C5 HP20 causing grief

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CitroJim
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Re: C5 HP20 causing grief

Post by CitroJim »

Hi Regan, yes the 'box does need to be drained to get the valve block out. It's a difficult job as there's not much space to work in.

It's worth looking over the valve block in case there's a broken valve spring the may have caused two clutches to engage simultaneously and cause the bang/lockup.

Proper LT71141 looks a deep amber and not unlike fresh low-viscosity engine oil but a shade or so. It smells revolting - like cat's pee.

No option really but to have the 'box out and strip it.
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Re: C5 HP20 causing grief

Post by mongoose100 »

Hmmm well the sample I took out didn't smell like pee, and it looked like I'd taken the engines sump plug out, black with a slight red tinge :(

Is there concrete evidence that D3 will kill the HP20's in a short amount of time? Need as much ammo as I can before I take it up with "specialist"
Regan.

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Re: C5 HP20 causing grief

Post by CitroJim »

Did the sample smell burned Regan?

That it's dark and only tinged with red suggests that it's either been very hot or has plenty of debris from clutches or bearings suspended in it.

No concrete evidence that Dexron kills them but in use it will shorten their life considerably and I don't believe, comparing Dexron and LT that Dexron has the same lubricity properties and I reckon this might lead to bearing failure if it runs with Dexron for too long.

Trouble is, I've stripped a couple that have been filled with the red stuff but i cannot know for how long they'd been running on it. Similarly I've stripped one that ran on LT and that was pretty wrecked too...
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Post by addo »

Is it the software which actually damages the boxes?

I mean, are the apply/release overlaps too great for input torque applied and thus excess friction material is shed, resulting in it damaging running clearances (including bearings) elsewhere?
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Re: C5 HP20 causing grief

Post by xantia_v6 »

It seems that the typical failures are due to lack of hydraulic operating pressure, lack of lubrication, as a result of the oil filter being clogged with wear debris from the torque converter lock-up clutch.

All of the clutches in the transmission (except, confusingly, the lock-up clutch), are generally fully released or fully locked up, so they don't ware much at all (in my transmission at 150,000 miles there was no visible wear to any clutch).

The lockup clutch however is intended to slip slightly in normal operation (from memory a 5% slip) so with a 2000 RPM input, it drives the transmission at 1900 RPM. This is apparently to smooth out the power pulses from the engine, but obviously gives opportunity of the clock-up clutch to wear. I think that this is the place where correct lubricant is critical, so that the lock-up clutch can slip slightly, but be protected by the lubricant sufficiently to last for 150,000 miles or whatever the design service life is.

Once the filter gets blocked and the line pressure drops, the lock-up clutch is probably destroyed in a short time.

The apparent software glitches which cause the transmission to momentarily lock-up and discomfort to passengers do not seem to cause any mechanical damage to the transmission.
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Re: C5 HP20 causing grief

Post by CitroJim »

I would very much concur with your words above Mike. The torque converter, by the fact it contains the lock-up clutch, is the precursor to a gearbox failure and underlines why it must be replaced as a matter of course when overhauling the gearbox.

Similar to yours Mike, the gearbox I recently rebuilt for Chris570's XM showed no clutch/brake wear after 171,000 miles.

He has a nice reconditioned TC on it.
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Re: C5 HP20 causing grief

Post by mongoose100 »

Probably get shot down in flames here but whats to stop someone somehow bypassing the internal filter when its apart and putting something like a remote spin on type external filter for the box???

More hassle than its worth?
Regan.

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Re: C5 HP20 causing grief

Post by CitroJim »

mongoose100 wrote:Probably get shot down in flames here but whats to stop someone somehow bypassing the internal filter when its apart and putting something like a remote spin on type external filter for the box???

More hassle than its worth?
It would be bloody difficult Regan, although not impossible...

The base problem is that the filter connects directly to the oil galleries leading to the pump so some clever adaptor would be needed. Also the filter is fully submerged. It may be necessary if an external device were fitted, to ensure it was always full of oil and the line between it the sump and pump was full at all times so there's never any risk of the pump pumping thin air...

Sadly, the external oil pipe that on first glance looks an excellent palkce for it is well after the filter...
Jim

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