Thanks for clarifying that Harvey. I was taking your previous post too literally.
Your new explanation makes more sense to me. If I understand correctly you are saying that the response time of the solenoid is significantly greater than the frequency of the duty cycle and therefore it cannot respond to each and every pulse. Instead it's effectively averageing out the signal.
Trevor - The armature may not bottom as a result, as I have pointed out, but it most certainly wil not, does not and can not, remain stationary, as is Harvey's exactly stated theory. Furthermore the signal, in so far as current and therefore positional control of the solenoid, can not remain constant as would be required. This I have pointed out by giving several reasons in detail. N.B. including the presence of a variable parallel circuit involved.
Does the TCU decide that it wants the solenoid open by a given amount? This would surely necessite the spring in the solenoid to be calibrated to the duty cycle so that (for example) a 50% duty cycle corresponds with a 50% open valve. There is undoubtedly some tolerance, maybe it's not such a knife edge as Trevor suggests. Looking at how small the solenoid is, I can't see a couple of percent either way making a significant difference to the functioning of the transmission.
Trevor - Yes MOST surely the spring would have to be calibrated and most importantly remain so, to an impossible degree, along with other variables.. An exact balance IS required. There is NO force preventing the armature from moving either way. There is damping which is variable, in the form of the fluid, but this only introduces the element of time, not force or a continual fixed resistance. An error of .000000000001% either way will upset the suggested balance and the armature will move accordingly. The element of suggested balance is further made impossible, as a solenoid is hugely non linear, with force increasing at a hugely non linear rate, as the armature reaches the fully inwards position. At the near inwards position, the factors calling for balance are extreme.
Or does the TCU just gradually increase the duty cycle, 10% at a time, until the trigger condition which caused it to open the solenoid is negated?
Trevor - This is exacly what would be required and this action requires a balanced loop feed back system. Otherwise the "condition" required, can not be assesed or maintained. The "condition" can not be negated, or restored, so as to be kept in ACCURATE balance without such a system. A feed back loop could provide a means of continual adjustment in order to maintain a balance.
Coming back to the original point of the thread, none of this theory makes any real difference when it comes to repairing the transmission. The solenoid is a "black box" part and it doesn't really matter how it works internally. It either works within spec, or not.
The object of the error code is not to conclusively identify the problem, just to tell the mechanic where to start looking for the problem. In the event of a code 12, a mechanic will probably just check the wiring and fit a new solenoid as instructed in the service manual.
Phil.