diagnosing auto no 4th gear problem

I bought an 88 240 auto wagon a few months ago with 234K. The transmission
is quite smooth and fluid is clean and full. However last week I began to
experience times when the tranny would not shift into 4th gear when the 4th
gear inhibitor is off, i.e. no illuminated arrow on dash. Note the dash
light toggles as expected with switch presses. Another way of describing
this is that cruising at 50MPH and pressing the switch has no effect on
upshift/downshift and the engine sounds like it is working harder than it
would in 4th gear (no tach on this car). It may be a little early to
describe a pattern but it seems as if 4th gear is available when the car is
cold and then becomes unavailable.

I found lot of discussion of this problem in old NG posts but I am looking
for one diagnostic tip. Can I remove the relay and jump contacts in a way
that would disable inhibiting 4th gear? If I still can't get 4th gear that
would seem to isolate the problem to wiring from the relay to the solenoid,
the solenoid, or the tranny itself. If I can get 4th gear that would seem
to isolate the problem to the relay or perhaps even as some have suggested a
fuse. One NG article claimed the relay consists of a single switch that
sends voltage to both the dashboard light and solenoid when 4th is
inhibited. If true that suggests that simply removing the relay should give
me what I want.

RC

 

Removing the relay will not restore 4th gear. Check the wiring under the car
to the solenoid, also it's possible for the solenoid itself to fail and
stick closed.