As the cold weather returned this year my '90 SR5 w/automatic OD tranny developed a problem shifting. An old thread by bilsif descibed the problem exactly. When the ambient temperature is cold, it takes too long before the shift to OD occurs. The colder it is the longer it takes. Some of the responses suggested that this was normal. It is not. It had never occurred in the past.
My analysis so far: A weak OD shift solinoid. I dissmissed this because it works perfectly once the first shift to OD is made. My current theory is that the ECM (or whatever else Toyota may call the computer) is blocking the shift due to receiving bad information from a sensor. The temperature guage on the dash responds normally.
Does anyone have a source for Toyota ECM operation?