Lock up Auto's

Submitted: Monday, Jan 31, 2005 at 19:24
ThreadID: 19974 Views:1961 Replies:2 FollowUps:5
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Why do they slip?
Just wanted to know what they exactly mean by a lockup converter when clearly if you back off the throttle the revs will drop to just above idle even when your going down a hill @ 100km/h.

Also anyone noticed on a TD prado90 when it changes gears either up or down, the revs settle shortly after the change, then 1/2 a second later they drop again & it appears that the auto is locking up in 3rd or 4th. Anyone know whats going on?
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Reply By: Tuff60 - Tuesday, Feb 01, 2005 at 01:04

Tuesday, Feb 01, 2005 at 01:04
As you said, after a gear change to 3rd or 4th, the revs come down a bit when you coast, that is the torque lock up coming on, for heat and economy benefits. droping back to idle at 100 seams strange but I have not driven an auto prado.
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Follow Up By: StormyKnight - Tuesday, Feb 01, 2005 at 19:21

Tuesday, Feb 01, 2005 at 19:21
No under hard acceleration, it will drop down a gear, the revs climb as you would expect, then even under full throttle, 1/2 sec later, rpm will drop again a couple of hundred revs. I assume that this is the lockup happening, but why when you have say the cruise on & I don't think this is only when, and you start to go down a steep hill, the throttle backs right off as you would expect. If the hill is steep enough & you can maintain 100 without throttle, the rpm drops toward idle even though the lockup transmission should I would have thought still be engaged.
This is what I mean by slipping & I can't see why it does it.
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Follow Up By: pjchris - Wednesday, Feb 02, 2005 at 11:32

Wednesday, Feb 02, 2005 at 11:32
Fuel economy...disengage drive and return engine to idle=less fuel use. If the clutch stayed locked then the vehicle would slow down and the cruise control would have to open the throttle again using more fuel than if the engine was idling.
My transmission detects that the vehicle is speeding up when going down hill and will engage the lock up clutch to slow the car down. Stupidly though, it does not do this when the cruise control is on!


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Reply By: Chaz - Tuesday, Feb 01, 2005 at 01:17

Tuesday, Feb 01, 2005 at 01:17
Hi StormyKnight,
A lockup converter is just like a normal one, except is has the addition of a clutch installed. It is a friction disc located at the front of the converter and applies to a metal plate that is part of the base that is fitted to the ring gear or fly wheel. It is applied by hydraulic pressure that is controlled buy a solenoid valve in the valve body.
Lock up is controlled by the ECU or transmission control unit and usually happens around 80kph, depending on vehicle.
What you can feel is the Trans shifting up and then locking the converter, (if you’re doing over 80kph). The Trans always unlocks during the shift and then locks again because the shift would be too harsh otherwise.
Also, the reason your revs drop so much at speed is because the converter unlocks as soon as you get off the throttle and you have no engine breaking in overdrive (4th gear), so go back to 3rd manually and you'll slow down quick enough. That's why you shouldn’t drive around town in 4th too much, no engine breaking and you'll have premature break wear.
Some people fit a converter over-ride switch so that they can control the converter manually which will eliminate the hunting or locking and unlocking that you can experience when towing with an auto, and this also makes for quicker acceleration, because you can lock it as soon as you go into 3rd and keep it locked, even under load.
Hope this helps explain it.

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Follow Up By: Chucky - Tuesday, Feb 01, 2005 at 13:35

Tuesday, Feb 01, 2005 at 13:35
Well said and written!
even I understood that explaination.
So if you had the override switch on and stopped the car wold it stall? If so would it help engine braking on the step down hill decents?
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Follow Up By: pjchris - Tuesday, Feb 01, 2005 at 15:20

Tuesday, Feb 01, 2005 at 15:20
One more point to be aware of is that often the lock up clutch is not built to withstand the full torque of the engine and locking it while towing heavy loads could lead to premature failure of the lock up clutch and it is very expensive to replace. Also having the converter locked during gearshifts will place enormous stress on the clutch/transmission etc.
Locking the convertor will probably lead to slower acceleration times as you lose the torque multiplication effect of the converter.
It will, in normal driving, lead to better fuel economy though.
As an example of modern transmission design, in the current 5-speed Pajero auto the computer will attempt to maintain the engine revs at the torque peak...approx 2300rpm on mine, and control the slip in the gearbox to provide acceleration. The effect is that the vehicle accelerates very well but a glance at the dash shows rpm stuck firmly to 2300rpm as the speed increases.


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Follow Up By: Chaz - Tuesday, Feb 01, 2005 at 20:27

Tuesday, Feb 01, 2005 at 20:27
Thanks Chucky,
My last 4by was an Isuzu traytop with Chev V8 and Turbo 700 trans that I fitted with a Fairbanks shift kit. http://members.bettanet.net.au/~conody/Isuzu%204X4.htm
This kit didn't allow the converter to lock in 1st and 2nd, but another way is to put a relay into the brake light circuit to unlock it when the brakes are on and you can use the brake pedal as a kind of clutch pedal by adjusting the switch so that the brake lights come on at the touch of the pedal. As peter said, it's not good to lock the converter under full throttle, but I did 80,000ks on that setup with no problems. Also, I would always back off the throttle slightly during gearshifts. My current 3.0L GU Patrol won't lock in low gears as well. I haven't yet fitted an Over-ride switch, but I will because another advantage in having control over your converter is when doing downhill decents, it drasticly improves engine braking.

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