Leak Detection Pump OBDII Code R1494

I have a 1997 Dodge GC with a 3.3 engine, 163,000 miles. It has started to throw a P1494 code which indicates an Evaporative Emissions Systems specificly a problem with the leak detection pump switch or mechanical failure of the pump. I installed a used pump from a junkyard and still have the same problem I have tested the pump...to the best of my limited knowledge, and the pump does seem to work if current gets to it (I've jumped it to the battery and can get the pump to build and hold pressure)

I am assuming that something is wrong with the circuit/wiring to the pump. 1. Can anyone tell me where and I how I should check this. I belive that the + comes from the 12v from the ignition key circuit... visually/physically where can I check that back to? 2 Where does the Ground circuit for this route to or from is there a trouble spot where this ground may have gone bad.

It may just be a problem with the ground or the + supply Has anyone got any experience in trouble shooting this?

Am I correct in thinking that the computer controls the ground and pulses it open & closed to build pressure? or is it the + line that it pulses?.

One of the problems trying to troubleshoot this is that the system/pump only kicks on under a certain set of circumstances ..cold engine, gas tank less that 85% full more than15% full etc. So doing a number of checks in a row causes the coolant to get warm enough that the system won't turn on. Maybe there's a old MOPAR tech out there who can demystify all of this for mer and give me some troubleshooting techiques?

Any advice would be appreciated.
Thanks
Spit

I may have this fixed!! went to a Dodge Dealer and had them pull a pump out of stock abd checked it with a volt ohm meter, they want $135 for it!! I found out the one I had put in from a junkyard was bad. Then I went to a junkyard and got another pump checked it with my volt ohm to make sure it worked like the new one at the dealership...Put it in Saturday and no codes thrown as of yet..(knock on wood). I did learn somethings about how this pump is supposed to work and how you can test for electical problems with the internal reed switch.

FYI
1st postion terminal/wire 12 volt common (12v in to pump)

2nd position " " ground for vacuum solenid this grounding is controled by the PCM..the PCM only grounds the solenoid when it wants to pump pressure in evap system to check for leaks and in fact it pulses the ground on and off fairly quickly to make the diaphram in the pump rise & fall and build up pressure.

3rd position " "12v from the pump back to the PCM..this is controled by a reed switch in the pump and is normally closed...So that when the pump is in it's normal un-energized state the 12v going into the pump is going back to the PCM. Once the pump has pumped up some pressure the reed switch opens and the PCM knows that the system is pressurized and it monitors how long it takes for the system to leak down... too fast and it throws a minor leak code, way too fast and it throws a major leak code

So if you test your pump with a meter...between 1 and 2 should be a small resistance. and if you put 12v on 1 and a ground on 2 you should hear the solenoid click If you test between 1 and 3 there should be continuity.


My pump did not have continuity between 1 and 3...I don't think that the 12v that needs to get back to the PCM was getting there and hence the P1494 code.
Spit