Son reports in from afar with these symptoms:

-OD was becoming intermittent and switches out on its own. Light goes out, OD disengages.
-Wiggling the gear lever while pressing the engagement button could get it to reactivate. (light and OD on)
-Became more and more difficult to engage and hold in.
-Checked the gearknob switch - wires connected firmly.
-Lots of fluid in trans.

I'm guessing the 4th gear switch is loose/bad. Do they really fail or just need tightening? How tough is it to tighten or remove and replace this thing?
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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD)

Thanks everyone for the replies - very helpful! Our daughter was home from college with her '83 for a brief visit and I pulled out the glovebox exposing its OD relay/harness and have fwd'd a slug of photos to #1 son along with an electrical troubleshooting procedure to hopefully narrow it down to the 4th gear sw.

I've also shown him how to short the relay to ground to bypass the 4th gear sw. so he has OD function for long trips, until we get the sw. replaced. And I've told him in stern tones (and big block letters in the email) to never, EVER forget to click the OD 'off' when shifting out of 4th. I"m aware of the self-destruct-in-reverse scene.

Jim Holst: like you I'm a believer in the Redline MTL, and it's already in both 240's gearboxes. It definitely improves gear shifting smoothness and OD operation, but can't solve an electrical problem - which is what we've got, it seems.
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Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD)

I had a similar symptom on my '86 (230,000 mi). The OD wouldn't work until I'd driven a few minutes or if I'd jiggle the shift lever. Because the OD was also slow to shift, I drained the tranny and overfilled with Redline MTL. To my delight the OD started working like new and the 4th gear switch also quit sticking. The tranny shifts more smoothly as well.

I went through those same symptoms on my 86. It has the tranny mounted switch on the exhaust side of the car. I replaced the switch and it made a world of difference. The car now will shift smoothy and anytime I hit the button. I first notice my car would only shift into overdrive when it had been on the road for about 10 Mins.

The switches seem to last about 175k to 200k miles.

Brian
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86 240GL, 86 740GLE & others

As the car is an '81 I'll assume, it's got the M46 trans. Ie. four forward mechanical gears and electric 5th (overdrive) activated by a sliding switch on top of the gear lever. I'm not familiar with the "4th gear switch" though.

I think it may be the wire running inside the gear shaft up to the switch that may be broken. Did wobbling the the gear shaft while it was in overdrive cause overdrive to cut out? If so, then he'll need to replace/rejoin the wire. It's a tight fit in there.
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'80..245..M46...b21a..SU carb..

If you clean the fuse number 11 ends and the socket and the problem does not go away... its the 4th gear switch for sure. It is so far gone that pulling to the right and rear is no longer effective.

Switch is on top cover of tranny on right side. A PITA but doable.

To make more accessable, with a floor jack under tranny supporting its weight, remove the left side tranny cross member support bolts and replace them THREE FULL TURNS. Remove the right side bolts and keep them safe. Lower trans with jack until the left support hits the lowered bolts. Slowly continue lowering trans which will cause the engine and all to roll down to the right exposing the upper right side of the trans more. Lower just enough to get a socket on the switch and remove it. Too low and the engine will hit on the firewall where there are all sorts of wires and such to get damaged.

After the switch is replaced, use a long drift or long shank phillips to guide the right trans mount holes to the body as you jack the tranny up. Torque the bolts and your done.

Duane

Check the wires from the od switch to where they run under the shifter boot. Normally, as Duane points out, if it is the 4th gear switch, pulling the shifter down and to the right while in 4th gear is a good check.

Justin B.