| Re: [GMCnet] tranny kick down switch [message #154060] |
Thu, 22 December 2011 12:30 |
Gary Casey
Messages: 448
Registered: September 2009
Karma: 0
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Senior Member |
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Bob,
I agree, and the only real problem (in my opinion, of course) is that cruising in high gear might be at an inefficient torque converter operating condition with a very low final drive ratio. When you really need a higher final drive ratio is when you can't get moving, forward or in reverse. All of my driving has lately been above 7,000 ft and I think my 3.42 final drive is more than adequate - but then my coach is about as light as they get. The thing that a different final drive does is change the speeds at which the lower gears can be used. An advantage of the 3.07 is that second gear runs out to a higher speed that can be very useful in the mountains. I'm going by memory from 40 years ago, but here's how I remember most all THM transmissions work: In manual second ("S") the transmission will downshift to second and stay there regardless of speed. 1-2 and 2-1 shifts will be almost at the normal speeds, but at maximum line pressure,
making the shifts jerky. In manual first ("L") the transmission won't downshift until it wants to, which is usually at a lower speed than one might like. Once in first it won't upshift until at a quite high speed, usually at a little higher speed than the normal WOT 1-2 in Drive. At least that's the way I remember it. Only THM-400's in Chevrolet's were much different than that.
One caution: Some of the modifications (as implied by Bob) will override the 2-1 downshift point, allowing the transmission to make the shift at any speed. So unless you know the transmission is unmodified, don't drop it into Low at 70 mph - you might get surprised.
Gary
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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