Like Bill says...Replace the 4th gear switch only after verifying that the terminals on the solenoid are clean and tight, with no broken wires at the crimp. Ignition ON, Enggine OFF, 4th gear, OD light ON. If no light with button push, pull shifter further to right and rear and try again. Onve light is on, under auto, wiggle wires and terminals at solenoid, including ground wire. Any click of solenoid is a bad connection.

Switch on tranny.

When cold, whatever fluid that has splashed onto switch inside the trans keeps switch from moving enough and also tends to inuslate a bit.

Fluid and trans get warm, things move easier.

Bumps and accelleration roll engine trans a bit and twist everything. Border line switch will open. Relay needs ground through switch to stay closed.

While under there, take a good long look at your engine and tranny mounts. One or more may need some attention.

Duane

Replace fourth gear switch,on right side of trans top cover.Hard to access without lowering trans a bit. Bill

funny you should ask if anyone else had this prob. i had an 86 245, stick w/od and it did the exact same thing. from a cold start it would not engage until i had been on the road for a bit. one thing i did notice was, when in 4th on the highway i'd push the stick to the right and into the bottom corner of the gearbox. this would allow the od to engage, i'd have to hold it there for a minute or so and all was fine. once it would stay in od it would engage and dis-engage like normal for the rest of the day. it never bothered me enough to fix, and then the beast died. it had a ton of rust so i just crushed it. always wondered if others had the same prob.

Two likely problem areas:

1) OD relay going bad. To access it, remove glove box. Relay is under the center vent duct. Solder joints like to go bad, likely there's a crack. When it expands after warmup the crack closes temporarily. Standard tranny takes a different relay but I think the location and failure mode are the same.

2) Wire under floorpan. I don't know about the standard tranny OD wire path. In auto version, relay output wire goes to the shifter housing, out the front of shifter housing to the tranny tunnel. Wire drops to right side of driveshaft. That's where it breaks, where it passes the driveshaft. Very close to the first u-joint, if I recall. Crawl under and take a look.
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Sven: '89 245 NA, 951 ECU, expanded air dam, forward belly pan reaches oem belly pan, open-front airbox, E-fan, 205/65-15 at 50 psi, IPD sways, no a/c-p/s belt, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245