| Re: [GMCnet] Transmission shifting [message #86587] |
Sun, 30 May 2010 08:37  |
Gary Casey
Messages: 448
Registered: September 2009
Karma:
|
Senior Member |
|
|
This is an answer to a relatively old post, so pardon me if it has been answered.
If there is a weak link, achilles heel, glass jaw in the 400/425 it is the 2nd overrun band. In a high-mileage car this band will either be still new (never used) or totally frazzled (has been used). This band is never applied in Drive, but is applied in manual second (S). When under load (positive torque) the 2nd one-way (Sprague, named for the company that invented the type that was originally used) clutch absorbs the reaction torque. Using manual second during acceleration doesn't do anything to help this Sprague clutch - it is very robust and in any event the band won't accept any of the torque even if it were applied. Yes, the line pressure is higher in manual 2nd, but that doesn't do much one way or the other - once the clutches are applied they don't slip anyway.
The problem comes on overrun downshifting (shifting from D to S with the throttle closed). The engine has to be accelerated to 2nd gear speed and the friction element that does that is the 2nd overrun band, which is drastically undersized (my opinion) for the job. What to do? First, do that downshift early when going at low speed. At low vehicle speed the energy of the shift is small and is no factor. Avoid overrun downshifts at high speed at all cost. Well, if you NEED the lower gear there is no choice. If you know you are going to use second gear for engine braking you can always go to full throttle so it automatically downshifts under load and then drop the selector into S. I do this sometimes in mountain driving. Another way is to put it under load, short of full throttle and downshift manually. With the engine putting out power much less energy is absorbed by the 2nd band. Don't want to go to full throttle, even momentarily,
when going downhill? Well, that's the result of a lack of planning. I often manually downshift and punch the throttle during the shift.
Downshifting into 1st isn't nearly the problem for two reasons. First, in my experience it is much less likely to be required and therefore is done less often. Second, the band that absorbs this energy is the reverse band, which is very robust and much more able to stand up to the task.
The shift schedule built into all factory 400/425's is very conservative, keeping the engine speed low (keeping your Toro quiet, you know), but this isn't a very good approach for such a heavy vehicle (in my opinion, again). Higher upshift speeds will produce higher engine noise, but will keep the converter in a more efficient range. Manually shifting can simulate this, but with a more abrupt 1-2 shift, not good or bad.
Gary Casey
previous post:
> this came up before and someone said that when climbing a hill it was
> better to manually shift to "S" than to use the kick down switch. more
> clutches were used manually shifting.
> can anyone confirm that?
>
> --
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
|
|
|
|
GMCforum
Search
Help
Register
Login
Home
