Hi!

I just picked up an '88 740 Turbo with a 4spd + O/D manual transmission. Taking it out for its first long drive I noticed something unusual: After almost an hour on the highway, while going downhill in O/D, I lifted my foot off of the accelerator and the engine dropped to idle as if I'd put the car in neutral. However, the "5" light was illuminated, indicating that the O/D was still engaged. I got back on the throttle and the car clunked back into "gear" when the engine RPM reached a speed typical of highway cruising. I did this several more times until the "clunking" became more of a "shuddering" and then finally, the O/D wouldn't even reliably engage (thought the "5" light ALWAYS came on just as it should). I drove the rest of the way home in 4th.

The next day, I drove the car around for a few short trips and even a 1/2 hour drive on the highway - the O/D worked fine the wole day! It seems the problem only occurs after long periods of engagement...

Any insight on this?

Could the answer be as simple as changing to a synthetic trans-fluid with a more stable viscosity?

Thanks for the help!

(here's a pic of the car, BTW)