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GKN Overdrive Strip Down

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8 hours ago, western said:

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

That'll reveal if it's broken, but will give no information on the pressure it's applying, especially as temperature increases.  Ideally, it needs testing of its stiffness.

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Boroscope exam this morning didn't show much, all 4 springs look intact but I think further dismantling will be needed at the weekend, 

 

I'll have a read of the link, 

cheers all for the suggestions/advice so far :i-m_so_happy:

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Managed to get some time to open up this overdrive, it's quite clean inside, the 4 springs are the same length as the 4 from my previous unit, haven't found any broken parts, the friction material is intact on both sides, the inner & outer brake ring faces look fine,

only thing I'm not sure of the brake rings & cone friction faces were all coated inPAS fliud is that correct ?

took these photo's & a short video, only thing I can think of is the 2 piston seals  the 4 springs may need changing, can get those from Devon4x4.

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The whole lot runs bathed in ATF Ralph so thats normal. 

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The unit behaves the same way as a two-speed automatic, except that automatics usually use hydraulic pressure in both directions, holding each gear in drive, not using springs in one direction.  They always have the clutch packs immersed, so I can't imagine for a moment this would be different - the linings would overheat and break up in no time if they weren't lubricated while in their slipping mode.

I still think you need to test the springs' stiffness, not just their length, and check whichever clutch pack is compressed by the springs for wear, if just to rule them out and point the finger squarely at the piston o-ring leak.

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Given you have a spare OD, I think you have three options Ralph

1. Use the springs from your old OD in the one you bought from Neil and see if that fixes it

2. Use the rear casing from your old OD on the unit you bought from Neil - this would rule out the pump and solenoid bore then

3. Replace the broken part in your old OD with the part from this one

Point three has the best chances of success and is least faff as it really ought to work, but having tried to help Neil fix that OD you have now, I am curious if 1 or 2 would fix it as I have a slipping one on the shelf!!

The springs came from D44 I think so one would expect them to be ok, but as pointed out above, no guarantee

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Been skimming this with interest, the actuation method on these seems to be a weak point, is there any way to convert them to just be on/off using a mechanical linkage, lever, cable, or clutch-slave style actuator?

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FF. I have no idea if any of your suggestions would work, I can't have my 110 off the road to sort out a none hydraulic/ electrical system, it's my only vehicle. 

 

Swapping the rear case with pistons in might work but I would want to replace the seals first & swap the filter as one is now very clean, my old unit is stripped & hasn't seen any oil for a year, so wouldn't trust the seals, if it doesn't fix the problem.

I would also need to use the planet gear set from this overdrive as my old one was siezed. 

Think I will order a set of piston seals & 4 springs & see if that works, my old unit started slipping as well but the might have been down to the broken part inside. 

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I presume, that the solenoid on these turns on/off the flow of fluid which moves the friction surfaces apart, thus giving you overdrive.

So in simple terms, if it slips when disengaged, surely it can only be:

  • Friction surfaces failed
  • Springs too weak (that hold the friction surfaces together)
  • High pressure ATF getting past the seals in the solenoid

Have I missed anything there?

 

 

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That's about it AFAIK, I need to get the solenoid off to look at the bore but need a spanner to modify as my existing spanner is to wide to fit the gap where the solly nut is. 

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2 hours ago, Bowie69 said:

I presume, that the solenoid on these turns on/off the flow of fluid which moves the friction surfaces apart, thus giving you overdrive.

So in simple terms, if it slips when disengaged, surely it can only be:

  • Friction surfaces failed
  • Springs too weak (that hold the friction surfaces together)
  • High pressure ATF getting past the seals in the solenoid

Have I missed anything there?

 

 

I think thats all. The clutch was new on this OD so i think Ralph can rule that out for sure. 

 

My suggestions above would deal with the other two issues. 

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I can swap the solenoid over from my old unit as it was working, bit reluctant to use the springs from my old unit, would rather use new springs, 

The cone friction material is undamaged as are the 2 surfaces it works with, there isn't much else that can be changed, other than new seals in pistons & pump part. 

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We tried three solenoids on that OD Ralph so i wouldnt waste your time on a fourth! That solenoid on it is new. 

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Just to add - You can test the solenoids easily with 12v across the terminals

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Is there a pressure relief valve?

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Think so - its under one of the circular plates in the bottom of the unit.

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If it was giving an over-pressure, could be forcing fluid past the seals in the solenoid/somewhere else.

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So a jammed pressure releif mechanism?

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Essentially, yes.

 

Sorry, head has been buried on code all day, my English is starting to read like a Linux manual ;) 

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Relief valve is clean & free, might be woth swapping the little spring from my old unit, the bore & valve itself look in good condition, 

as to the solenoid I was thinking it may be sticking not fully releasing the pressure.

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We thought that, but tried three solenoids and it slipped with all of them. The springs really seemed to solve it. 

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The fact it slipped at the first move of the day would suggest it's not hydraulic.... Sometimes just dipping the clutch and going again solved it for days. Hence focus on the springs eventually... (As well as the extensive overhaul...)

I was wondering if there was a way to pack the springs and add a little extra force to them for testing? 

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