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P38 Torque Converter Lock-up


donaldh

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I've noticed recently that the torque converter on my P38A doesn't start locking up until I have travelled a few miles, usually well after the engine is fully warmed up. I'm curious to find out if this is normal or there is some kind of fault.

The workshop manual has this to say about torque converter lock-up:

"Lock-up clutch operation is dependent on throttle position, engine speed, operating mode and the range selected on the transfer box".

That seems to describe its operation after I am a few miles into my journey, but doesn't help to explain the warm-up behaviour.

Any ideas?

Donald.

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As I understand it:

The valve work to enable lock is supposed to be smaller ( and weaker? ) than the valve work used to change gears. For this reason, there is an internal temperature sensor in the gearbox which the ECU uses to monitor the oil temp, and the ECU wont let it lock up until the oil temp has reached a certain value and so becomes thin enough to properly work the valves for the lock up mechanism. Supposedly, since the oil is thicker when cold, there is possibility it can damage the valves.

Its meant to be a wholly internal feature to the gearbox and ECU which cant be influenced by anything external ( like the main ECU, gear stick position, road speed etc etc )

funny you should bring this subject up.

I was about to ask whether anyone knew of a way it would be possible to manually disable the lock up. To me, its a bloody pain in the arse how it locks up at 36mph . . .esp when towing. You immediately lose acceleration when you pull a heavy load. It'd be great to temporraily disable through a switch once in a while.

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As I understand it:

The valve work to enable lock is supposed to be smaller ( and weaker? ) than the valve work used to change gears. For this reason, there is an internal temperature sensor in the gearbox which the ECU uses to monitor the oil temp, and the ECU wont let it lock up until the oil temp has reached a certain value and so becomes thin enough to properly work the valves for the lock up mechanism. Supposedly, since the oil is thicker when cold, there is possibility it can damage the valves.

Its meant to be a wholly internal feature to the gearbox and ECU which cant be influenced by anything external ( like the main ECU, gear stick position, road speed etc etc )

funny you should bring this subject up.

I was about to ask whether anyone knew of a way it would be possible to manually disable the lock up. To me, its a bloody pain in the arse how it locks up at 36mph . . .esp when towing. You immediately lose acceleration when you pull a heavy load. It'd be great to temporraily disable through a switch once in a while.

I belive the one to ask woud be lara he has fitted a switch to his to do the opposite, so I guess he would have better understanding of these than most people.

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Thanks for the info. I guess it's perfectly normal behaviour then. It's unfortunate that on my daily commute I can be doing 60 and the torque converter won't lock up.

To override the EAT ECU it should just be a case of identifying the wire for the solenoid then adding a switch to either supply power, or break the connection to the EAT ECU, depending on whether you want to force it on or force it off. I guess you could use a tri-state switch to allow ECU <-> Off <-> On.

Cheers,

Donald.

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  • 1 month later...

As I understand it, lockup is disabled when cold as part of the "fast warm up" strategy - to get the powertrain to operating temperatue as soon as possible.

IIRC the wiring diagram shows the lockup being controlled by a pair of wires, so a manual control might be possible - or might just throw a gearbox fault.

I wouldn't have a switch to manually enable lockup, though. If it tried to change gear with the TC locked that would probably be a Bad Thing.

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Can I be thick? How can you actually tell when driving that it is locking up? Maybe it's because I have only just got an Auto but I never know what the gearbox is doing really, it just sort of gets on with it.

I tend to drive in one of two frames of mind. If I'm just puttering along, I can't tell you what gear it's in or if it's locked up. When I'm paying attention, I can tell you. The lockup feels similar to a gearchange with less effect on overall revs - and with the sports exhaust you can hear the engine load reduce when it happens.

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SNIP

I was about to ask whether anyone knew of a way it would be possible to manually disable the lock up. To me, its a bloody pain in the arse how it locks up at 36mph . . .esp when towing. You immediately lose acceleration when you pull a heavy load. It'd be great to temporraily disable through a switch once in a while.

Have you tried using Sport mode when towing?

The manual shows that on a Diesel, or pre-'99 Petrol, pin 42 on the gearbox ETC is the output which controls the solenoid valve that switches the lock-up on.

It might be interesting to monitor this with a voltmeter to see what condition is 'normal' and what condition is present when lock up occours. Then you will know what condition needs to be keep 'on' to stop the lock-up occouring.

David.

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David

Sport mode -yes, i do often tow in sport mode as it likes to hold onto a gear a little longer. Trouble is that i find is that its a fine balance between pressing the accelerator hard enough that your stopping lock-up, but not too far to make the gearbox think it wants to change down to 3rd.

Since i'm towing a horse trailer, my aim is smooth acceleration and not necessarily get upto speed as fast as possible. The issue i'm mainly having is that i can accelerate upto 36mph fine and smooth, but the lockup kicks in ( even in sport mode ) and it just drops the revs to low and i lose power and acceleration. Pressing the accelerator down further can sometimes unlock the gearbox bringing the revs up and bring back acceleration, but sometimes it drops to 3rd harshly and jerks the the trailer - which is what i look to avoid when towing the horse.

I didn't know about the pin 42 . . . i'll investigate that further. thanks,

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