To me, lockup refers to the torque convertor and OD is 4th gear. Does the lockup occur only when in 4th? I am a life long manual trans guy, have owned a few (very few and never really got into them) automatics and am finding the A4LD to be a mystery.
Bill
Bill - my knowledge of the A4LD could probably be written on the head of a pin but, yes, the OD is essentially the 4th gear. The purpose of the torque convertor lockup is to reduce slippage and increase efficiency when in the 1:1 OD gear.
All A4LD's have a lockup solenoid that, in the original vehicle such as a Ranger or Bronco, is triggered by a signal from the EEC computer which applies 12 volts to the solenoid....I'm not exactly sure what the signal parameters are but I believe it's based on a combination of rpm and throttle position. Basically, all that the solenoid does is initiate a hydraulic operation within the valve body that triggers the convertor lockup clutch by the plunger simply moving in or out.
Since we don't have a computer in the Alpine we need a work around to apply the 12 volts to the lockup solenoid....the two common ways are via a manual switch or a pressure activated switch. The pressure activated switch method is well documented on various sites such as
www.therangerstation.com.
Basically, the pressure switch is installed in the governor portion of the transmission which is in the tail extension. When the fluid pressure reaches the level that the switch is calibrated for (typically about 50 PSI) the switch closes and triggers the solenoid. It's not a perfect science and may require experimenting with a couple different pressure switches to achieve results but the target is to have the lockup engage in the 45-50 mph range.
As to your question about the lock-up engaging in other gears, it is my understanding that the lockup clutch is by-passed in 1-2 but not in 3-4. I personally don't see an issue with lockup engaging in third gear...the downside might be a rougher shift to 4th than otherwise but I think with normal operation you would pretty much always be in 4th gear by the time the lockup is engaged.
With regard to braking or forgetting to unlock it....when the car slows down, the pressure drops and the switch opens up so you don't need to do anything else.
But, to add a little confusion to the mix, there are two versions of the A4LD. The earlier versions (I believe 1985-1988) employed a single solenoid for the lockup function and the 3-4 shift function was vacuum driven. The later versions employed a second solenoid that triggered the 3-4 shift with another signal from the EEC. The way you can tell if you have a single solenoid or dual solenoid is to look at the electrical plug on the side of the transmission...if there are two wires you have only the lockup solenoid....if there are three wires you also have the shift solenoid. So, to get into 4th (or OD gear) you have to also trigger that second solenoid.
I'm not exactly sure what the course of action should be for the later versions.....perhaps you just wire both solenoids in parallel and trigger them simultaneously either manually or via the pressure switch....I don't know enough about the A4LD to know if it's a problem to do both functions at once or if they should be initiated separately....i.e. shift first and then lockup. I have an earlier version so I haven't had to address that issue.
My transmission is at the rebuild shop and I'm probably still a year away from having the car put together so this is still somewhat all theory in my case. But, as I said, this is all well documented and really not rocket science.