Home » Public Forums » GMCnet » Re: [GMCnet] Transmission shifting
| Re: [GMCnet] Transmission shifting [message #86587] |
Sun, 30 May 2010 08:37  |
Gary Casey
Messages: 448
Registered: September 2009
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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?
>
> --
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| Re: [GMCnet] Transmission shifting [message #86593 is a reply to message #86587] |
Sun, 30 May 2010 09:52  |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Gary,
Is there any documentation available that I can read that discusses the
shift schedule of the 400/425 transmissions?
I'd like to learn if you "tune" the transmissions shift points to your
engine's torque / power curve?
Regards,
Rob Mueller
Sydney, Australia
AUS '75 Avion-The Blue Streak TZE365V100428
USA '75 Avion-Double Trouble TZE365V100426
-----Original Message-----
From: Gary Casey
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.
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] Transmission shifting [message #86597 is a reply to message #86593] |
Sun, 30 May 2010 10:11 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
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Senior Member |
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Rob,
Interesting reading on Caspro re transmission longevity and their shift re-porgram kit.
http://www.casprogmc.net/troubleshootingTransmissionLongevity.html
http://www.casprogmc.net/reprogramKit.html
Not intended as a statement of my opionion -- just information.
Dennis Sexton
73 GMC
Germantown, TN
USA
-----Original Message-----
From: Rob Mueller <robmueller@iinet.net.au>
To: gmclist@temp.gmcnet.org
Sent: Sun, May 30, 2010 9:52 am
Subject: Re: [GMCnet] Transmission shifting
Gary,
Is there any documentation available that I can read that discusses the
hift schedule of the 400/425 transmissions?
I'd like to learn if you "tune" the transmissions shift points to your
ngine's torque / power curve?
Regards,
ob Mueller
ydney, Australia
US '75 Avion-The Blue Streak TZE365V100428
SA '75 Avion-Double Trouble TZE365V100426
-----Original Message-----
rom: Gary Casey
The shift schedule built into all factory 400/425's is very conservative,
eeping the engine speed low (keeping your Toro quiet, you know), but this
sn't a very good approach for such a heavy vehicle (in my opinion, again).
igher upshift speeds will produce higher engine noise, but will keep the
onverter in a more efficient range. Manually shifting can simulate this,
ut with a more abrupt 1-2 shift, not good or bad.
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MCnet mailing list
ist Information and Subscription Options:
ttp://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
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Dennis S
73 Painted Desert 230
Memphis TN Metro
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| Re: [GMCnet] Transmission shifting [message #86634 is a reply to message #86633] |
Sun, 30 May 2010 16:31 |
Dennis S
Messages: 3046
Registered: November 2005
Karma: 2
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Senior Member |
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Rob,
I seem to recall that Arch installed one of their kits -- an archive search may find it -- but I don't have any more info. I'm not sure of the status of Caspro either -- didn't Manny buy the rights to their belt dirve?
Dennis Sexton
73 GMC
Germantown, TN
USA
-----Original Message-----
From: Rob Mueller <robmueller@iinet.net.au>
To: gmclist@temp.gmcnet.org
Sent: Sun, May 30, 2010 4:26 pm
Subject: Re: [GMCnet] Transmission shifting
Dave,
Thanks, Mate! That is really good info!
Has anybody installed their re-programming kit and what were the results?
BTW their info on trans leaks is also excellent!
Regards,
ob Mueller
ydney, Australia
US '75 Avion-The Blue Streak TZE365V100428
SA '75 Avion-Double Trouble TZE365V100426
-----Original Message-----
rom: dennisfsexton@aol.com
Rob,
nteresting reading on Caspro re transmission longevity and their shift
e-porgram kit.
http://www.casprogmc.net/troubleshootingTransmissionLongevity.html
http://www.casprogmc.net/reprogramKit.html
Not intended as a statement of my opionion -- just information.
Dennis Sexton
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MCnet mailing list
ist Information and Subscription Options:
ttp://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Dennis S
73 Painted Desert 230
Memphis TN Metro
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