I am transposing all my dialogue from Turbo Bricks in hopes someone here might have some relevant advice.
...I'm experiencing the same thing with my O/D this week. New rebuilt J type, using recommended 30 weight non detergent. Definitely feels hydraulic, if it was electrical I would imagine it would throw me back into 4th but instead it free revs until it builds pressure and catches again. More noticeable when at operating temperature, never under load and usually on light or heavy decel. I've checked level twice now and it's good. It was a dry fill though, is it true to lift the nose a bit while filling, let it sit for a few minutes and then level it out again for overfill runoff?
The British shop down the street from my shop likes 80/90 in some of the older OD units but I think that's just a band aid in my opinion and my trans/OD was rebuilt with all new seals and such.
...Well things just keep getting worse. I drained out the 30w and it looked like hell, like it had been in there 30,000 miles but it has around 300 miles on it. So I decided to drop the OD sump and just flush out everything with fresh oil. I noticed in the bottom of the sump plate what looked like black hard rubber or plastic particles outside of the filter. The filter looked fairly clean with no particles. So I buttoned it back up, refilled it by the recommended process, rechecked the level and now nothing. Where I at least got engagement for 5 or 10 minutes cold I get nothing now. With OD engaged it free revs in 4th until RPMS catch up to engine speed at whatever speed I'm traveling. So, I'm kinda baffled. Sounds like this "rebuilt " overdrive needs a overhaul. Such a bummer because I paid good money for it under the assumption it had been gone through by a highly recommended professional shop prior to my purchase.
...So to follow up on this issue and maybe help others in the same boat. After dropping the sump again and inspecting the filter/ relief valve and pump and not really finding anything of issue I hooked up a pressure gauge to the O/D and drove it again cold, it was making marginal operating pressure cold and it worked seemingly well for about 5 miles and then dropped out and started surging once it warmed up, pressure wasn't even close to specification.
After talking to a few more people with experience with these later overdrives and then digging a little deeper on the internet I think I've narrowed it down to the two operating pistons that are not accessible except with the O/D opened up on the bench. They are held in by two retaining plates and are responsible for the heavy lifting as far as pressure needed for proper operation. Volvo in fact updated these operating pistons in later M46 overdrive applications with slightly larger pistons with much larger sealing O rings. Luckily these are still available for a very reasonable price from VP Autoparts and are a direct swap OE upgrade to the J Type on the M41s, but this is probably not new to some of you. I'll be splitting it open next week so I'll let you all know how it goes.
More info here (go to bottom of article):
Original style with smaller O ring, updated piston with large black O ring.
Well, things took a turn for the worse. I split the overdrive a month ago to inspect seals and mechanical components. Everything looked ok so I proceeded to swap out the operating pistons (as I suspected these seals were failing when warm but had no proof). One item of note, the pistons were slightly taller then the originals (a few millimeters), I don't know if this had bearing on the outcome? I put everything back together, filled with straight 30, ran on lift, rechecked level, did this two times before oil level stabilized. I then proceeded to back the car out of the shop, but as I let the clutch out in reverse I heard a whizzing noise from the transmission area and reverse movement was feeble at best. I engaged first gear and that seemed normal, so I then pushed the car out the shop and went for a short drive around the neighborhood. Right away I knew something was wrong because no matter which gear I was in as soon as I would let off the throttle in decel, instead of having mechanical and gradual RPM drop, the RPMs would immediately drop to idle until I applied throttle again whereas it would rev freely until "catching up" or matching the previous RPM where I had left off. So, back to the shop I went. I brainstormed for awhile and really couldn't think of anything I did during this operation to cause this so I pulled everything back out and apart (the next day) and replaced the old operating pistons as they were the only part I had exchanged (also thinking the piston height difference may have been the cause still). Buttoned it back up and went out for another test drive. Nope. Whatever damage had been done was done, exact same symptoms.
So here I am a couple weeks later finally ready to tackle this again (4-5 hours each time was pretty disheartening so I walked away from the car for awhile). In the interim I luckily came across a later J type overdrive that we had buried in our used parts storage at work (until this point I did not realize that there was an early and late style J type). I disassemble it and inspected it and found it to be in excellent shape, looking like it was removed for other reasons from what I assume was a failing M46. This also gave me an insight into a few differences between the early and late J type units. The operating pistons and seals were definitely in need of replacement, some scoring which I carefully polished out on the piston walls and replaced with my new pistons. Again, this were slightly taller in dimension then the originals. I buttoned everything back up but I have yet to install it and test in car yet.
Before I proceed again I am looking for some feedback. Has anyone ever experienced an M41 in this particular failure mod? Has anyone here swapped out the "upgraded" operation pistons? Am I going to cause a similar failure by swapping out these pistons in the newer unit or did I make another mistake during my original work in the earlier unit? Will I encounter any issues with swapping over to the later J type? Mainshaft compatibility?
Late J type (M46):
Early J type (M41):
...Is it possible that I mechanically engaged the overdrive somehow and it was engaged when I first stuck the car into reverse to leave the shop? I do have it wired up as to not engage the solenoid except for in 4th gear. Believe me I'm bummed out about it as well. I'm getting a really strong desire to just adapt and swap in a true 5 speed from something. A good working overdrive is great, when it's a good working overdrive. The British curse is strong with this one.
The last thing I did before removing and dismantling was the recommended stalling it in reverse to decouple the cone on the shaft. Other then that I didn't use the OD prior to the operation.
..."I wonder if the taller replacement pistons are holding the OD partially engaged?"
From my understanding, it takes very little torque to damage one of these if they are engaged in reverse.
I was thinking the same thing. When I took it out for inspection though I checked the piston movement to the bridge and there was no preload, yes the older shorter pistons had more travel to engage the bridge but from my understanding it's not engaged until pressure is applied to the bridge. The only change is less volume in the chamber at maximum piston travel. But fluid only enters the chamber when the OD is engaged right? If it does enter the chamber and bypasses then I could see how the change in volume might partially engage the OD but I don't think this is the case.
Has anyone ever wiped out reverse in an OD before and can describe the symptoms post damage? I suspect that maybe the relief valve got jammed from lack of lubrication on reassembly and some how allowed the OD to engage without the aid of the solenoid and thus destroying the OD when I engaged reverse. Other then that I'm baffled on this failure.
...Oddly enough when I review the resource info I used when doing this upgrade, their original pistons are different then both mine and the replacements (the green oring ones are their originals which appear to be the same height but only different orings, hard to tell from there photo angle though).
Are there multiple variations of the J type besides a late and early casting???