For three years of messing with this car I've changed out the transmission fluid twice. In the fall I cleaned the bottom elongated screen and one tubular screen. Never helped. Getting on the interstate today on a unusual warm and sunny winter day. The thought came to me to try and use the overdrive. I felt some hope that it might work for some reason.

Attempted it and the overdrive kicked in. Cruised over 70 somewhere around or below 2500 rpm. Over 75 was still quietly under the 2900 rpm mark. Quiet motor and the feeling of wanting to go faster. Never felt that yet in this car. It handled pretty well other than the front suspension needs replacing and the pan hard bushings are shot. Not sure how the IPD Cam gear is adjusted but it kind of feels like its got a little less torque than my 86 but with the rpm's staying lower too. Maybe I'm imagining things.

On the way home the OD slipped out of gear when I hit the brakes dropping my speed from 70 to 60. It seemed like the transmission was in neutral with my rpms having no or little effect on the driveshaft. I shifted out of gear and then back into 4th at 55 mph. After the construction zone I got back into OD and it ran fine. 2200-2400 rpm at 70 (might be more since speedometer is under by 3 or 4. My guess is that its still in need of servicing or possibly it needs more fluid. Drips a slight bit. But I've now got overdrive.

Do you have Redline MTL in it? In my old '86, the overdrive was not working well at all. I put MTL in it, and the overdrive ran perfectly for the two more years I drove the car. It is thicker than regular Type F ATF, so it creates more pressure internally even when seals are bad. More of a band-aid than a fix, but it's much cheaper and less time-consuming than rebuilding the overdrive!

Nate
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1997 850, 1979 242GT, 1990 740GL

Ahh, the old Laycock overdrive. Quirky little beast but actually very simple in design and function.

My guess is that you need to pull the unit and have it rebuilt. This usually entails replacing seals and an o-ring inside of the box. The overdrive is essentially a one speed automatic transmission. If it can't build up enough fluid pressure, the gear will not engage, or slip out as you have experienced. If the OD is physically "good", there should be enough pressure to engage OD at 35-40mph. You're lugging the engine at that speed but it's a quick and dirty test.

The other problem on a high mileage car is that the cone clutch is worn out. Your freewheeling symptom kind of points to this (but not necessarily). This is more serious as the clutch is hard to find. If you get to that point in your diagnosis, you're better off buying a good, used OD and swapping the entire unit.

Your speedo might be off a bit. I get 2500 at 70 on the dot, and 2800 at 75. I hit 3000 rpm at 80, give or take.