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Vintage M41 Overdrive Troubleshooting

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 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.

:oops:
 
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???

https://www.volvoclub.org.uk/faq/OverdriveRebuild.html
 
Update:

OK, making some great progress! Ripped out the old OD and installed the replacement and it works like a dream! Will dissect the old one over the weekend to determine the failure mode. Also, contacted VP Autoparts and these pistons are the correct replacements and they've sold over two dozen with no reported issues. This solidifies that I had a pre-existing failure in my overdrive, the new pistons and seals finally finished it off with the renewed operating pressure I suspect.

Here's a few more things I learned about the differences in early and late J Type's that should be noted for reference:

First item (forgot to photograph) was that the speedometer housing drives are different, gears are interchangeable (along with corresponding gear on mainshaft as I recently found out), early J's require threaded adapters for drive cable where as late J's have the threads on the housing.

Second item was mounting flange surfaces are slightly different on both OD housing and intermediate housing. At first this worried me but I found out that this makes no difference, they are completely interchangeable. I did find that the later OD housings have much tighter tolerance then the older ones and the flange on the intermediate housing must be free of all defects as the fit is very snug. At first I thought I was hung up on something when bolting together but after test fitting the other intermediate housing to both I figured out it was a tolerance issue. See above post for OD housing flange pictures.





Last finding and probably the most significant was that over the production of the J Type overdrives there were three possibilities of gear reductions available. This is easily identified in the production number as the first two numbers denote the percentage of reduction. In my case the early OD was 25/115xxx and the later replacement was 27/115xxx. Apparently there is also a 28/115xxx as well but I'm unsure of original application. This was also very noticeable on my first road test as I gained a few hundred RPM in overdrive which heck yes, I'll take it!





So really the only negative at this point was the change in speedometer accuracy. I find it odd though that both plastic gears were identical so I'm assuming the metal gear on the shaft is to blame or the change in gear reduction comes into play, either way I'll live with it as I usually drive by RPM and flow of traffic anyhow.
 
Always nice to put in a part that works. Congrats.

Are you trying to tell us that there are three different ratios in J-type O/D's, none of which are 0.8:1? Citation is needed on that.
 
Always nice to put in a part that works. Congrats.

Are you trying to tell us that there are three different ratios in J-type O/D's, none of which are 0.8:1? Citation is needed on that.

Thanks.

I'm just saying that according to the reference material I found the first two numbers denote the percentage of reduction in whatever gear operated.
 
The green book I looked at said that all J-types in Volvos have a ratio of 0.8:1. What is your source and could you elaborate on what it said about ratios.
 
The early cars perhaps, but the later cars that were equipped had a higher ratio due in most part to increased performance and top speed. This late J type came out of a 200 series. Ive documented here my many findings that substantiate an evolution in the architecture of the J Type as time went on. A change in ratio seems logical, I believe the green book was just generalizing (green book for which model by the way?).

Besides multiple ratios being offered, remember that the J Type superceded the D Type as well as the lesser known A Type (never offered in a Volvo).
 
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So with the old OD out I had time to dissect it over the weekend. I've never had one full apart or know what exactly to look for but I found what I think are three significant things. Any advice welcome as I'd like to rebuild this unit and keep it around for a spare, if it's too messed up I'll just keep it for core parts nonetheless.

First was the broken stud for the retaining bridge, the chunk of aluminum was never found and most likely circulated throughout the OD. Second item was the surface of the cone looks heavily friction machined and the oil seal disc deems a little banged up. Lastly, the friction braking surfaces look fairly burnt, friction material seems reasonable but pitted/chunked in some areas.







 

Laycock manuals, two local British sportscar restoration shops, also doing a Google search yields multiple results in various British car forums. While not Volvo specific, deciphering the serial number uses the same method no matter which car it's bolted to. I believe the green book, while not incorrect, is a point in time reference and there may have been production changes not accounted for.
 
The owner's manual for my (long gone and dead) '71 1800E states that the OD gear ratio is 0.797:1
 
The owner's manual for my (long gone and dead) '71 1800E states that the OD gear ratio is 0.797:1

That's what the general consensus is. All the Volvo literature I have found states the same. That's why I'm glad we're having this discussion as what I've found contradicts this not only in facts from the serial plate but in actual real world comparison in that the overdrive swap netted noticeable actual drop in RPM at the same speed with no other changes.
 
Laycock manuals, two local British sportscar restoration shops, also doing a Google search yields multiple results in various British car forums. While not Volvo specific, deciphering the serial number uses the same method no matter which car it's bolted to. I believe the green book, while not incorrect, is a point in time reference and there may have been production changes not accounted for.

Thanks for the reply. I can easily buy that British cars may have had different ratios from what Volvos had. I am not seeing the evidence that J-types in Volvos had any ratio other than 0.8(.797):1.

I am open to persuasion on this, but will not change my mind due only to hearsay.
 
Thanks for the reply. I can easily buy that British cars may have had different ratios from what Volvos had. I am not seeing the evidence that J-types in Volvos had any ratio other than 0.8(.797):1.

I am open to persuasion on this, but will not change my mind due only to hearsay.

In testing I confirmed this "theory". With nothing else changed, only OD units, my results were this:

4th gear, no OD, approx. 4000 RPM @70mph
4th gear, w/ early OD 25/115xxx, approx. 3400 RPM @70mph
4th gear, w/ late OD 27/115xxx, approx. 3200 RPM @70mph

For further reference I have a 4.10 rear diff and 205/60/15 tires (slightly shorter then stock).
 
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