I had two issues with the servotronic control unit on my 1989 735, which now seem to be fixed.
In colder weather, I would get minimum assistance until I had driven a few miles, then the steering would suddenly go to normal weight. If I re-started the car after a couple of miles, it would also usually sort itself out.
The other problem was a sort of groaning noise and vibration through the wheel, like a rubbery feel. I'm certain this has been fixed, the cold weather issue will have to wait a bit but I'm pretty confident.
The problem, as always on these cars, is bad solder joints, in this case on the microcontroller card in the servotronic unit.
The cold start problem was a bad joint in the reset circuity, so the board wouldn't boot up properly. As it warms up, the bad joint makes contact and the board will then boot up properly if the ignition is cycled, or will eventually restart itself.
The second problem is a bad joint on the capacitor across the steering output. The servotronic control unit works by pulsing the 12V output, the amount of 'on' time varies the voltage seen by the solenoid at the steering box. If it's on for half the time, you get about 6V out. The capacitor is there to smooth out the switching, turning the pulsed 12V into a steady voltage. If this fails, as mine did, the solenoid is rapidly pulsed between full on and full off, rather than getting a stable voltage and this gives the groaning and vibration through the wheel.
The board is covered with a conformal coating so I had to scrape this off the joints before re-soldering.
Now I've got the steering fixed, my heater matrix has started to leak and the EDC shocks are gone. The camshaft is a bit rattly as well but its been like that since I've had it (3yrs). I'm thinking of chopping it in against a newer 740 (93-4) rather than spend the price of the car fixing it.
What do you guys think, am I better with the devil I know, or putting the money towards a newer faster car which might also have expensive problems around the corner? The idea of 280+ horses is tempting, the 735s could do with a bit more poke.
1989 735 177k