I'm coming up on 350k miles on the original M46. I've had the overdrive replaced (used unit), but the tranny has never been opened up. How many more miles can I expect before I will have to tear it down? It is difficult shifting into first (reverse detent needs adjustment?) and sometimes "grinds" going into 2nd, but otherwise works perfectly.

I suspect part of the reason that it has lasted this long is that 90% of the car's mileage was done on the highway (not much opportunity for shifting). I also change the fluid every 25k.

Also, what usually wears out first in these boxes? Synchros, bearings, etc.?

TIA.

Hi Scott,


My son's 240-1981 is now 291k miles and is fully original. Never developped any leak except at the speedometer cable connection. It is still silent and shifting great is all gears. I used slight clutch pedal pressure when engaging or releasing overdrive; I convinced my son and he does the same. Also I can say we are not agressive drivers.

Oil was replaced at about 50k miles over the 21 years with automatic transmission oil type F. Screen under overdrive rectangular cover has been cleaned every time with lacquer thinner and compressed air blown dry.

All what is needed:

1- a new rectangular seal for overdrive cover
2- occasional new seals at the fill/drain plugs
3- 3 liters of trans. oil type F
4- a pump that screws to the plastic oil bottle(the type used for outboard motor gearcase maintenance,OMC marine used to sell it about $10)
5- 45 minutes


You can use better quality oil and maybe extend the oil replacement frequency if you like. I never had any regret for the trouble I gave myself because I collected the benefits and now my son does. Periodic oil changes probably help oil seals to stay soft and operate in a dirt free environment.


This is our experience.

I know of an M46 approaching 600k.

As time goes on, you should start switching to progressively thicker oil.

At 350k, it might be time to switch to 10W30.

The guy with 600k is up to 90W145 gear oil. I think it might be rebuild time soon, as there isn't much thicker than that. :)