| Re: [GMCnet] Down shift switch [message #106111] |
Thu, 18 November 2010 05:52 |
Gary Casey
Messages: 448
Registered: September 2009
Karma:
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Senior Member |
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Bruce,
I think you have the right idea. The "overrun" downshift to second uses the
second overrun band to pull the engine speed up and hold it in gear. That band
is very small, almost like it was intended to be never used. Any "help" you can
give it to get the engine speed up will improve its life. On the other hand,
the 2-1 overrun downshift uses the reverse band and that is far more beefy and
tolerant of abuse. Still, the number of overrun downshifts the THM 425 can
tolerate is limited. I've done the same thing as you by forcing a power-on
downshift by momentarily going to full throttle and then locking it in the lower
gear manually. Takes less skill:-). And yes, the time delay is caused by oil
flowing through orifices to fill the clutches and band servos.
Gary Casey
I have been thinking that using the brakes during manual down-shifting will not
reduce the shock to the transmission. Instead what you want to do is bring the
engine rpm up to match the change in ratio. Same idea as double-clutching a
highway tractor. This evening I tried out my idea on the ford van I use. I found
that when traveling at 50 kph (30 mph)and shifting "D" to "1"("D" to "2" does
not produce the required effect at 50 km) that the right amount of accelerator
will produce no noticable feeling that a shift occured. For engine braking you
have to pause before letting up on the accelerator to let the pressure build up
on the required cluthes. My understanding that in auto transmissions the oil
passes through orifices for smother shifting, feels like 1/4 to 1/2 sec delay.
When heading home from work I have 8 stop signs, after the 6th one I got pretty
smooth at down-shifting with only using the brake the last 10 ft. to stop from
50 kph.
Bruce
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