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Pin 1 is ground, pin2 is NVLD switch signal and pin 3 is NVLD solenoid control. NVLD stands for natural vacuum leak detection. As the vehicle cools, the fuel tank naturally contracts creating a vacuum on a seal container. When this occurs the switch closes in the NVLD signaling to the PCM there is no leak.
This is what a 2004 Caravan has. A LDP is used on older vehicles and actually builds pressure in the evap system and monitors time to determine if a leak is present.
Newer vehicles use an ESIM, emission system integrity monitor. 
Thanks but are you saying that the system performs the test when the Eng and key is turned off ?
I always thought for the monitor to set it would have to be driven on the interstate above 50 mph with approx 1/2 tank of fuel .
for a certain time and the monitor would reset ?
What Teflon said is dead on right. The LDP is just that a pump and it pressurizes the system and then the NVLD and ESIM both use vacum to test for leaks. There are many different emission systems on the car that test many different ways. Some like EGR need to be driving down the road and some will need to be above a certain mph for a certain amount of time and so on. Thats why many emission systems may only test themselfs once a week or so depending on how you drive.
For your question is yes it will test with the eng off but it will not turn the lite on untill it fails so many times and the car is running. It will also run a backup test with the car running as it will close the tank vent and then open the purge solenoid to pull a vacum and see if the NVLD switch closes and then see how long it stays closed. One thing to remember is if you have the check eng lite fixed and the tech erases the codes with a scanner he also erases all the PCM self test results and thats why they tell you to drive it for a week so hopefully it will run all of the test again so when the emission people look at your car they see all the test have run and passed. Ron
Last edited by 383man; 01/19/12 07:36 PM.
