system pressurizes too quickly, a restriction somewhere in the EVAP system may be indicated.

Action : During portions of this test, the PCM uses the reed switch to monitor diaphragm movement. The solenoid is only turned on by the PCM after the reed switch changes from open to closed, indicating that the diaphragm has moved down. At other times during the test, the PCM will rapidly cycle the LDP solenoid on and off to quickly pressurize the system. During rapid cycling, the diaphragm will not move enough to change the reed switch state. In the state of rapid cycling, the PCM will use a fixed time interval to cycle the solenoid. If the system does not pass the EVAP Leak Detection Test, the following DTCs may be set:

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THANK YOU BOTH SO MUCH!!! So by reading this will I not have power to the orange wire because it is the signal going to the pcm from the LDP? This is alot to take in but I feel alot better about figuring this out. Also sounds like the jeep will have to be running to make everything work correctly. Again thanks!!!
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