| Re: [GMCnet] tranny kick down switch [message #153989] |
Wed, 21 December 2011 20:28  |
Gary Casey
Messages: 448
Registered: September 2009
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In spite of how it feels during a full throttle upshift, I'm sure it will not "destroy itself." However, downshifting from 3rd to second manually is significantly harder on the transmission than downshifting automatically by way of the detent ("kickdown") switch. When downshifting manually the second overrun band is engaged and it will try to pull the engine up in speed. When doing a full throttle automatic downshift the third clutch is simply released and the engine is merely allowed to increase in speed until the second freewheeling clutch engages - very gentle compared to trying to jerk the engine up with the second band. The second gear band is very, very fragile and it doesn't take a lot of high speed downshifts to burn it up. When I need to downshift I'll go to full throttle momentarily to trigger the downshift and then move the lever to the "S" position to hold it in second. Avoid manually forcing the downshift unless it is really
necessary. The upshift to third is no different whether selected manually or allowed to happen automatically.
A manual 1-2 shift, however is a totally different animal. When in either S or L the transmission line pressure is increased to the maximum - usually about 150 psi. So the manual 1-2 shift, even if done at very light throttle, is done at maximum line pressure. That can really jerk, but it still won't destroy the transmission. One way to "trick" it: With the transmission selector in L and the transmission in 1st, go to full throttle and immediately shift to D, back off a little on the throttle, allowing it to shift to second. Immediately pull the lever back into S, holding it in second.
I've seen some "shift kits" that make logical changes in the shift calibration to speed up the shifts without making them too harsh. But I've seen others where the builder seemed to think that if some is good a lot more is better. These guys will often throw away check balls that are there to control shifts, leaving the clutches with no fill restrictions at all. Just doesn't make sense. And then they will increase the pressure regulator spring force to the point that the shifts are impossibly harsh. Doesn't help the transmission, doesn't make it run cooler - all it does is make it obnoxious.
Just my opinion.
Gary Casey
I unplugged my kick down and only pull it into super manually. My tranny has a shift kit in it so it shifts harder then stock. On a hill it down shifted OK but at times the up shift was too violent. I unplugged it to save the tranny from destroying itself.
--
Roy Keen
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| Re: [GMCnet] tranny kick down switch [message #153997 is a reply to message #153989] |
Wed, 21 December 2011 20:57  |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
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Gary, I think you are on the right track on that personally. I am constantly in the mountains here and although I still have the 3.07 with a heavy coach, I feel I have adequate ratio coverage and tend to shift as you describe. There are some inclines and declines where I just leave it in 1st or 2cnd and try not to shift at all. Revving the engine does not bother me a bit and if I can maintain a gear I would prefer that. The other thing I do that may not be recommended is leave it in L (1 first gear) and let the trans upshift and downshift by itself when it reaches it max shift rev limit. It's not very traumatic and on some of those mountain roads, I don't have time to be fooling with that "balky" shift lever. Unless modified, these transmissions will override the selector position to protect the engine and transmission and frankly the revs don't get that high.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] tranny kick down switch [message #154029 is a reply to message #153997] |
Thu, 22 December 2011 04:30 |
Mr ERFisher
Messages: 7117
Registered: August 2005
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The reason I "un-plug" the kick down is, for the engine..
I know of 3 engines that were blown when the coach was going down hill,
and the driver punched it, (trying to pass) and over revved the 455 when
the tranny "kicked down".
and then there are these mountain driving tips, that show the tranny uses
more resources when you pull it into S , rather than the kick-down.
http://gmcmotorhome.info/transmission.html#shift
gene
On Wed, Dec 21, 2011 at 6:57 PM, Bob de Kruyff <NEXT2POOL@aol.com> wrote:
>
>
> Gary, I think you are on the right track on that personally. I am
> constantly in the mountains here and although I still have the 3.07 with a
> heavy coach, I feel I have adequate ratio coverage and tend to shift as you
> describe. There are some inclines and declines where I just leave it in 1st
> or 2cnd and try not to shift at all. Revving the engine does not bother me
> a bit and if I can maintain a gear I would prefer that. The other thing I
> do that may not be recommended is leave it in L (1 first gear) and let the
> trans upshift and downshift by itself when it reaches it max shift rev
> limit. It's not very traumatic and on some of those mountain roads, I don't
> have time to be fooling with that "balky" shift lever. Unless modified,
> these transmissions will override the selector position to protect the
> engine and transmission and frankly the revs don't get that high.
> --
> Bob de Kruyff
> 78 Eleganza
> Chandler, AZ
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--
Gene Fisher -- 74-23,77PB/ore/ca
“Give a man a fish; you have fed him for today --- give him a URL and
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Alternator Protection Cable
http://gmcmotorhome.info/APC.html
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| Re: [GMCnet] tyranny kick down switch [message #154056 is a reply to message #153997] |
Thu, 22 December 2011 12:09 |
roy1
Messages: 2126
Registered: July 2004
Location: Minden nevada
Karma: 6
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| Bob de Kruyff wrote on Wed, 21 December 2011 18:57 |
Gary, I think you are on the right track on that personally. I am constantly in the mountains here and although I still have the 3.07 with a heavy coach, I feel I have adequate ratio coverage and tend to shift as you describe. There are some inclines and declines where I just leave it in 1st or 2cnd and try not to shift at all. Revving the engine does not bother me a bit and if I can maintain a gear I would prefer that. The other thing I do that may not be recommended is leave it in L (1 first gear) and let the trans up shift and downshift by itself when it reaches it max shift rev limit. It's not very traumatic and on some of those mountain roads, I don't have time to be fooling with that "balky" shift lever. Unless modified, these transmissions will override the selector position to protect the engine and transmission and frankly the revs don't get that high.
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This is probably the best reason not to install a shift kit.I lost an engine awhile back when I accidentally pulled it into low thinking it was super. If the shift kit wasn't there the design of the trans would not have allowed it to go into low.
Roy Keen
Minden,NV
76 X Glenbrook
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| Re: [GMCnet] tranny kick down switch [message #154065 is a reply to message #154029] |
Thu, 22 December 2011 13:59 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""I know of 3 engines that were blown when the coach was going down hill,
and the driver punched it, (trying to pass) and over revved the 455 when
the tranny "kicked down"""
Unless the transmission has been modified (which they may have been) the trans will shift no matter what the kickdown switch is commanding or what position the lever is in. It will upshift at it's normal WOT shift point no matter what. Now I suppose a tired engine may blow it's guts out at something around 4500 rpm, but that's a different cause.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] tranny kick down switch [message #154068 is a reply to message #154065] |
Thu, 22 December 2011 14:28 |
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ljdavick
Messages: 3548
Registered: March 2007
Location: Fremont, CA
Karma: -3
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The oiling system in our 455's (as I understand it) has a flaw at higher RPM's. The oil does not return to the pan quickly enough and can cause oil starvation and engine failure. Mr. Onan, Joe Mondello had a fix for this but the details are lost to me at the moment.
Larry Davick
Fremont, California
The Mystery Machine
'76 (ish) Palm Beach
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Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, Ca
Howell EFI + EBL + Electronic Dizzy
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