How foolish of me! 88 740 non turbo, with AW70L tranny, sorry.

An update:
Since my original posting I decided to poke a piece of round plastic down into the 2 small passageways that pass fluid to and from the solonoid. Unfortunately I dropped the plastic into the hole corresponding to the outer rim of the solonoid. So, off came the pan and there it was in the bottom, retrieved it, replaced the pan, put in 4 qts of new fluid.

Guess what, the OD now works perfectly. Not sure what the cause was. The passageways did not seem blocked, in fact the insides of the tranny were pristine, screen was clear. Maybe the new fluid did the trick, the old fluid was somewhat brown but somehow I do not think that was it. In any event I plan to do a complete flush next weekend.

Fortunately for me where I dropped the plastic piece it was a hole which allows fluid to drain into the pan when solonoid energizes. I guess when that OD solonoid energizes it causes a drop in pressure to activate or deactivate a servo thereby engaging 4th gear.

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David Hunter