M46 first and second gear slip - bad overdrive?

jamesinc

So I've done a bit more driving today, and wired the overdrive for the first time. The overdrive works fine, seems to engage and disengage without drama, but I can definitely get something to let go in low gears.

I think the clutch is okay, and it's the overdrive that's the issue. I will now present my reasoning for your consideration.

Given:

the problem presents most in first, a bit in second, almost impossible to reproduce in third, and not reproducible in fourth; andthe clutch has the most mechanical advantage in first and the least in fourth; andthe overdrive, which takes the transmission output shaft as its input, has the least mechanical advantage in first gear and the most in fourth gear

It is reasonable to assume that the issue lies with the conical clutch inside the overdrive (the annulus).

From what I've been able to dig up online, it appears the most likely issue is either the friction material on the annulus has worn, or the springs that engage it onto the output carrier (when overdrive is disengaged) have fatigued.

It may also be something simple, like the fluid level being too low. I don't expect this to be the case as I did the recommended overfill but I will check this first.

So, because this car is my daily, you once again have the pleasure of watching me throw money at it. I've ordered an overdrive rebuild kit and new springs, as well as a J-type overdrive pressure test gauge.

I contemplated ordering a remanned annulus, but it is $300 ($190 after core refund, but add shipping to the UK and I think we're back to $300). I will buy one if necessary but I'd like to inspect the old one first.

Stay tuned...

Does it also "slip" in reverse? Might be kinda hard to test. Just wondering whether the OD would slip in reverse as well as first or not. I can't recall exactly how that sprag clutch works in the OD unit...

I don't completely understand the OD, but I expect that the sprag is there to help the wheels backdrive the motor, which would mean it also would engage when you are driving in reverse.

But I'm not quite sure if that's what it is for. I assume because while the engine and transmission delivers an engineered range of torque, the wheels could potentially deliver much higher torque through the driveshaft.

Just speculating. You know what, I will break this out into its own thread.

Are you suggesting I try planting it in reverse to see if the slip is absent? I'll give it a go tomorrow.

jamesinc

Are you suggesting I try planting it in reverse to see if the slip is absent? I'll give it a go tomorrow.

Yeah that's what I am suggesting. I guess if the slip is absent in reverse, it may indicate ???? I don't know now! :) But it's another data point! You'd think if it were the clutch, it would also slip in reverse...so if it DOESN'T slip in reverse, maybe that points to the OD, but maybe the OD slipping would also occur in reverse? IDK!

"A further function of the over-running clutch was to ensure that drive was maintained while overdrive was being selected. If it were not fitted there would be a temporary loss of drive when changing"

So maybe my theory about the OD not slipping in reverse is bogus. For some reason I thought that sprag clutch had something to do with reverse, but I guess not. So I guess if the cone is slipping then then the OD would slip in reverse as well.

https://www.uniquecarsandparts.com.au/how_it_works_laycock_overdrive

So yeah, based on that image, I guess if the springs (that hold the cone clutch against the annulus when the OD is disengaged) are fatigued, or the lining is worn, it should slip in all gears, but it probably depends on how much torque is required for it to overcome the lining frictional forces.

I think your thought process to discount the clutch is flawed. By your theory your more likely to get a clutch to slip in say 4th gear than 1st. Thats the opposite of my experience though. I need to do some more head scratching to explain why but I'm to tired to engineer right now haha.

It seams safe to say the clutch/pressure plate is ok, but perhaps it is not releasing fully. Maybe either due to clutch master not bleeding off pressure fully when you let off the clutch, or the wrong combo of throwout bearing/pressure plate/pivot ball meaning you are running out of travel in the slave cylinder.

I don’t get it, if the overdrive is not engaged, then its straight out the rear shaft.

I've never known or experienced a slip in lower gears where the OD is to blame. Only time is when in 4th and OD is engaged then one can tell if the cone clutch bites instantly or a more lazy slip a bit engagement.

Ultimately what im suggesting is something has changed in your pressure plate or carrier bearing area as full clutch plate engagement is not happening (or oil has contaminated the plate, pray not).

I already checked these things. The slave isn't operating near the limits of its travel, and it is immediately releasing when I take my foot off the pedal, and its resting position feels right, it is on the clutch fork but not really applying any pressure.

It could be contamination, although I cleaned everything pretty thoroughly.

From experience with the same unit on british cars it is the cone clutch in the overdrive or the springs that hold it out of overdrive.

Overdrive should always be off in first and second so hydraulics are all out of operation. Check you have no power to solenoid in first and second.

Flywheel clutch will slip in high gears first and overdrive cone clutch will slip in low gears first.

Good article you have probably seen.

If you have any doubt at all that the overdrive may be stuck on, do not reverse even a tiny bit. It will do a lot of damage to the internals. Destroys one way clutch.

Okay so tonight I put the front end up on stands and crawled under. Bone dry under there [pats self on back]. I took the clutch fork boot off and had a look inside the bellhousing, again no evidence of anything weird, no moisture, nothing splattered on the bellhousing, no evidence of any residues etc.

I checked the transmission fluid level and added maybe 200ml to raise it up to the fill port. Bear in mind the fill port is at the front of the transmission, and the car was slanted back quite a bit, so I think the level was probably fine.

Then I took it out for a drive to try and better characterise the issue. With everything cold, I was actually able to pull quite hard in second, although I could still get it to let go in first. But as it warmed up the slipping got worse. So I suppose that doesn't really tell me much, as the transmission and clutch would both heat up with use, but a data point is a data point.

No burnt clutch or oil smells ?

Oh yeah I forgot to mention, I did stick my head under the car after and have a sniff but nothing out of the ordinary.

jamesinc

Oh yeah I forgot to mention, I did stick my head under the car after and have a sniff but nothing out of the ordinary.

If the clutch were slipping I think you'd definitely smell it...so it all seems to be pointing to the overdrive. I have a disassembled overdrive unit - let me know if you want me to check the condition of any of the parts that you may need. I can't recall what condition the friction surfaces were in.

It sounds like overdrive clutch to me as well. You don't have a spare for an under car change I take it.

I encountered a similar issue with a m41 a few years back. Same overdrive unit. It appeared to have had a heavier gear or engine oil in it. People used to say this was correct for M40. Perhaps your box has had incorrect oil? If it is getting better as you describe, perhaps more driving to allow your fluid to penetrate and wash the friction surfaces? It may continue to improve. Some ATF (that I assume you are using) is high detergent and will wash out the residue over a few weeks.

Out of interest, what fluid are you using? There was a special Volvo fluid, and I know that most people use a version of ATF here. I use a Penrite product for the overdrive application because of its better wear resistance. Gear change is a little slower for the first five minutes, but overall much nicer after the oil warms, compared to ATF. I'm particularly interested in the greater wear protection as parts get harder to find.