Quote:
Originally Posted by 80stech
This is a very interesting thread!  As far as I can tell from your diagram on how the spool is laid out, just because the DTML can go through all it's tests doesn't prove the spool is all the way back in the non-test position when it's done.
If that diagram is right then I wonder where the 1mm orifice is ?? |
Even though the diagram doesn't specifically state it, I'm guessing the 1mm orifice is at the pump outlet itself.
That valve/plunger (is that what you're referring to when you say 'spool', or do you mean the coil/solenoid?) looks to me to only have two positions. The diagram shows the default (de-energized) view. Air can enter the filter and takes the right (up) fork at the y and goes on up to the canister. This is the position during engine operation.
The EVAP leak test is only done after the engine is shut off (and other conditions are met). It starts with the change-over valve in the same position, but the pump is activated to take the reference reading (pump through the .5mm orifice and store the current reading for future comparison).
Then the plunger is energized, and it moves to right in the diagram. This blocks off the port where the right (top) fork in the line from the filter meets the valve and opens the port at the valve where it meets the line heading straight up from the pump.
By monitoring motor current over time (the test takes a few minutes), the system can detect 3 conditions:
1) Leak > or = 1mm as the motor current falls and then doesn't rise.
2) Leak > or = .5mm but < 1mm as motor current falls then rises as tank pressure builds but eventually levels off at a level < or = the current stored from the 0.5mm reference reading.
3) No leak (or, more accurately, no leak or a leak less than the allowable 0.5mm standard) as the motor current falls then rises as tank pressure builds and ultimately surpasses the current of the 0.5mm stored reference measurement.
OK, whose head hurts after reading this?
