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reb78

GKN Overdrive Strip Down

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I dont think the filter will yield anything fruitful here Ralph - I think we tried it in Mo's truck and with a new one and it didnt help at that point. The springs seemed to make a real difference as it is these that hold the clutch in the 'off position'. When pressure from the pump is allowed through by opening the solenoid, the clutch is pulled back into the other cone and drive goes through the unit.

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Filter, relief valve & piston accessed, & some gunk removed from filter element, piston, springs look OK, 

Inside of OD looks clean too. 

IMG_20190521_175537.jpg

IMG_20190521_175523.jpg

IMG_20190521_175551.jpg

IMG_20190521_175809.jpg

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What is that?  It looks like it could be  broken up sealant.  Was it you who has to encase the sensor in epoxy due to a broken plastic casing?  Any of that on the clutch packs or in the solenoid system could cause slipping.

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It's not sealant, just black gunge, I haven't done anymore to it so far, need some more ATF, going tohave another look at it tomorrow or monday.

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Any idea on its source?

Whatever it is, if it's in the clutch pack, it could be holding the plates off one another or lubricating them.

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all that black gunk came out of the filter, I haven't opened the casing yet

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What is the oil pump made of?  Is it a fibrous gear like the LT77/R380?  If so, it could be breaking up, and the reduced oil from a worn pump and blocked filter wouldn't help with the pressure on the clutch packs.

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Its a piston I think with an o ring. Definitely worth checking but I am not sure the OD needs oil pressure in its rest/off position. 

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Piston & O ring are fine as is the relief valve parts. IIRC the 4 long springs move the brake cone from engaged to disengage position 

Haven't opened the unit up yet, been busy doing garden clearing work here at home

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Which position did you say it's slipping in?

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Slipping in when disengaged giving me no drive & occasionally wouldn't transmit drive when engaging, it worked OK on the drive up to wiltshire a week or so ago

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Its slipping at rest. If it wasnt it would be driveable. I am womdering if the cylinder the solenoid sits in is worn and letting pressure by when the OD is disengaged perhaps. Just enough to give enough pressure to pull the cone from the rest position. 

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I haven't removed the solenoid yet, I'll pop it off tomorrow morning & take some photo's I have my old solenoid & a spare.

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Posted (edited)

IIRC - Neil tried new seals and then a new solenoid on that unit. Thats why I am wondering about the bore it sits in. 

Edited by reb78
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I can compare the bore with my original unit in morning 

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I've pretty much replaced all but the case itself at one point...

Rich is right, the oil pressure isn't used when not in Overdrive mode as far as I can work out...

Therefore, the springs are important to keep the pressure on the brake cone and give you normal drive.

When the solenoid is activated, it pumps oil to move the brake cone to lock the internals and force the drive through the three planetary gears.

It should just be clamping pressure from the springs needed for normal drive. So unless oil pressure is getting by the solenoid and acting against the spring pressure, it "should work".

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That's my basic understanding of these work too, the springs do the had work when disengaged.

Only gleaned from my Domite days when I was younger.

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Thats as I thought to, 

Cheers for the extra info, pretty sure we can cure it.

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Although already under pressure when fitting the nuts to the spring retainers, it may be worth trying spacers / washers under the nuts to add a little more spring pressure and see if it make the difference you are looking for?

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Tried to remove the soleniod but I haven't got a spanner thin enough or the correct size [26mm]  to fit between the casing & solly body, so it'll have to wait.

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Domite should have been Dolomite, sigh.

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9 hours ago, V8 Freak said:

 

Rich is right, the oil pressure isn't used when not in Overdrive mode as far as I can work out...

Therefore, the springs are important to keep the pressure on the brake cone and give you normal drive.

When the solenoid is activated, it pumps oil to move the brake cone to lock the internals and force the drive through the three planetary gears.

It should just be clamping pressure from the springs needed for normal drive. So unless oil pressure is getting by the solenoid and acting against the spring pressure, it "should work".

That makes sense - it would be a fail-safe to leave the car operable if there was an electrical or pump failure.

I'd be surprised if enough oil could get pas a small leak on the o-ring to do this.  Does it go into a sealed actuator?  In that case, a small leak would retain pressure and I can see how a gradual slackening on the clutch pack would occur.  I would have been looking for a problem with the clutch pack linings or the spring pressures, but I'm no engineer and have never used, let alone rebuilt, one of the these.

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I agree but the clutch pack and springs are new on that unit ^^^^^.

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I have a boroscope thing which I can have a look inside at the spring. 

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9 hours ago, reb78 said:

I agree but the clutch pack and springs are new on that unit ^^^^^.

"New" doesn't guarantee "good", as we know from certain brands (and unbrands).  Even genuine stuff sometimes has flaws, though less common, and there is the possibility of wrong spec replacement parts, given how long the overdrive was in production - a different suffix unit might have subtly different components.  There's no way to tell without close examination of the parts, but I wouldn't rule them out.

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