I'm the proud (new) owner of a well used 1961 SII which has had a persistent overdrive slipping problem. From previous forums I know that it is due to the viscosity of the oil I have been using - I've tried several brands culminating in a mineral based SAE 30 which did the job for about an hour before the overdrive started slipping again (it was a long drive and a hot day). I'm now thinking about draining some of the 30 and topping the gearbox up with 80-90 to see if increasing the viscosity solves the problem. Has anyone tried this before?
Also does a noisy diff (whine) come with the territory or should I get it looked at?
Gosh, I dont know what forums you have been watching, but no one round here would steer you to run anything but motor oil in an overdrive.
The viscosity of the oil will not improve the OD function unless it is grossly worn internally, and even then thats a very temporary fix.
That viscosity of oil would likely be gear oil, which is very bad for the OD internals due to the extreme pressure additives, and often sulfer compounds.
Your problem will be one of the following:
1) Oil pump is worn out.
2) The clutch linings are worn.
3) The pressure regulator is worn or leaking.
4) The actuator piston seals are worn and leaking.
5) You have an internal oil leak.
6) The solenoid actuator is mal-adjusted and you are not getting full flow to the pistons.
I'd get a pressure gauge on the OD and and see where you sit, then decide what to do next (if the pressure is in specifications, the your linings are shot).
Your axle will have no bearing on your OD problem.
