Lock up Auto's
Submitted: Monday, Jan 31, 2005 at 19:24
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StormyKnight
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?
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.
Chaz
http://members.bettanet.net.au/~conody/index.htm
AnswerID:
95961
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?
FollowupID:
354808
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.
Peter
FollowupID:
354828
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.
Chaz
FollowupID:
354898