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Old 09-22-2008, 09:36 AM   #51
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I'm suprised only one person mentioned the cruise maintaining speed down hills. On my truck and my father (GMC, so same as mine), if cruise is set, regardless of hill size it seems to try to maintain speed. You can actually hear and feel it downshift in some cases. I'm not sure if it's in DFCO or not, but it's definately trying to prevent runaway. We've both got the towing package so maybe that's a feature that's assuming you have a large load to prevent runaway.

And before I replaced the tranny on my truck it was only shifting from 2nd to 3rd. Supposedly it needed a shift solenoid to get to first and lock overdrive, but the thing was pretty beat and filled with Lucas fix a tranny stuff and probably never had it's fluid changed ever, so I just accepted it as a loss and waited for the inevitable. The funnest thing was flatbedding a car from NH to NY on back roads through the berkshires and VT. Only being able to pull away in 2nd and climb hills in 2nd really sucked, but somehow it just kept going and lasted another month after that too.
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Old 09-22-2008, 03:57 PM   #52
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Originally Posted by itjstagame View Post
I'm suprised only one person mentioned the cruise maintaining speed down hills. On my truck and my father (GMC, so same as mine), if cruise is set, regardless of hill size it seems to try to maintain speed. You can actually hear and feel it downshift in some cases. I'm not sure if it's in DFCO or not, but it's definately trying to prevent runaway. We've both got the towing package so maybe that's a feature that's assuming you have a large load to prevent runaway.
This is what I've found. The engine will brake if I'm in cruise control going downhill and keep me within 4-5 mph of the speed setting. But sometimes if I'm just using the constant constant throttle technique and let off on downhills, it will shift into neutral and my RPMs will drop to about 1000. It seems weird, though, because it doesn't like to do it in overdrive. Only in 4th gear or lower. There's one hill on my drive that's in town and it'll go into neutral every time on the downhill when I'm in 4th. But there's an even bigger hill on my highway drive in when I'm in overdrive but it'll keep the RPMs up near 2000.

I also don't like using my cruise control when there are quick hills without subtle changes between. If there are a lot of up and downs in succession, my Explorer will pickup speed on the downhill. But then it won't gradually apply power as we start going uphill. It will wait until the speed goes to 2 mph below my setting and then slam into gear and really rev it up to catch up. I hate that. It does this when I hold the decelerate button too, like if I'm getting into a construction zone or something. I'll hold the button to slow down to 60 then let go, but then it'll just raise the RPMs up to like 2800 and apply a lot of power before settling back down. So now I just turn it off then back on again with the new speed setting.

Also, I know this is a bit off topic, but does anybody know what my engine is doing when it roars but stays at the same RPM? I'll be going up an incline and losing a bit of speed with the cruise control on. Usually at about 1900 RPM. But before downshifting to gain power, it will make a lot more noise and produce more power but still at 1900 RPM. It sounds like it's sucking in a lot more air or something. On about 75% of the hills this extra power is enough to avoid a downshift.
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Old 09-22-2008, 04:09 PM   #53
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My dad says in his new motor home (I think its an E450 Super Duty chassis) that if he taps the brakes while going downhill the transmission will downshift. If he continues to gain speed and taps the brake again the tranny will downshift again. I think this is to prevent people who don't know how to drive large vehicles from overheating the brakes going downhill.

-Jay
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Old 09-22-2008, 04:29 PM   #54
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But sometimes if I'm just using the constant constant throttle technique and let off on downhills, it will shift into neutral and my RPMs will drop to about 1000.
For reasons heavily discussed in the first half of this thread, that is almost certainly not what's happening. It is more likely that your torque converter is unlocking but your transmission is staying in gear.

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My dad says in his new motor home (I think its an E450 Super Duty chassis) that if he taps the brakes while going downhill the transmission will downshift. If he continues to gain speed and taps the brake again the tranny will downshift again. I think this is to prevent people who don't know how to drive large vehicles from overheating the brakes going downhill.
It's a feature of heavy-duty automatic transmissions (I know, for example, that modern Alison transmissions in GM full size trucks have it -- but instead of depending on the brake pedal, it's pretty aggressive with it when in "Tow/Haul" mode) specifically to provide automatic engine braking, though I don't think it's designed to help ignorant people...it just helps anyway.
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Old 09-22-2008, 04:55 PM   #55
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I don't remember any of my old ambulances doing that... I used to order ambulances on International DT 4700 Low Profile chassis because I could customize the heck out of the truck. When Ford says "This is the ambulance package, take it or leave it." I left it.. International let me specify exactly what engine and transmission I wanted, with custom suspension. I always ordered trucks with the Allison MD 3060 pushbutton automatic. Sweet transmission. As soon as I took delivery of a truck I'd take it down to the Allison shop and have the transmission reprogrammed with a custom program so it would shift smoother, and sooner as we never had the truck anywhere close to the 20,000 pound rating. I miss those days... I loved ordering ambulances.

-Jay
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Old 09-22-2008, 05:52 PM   #56
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Ah, here's the info:
http://www.allisontransmission.com/s...?CategoryID=11
See "Automatic Grade Braking". I think it's only been around for a few years. When was the last time you ordered an ambulance?
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Old 09-22-2008, 06:00 PM   #57
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The last one I ordered was a 2000 or a 2001 model.
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Old 10-08-2008, 04:59 PM   #58
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what's better hard firm shifts or smooth as butter shifts you can barely feel?
I always figured the barely felt ones were better since it would seem they match RPMs better, allowing for less stress on the gears at the moment of engagement. But I take it this is not the case (I'm no tranny expert). Can you explain?
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Old 10-08-2008, 05:13 PM   #59
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Harder shifts are usually better. All the effort required to soften the shift for the comfort of the driver costs fuel and transmission wear. The clutches/bands that are feathered to soften the shift spend time rubbing for a soft shift, or just kick into place for a harder shift. Not only does the rubbing wear them more, but the engine is still running and producing power without accelerating the vehicle during that time; that wastes fuel.

GM 4L60E transmissions fitted with a replacement servo from a Corvette shift harder and last longer, or if it's already slipping, the Corvette servo will cure the slippage and buy some time.

Softening the shifts, for the most part, compromises everything else for the sake of comfort. There are some conditions where a hard shift is bad (usually when it's a symptom of something being broken) but mostly the shock of a hard shift is better than the wear of a soft one.
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