| Down shift switch [message #105809] |
Fri, 12 November 2010 20:55  |
Bruce S
Messages: 13
Registered: November 2010
Location: Winnipeg,Manitoba,Ca
Karma: 0
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Junior Member |
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I got my GMC MH last May and have use it for only one long trip. Being that I am a flat-lander I like to force down-shift when going up or down a steep grade, also feel that the transmission and brakes run cooler when doing so. I have problems shifting back to "D" and often miss it and go to "N". I noticed that when the accelerator is floored the linkage operates a switch, which I think down-shifts the trans . I am going to install a 2 position push button switch on the shifter leaver and wire it accross the switch that the accelerator operates. Then when I push it the transmision down-shifts, push it again transmition go back to "D". I would like to know if anyone has tried this?
Bruce
http://www.facebook.com/album.php?aid=9526&id=100001809171788&l=5ca2c69fc9
78 Eleganza II, more demanding of attention & the pocket book than the wife but less bitchy (my better 1/2 suggestion).
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| Re: Down shift switch [message #105812 is a reply to message #105809] |
Fri, 12 November 2010 22:13  |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
Karma: 12
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It's the same gear, but not the same as a manual downshift as far as pressures internally. That is the consensus I've gathered here. Also not sure if you get actual engine braking in that electric downshift mode. My kickdown switch works, but I use the lever to downshift.
John Lebetski
Woodstock, IL
77 Eleganza II
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| Re: [GMCnet] Down shift switch [message #105891 is a reply to message #105812] |
Sun, 14 November 2010 16:53 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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I'm no transmission expert by any stretch of the imagination. But I have
heard Manny and other experts say that the internal clutch pressure is
considerably higher when the lever is used for downshift; therefore, less
clutch slippage and heating.
The switch is a simple solution but probably not a good one.
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI+ & EBL
www.gmcwipersetc.com
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Ken Henderson
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www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: Down shift switch [message #105895 is a reply to message #105809] |
Sun, 14 November 2010 18:00 |
armandminnie
Messages: 864
Registered: May 2009
Location: Marana, AZ
Karma: 2
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If I understand Manny correctly - I have asked him about this in person - he disables the automatic downshift and recommends that the shift lever be used to do the job because of the pressure difference on the clutches and/or bands. Lower pressure means more slipping and more heat.
As far as using the lower gear going downhill, Manny says that dropping the transmission into second at speed is bad. The best thing is to slow way down using your brakes and then downshift - unless it is an emergency, of course, and then anything goes.
Armand Minnie
Marana, AZ
'76 Eleganza II TZE166V103202
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click here to visit gmcws.org
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| Re: [GMCnet] Down shift switch [message #105905 is a reply to message #105891] |
Sun, 14 November 2010 19:02 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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G'day,
Here's what Rick Denny had to say about this in 2008:
Regards,
Rob M.
USAussie
The problem is downshifting when the input and output shafts of the
transmission will be a much different speeds. That will cause the torque
converter to slip while the clutches in the tranny engage.
Manny has showed us that clutch--it's not as big as brake shoes.
But if you brake such that your speed is well within normal for the
destination gear, and then downshift, it's not much of an issue, in my view.
For example, if you use the brakes to slow down to 45 and then downshift to
S, the engine will be down near the stall range of the torque converter
where it can slip more easily. You'll end up in the low 2000's RPMs and the
engine will help you maintain speed.
Contrasting that technique is to downshift to S at 65 mph to let the tranny
slow you down to 45. You'll be in the middle 2000's RPMs before the
downshift, and well into the 3000's after downshift. The torque converter is
a lot tighter there and that puts much more of a shock on the system.
So, it seems to me reasonable to use the brakes to slow down, but use the
tranny to help maintain lower speeds so that you don't have to ride the
brakes.
Of course, it's not good for the tranny if you run into the side of a
mountain because you lost the brakes on a steep descent!
The rule of thumb, of course, is to descend in the same gear you used for
the ascent.
Rick "expressing opinion" Denney
'73 230 Ex-Glacier "Jaws"
Northern Virginia
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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] Down shift switch [message #105907 is a reply to message #105905] |
Sun, 14 November 2010 19:18 |
Mr ERFisher
Messages: 7117
Registered: August 2005
Karma: 2
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Senior Member |
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and here are some hints, tried and true
http://gmcmotorhome.info/transmission.html#shift
gene
On Sun, Nov 14, 2010 at 5:02 PM, Rob Mueller <robmueller@iinet.net.au>wrote:
> G'day,
>
> Here's what Rick Denny had to say about this in 2008:
>
> Regards,
> Rob M.
> USAussie
>
> The problem is downshifting when the input and output shafts of the
> transmission will be a much different speeds. That will cause the torque
> converter to slip while the clutches in the tranny engage.
> Manny has showed us that clutch--it's not as big as brake shoes.
>
> But if you brake such that your speed is well within normal for the
> destination gear, and then downshift, it's not much of an issue, in my
> view.
> For example, if you use the brakes to slow down to 45 and then downshift to
> S, the engine will be down near the stall range of the torque converter
> where it can slip more easily. You'll end up in the low 2000's RPMs and the
> engine will help you maintain speed.
>
> Contrasting that technique is to downshift to S at 65 mph to let the tranny
> slow you down to 45. You'll be in the middle 2000's RPMs before the
> downshift, and well into the 3000's after downshift. The torque converter
> is
> a lot tighter there and that puts much more of a shock on the system.
>
> So, it seems to me reasonable to use the brakes to slow down, but use the
> tranny to help maintain lower speeds so that you don't have to ride the
> brakes.
>
> Of course, it's not good for the tranny if you run into the side of a
> mountain because you lost the brakes on a steep descent!
>
> The rule of thumb, of course, is to descend in the same gear you used for
> the ascent.
>
> Rick "expressing opinion" Denney
> '73 230 Ex-Glacier "Jaws"
> Northern Virginia
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
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
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| Re: Down shift switch [message #105985 is a reply to message #105809] |
Mon, 15 November 2010 20:32 |
Bruce S
Messages: 13
Registered: November 2010
Location: Winnipeg,Manitoba,Ca
Karma: 0
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Junior Member |
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Thanks for the great replys. I did not know about the lower pressures on the bands re downshift switch verses manual shift. Will have to check the shift cable and shift lever for lube and wear. I also like the idea of using the brakes during down-shifting when starting to go down a steep grade. In the past I straight down-shifted for steep downgrades the truck/camper that I traveled many many miles with no trans problems, but it was 1/2 the weight of the GMC.
Again thanks
Bruce
78 Eleganza II, more demanding of attention & the pocket book than the wife but less bitchy (my better 1/2 suggestion).
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| Re: Down shift switch [message #106101 is a reply to message #105985] |
Wed, 17 November 2010 20:32 |
Bruce S
Messages: 13
Registered: November 2010
Location: Winnipeg,Manitoba,Ca
Karma: 0
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Junior Member |
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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 clutches. My understanding that in auto transmissions the oil passes through orifices for smoother 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
78 Eleganza II, more demanding of attention & the pocket book than the wife but less bitchy (my better 1/2 suggestion).
[Updated on: Wed, 17 November 2010 20:39] Report message to a moderator |
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