Rpm's at 2k even when coasting in neutral - Chevy and GMC Duramax Diesel Forum
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Old 11-09-2009, 10:54 PM   eBay Motors  #1 (permalink)
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Rpm's at 2k even when coasting in neutral

Can some one tell me if it’s normal for my truck to still be at 2k rpm's when I'm on the highway and coasting downhill or coasting in neutral? Why wouldn't the rpm's drop when I shift into neutral. Also when I let off the throttle when cruising at any highway speed, the truck starts slowing down immediately, I'm new to diesel trucks and something doesn’t seem right with that!!! Shouldn’t it “freewheel” when I let off the throttle?
And no I’m not in the tow/haul mode when this happens.
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Old 11-10-2009, 12:05 AM   eBay Motors  #2 (permalink)
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Its the wonderfull allison! It may be dumb now but throw 10,000 behind it and start going down a hill. You wont even have to touch the brakes!

As for the 2000 RPM when in neutral. Im not sure about that. What year is your truck?
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Old 11-10-2009, 12:10 AM   eBay Motors  #3 (permalink)
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Im assuming he has an LLY since he posted in the LLY section but Im not sure Jason. Yeah the Allison is wonderful when it comes to slowing down I personally like it less ware and tear on my breaks.
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Old 11-10-2009, 12:35 AM   eBay Motors  #4 (permalink)
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The allison is a smart transmission..it wont shift into reverse or neutral going down the highway. The reason the rpms stayed there is because the torque converter is locked. The allison is electronically shifted.
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Old 11-10-2009, 07:45 AM   eBay Motors  #5 (permalink)
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Quote:
Originally Posted by ob_1jr View Post
The allison is a smart transmission..it wont shift into reverse or neutral going down the highway. The reason the rpms stayed there is because the torque converter is locked. The allison is electronically shifted.
Thanks for the replies! It is a 2004.5 LLY/5spd Allison CCLB SRW.
I thought it might have something to do with the torque converter, but I was thinking there might be a problem with it.
So that is considered normal and nothing to be concerned about?

Would probably get a lot better mpg's if it could coast a little when you back off the throttle instead of slowing down so quickly. I wonder if there's a tune for that???
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Old 11-10-2009, 08:02 AM   eBay Motors  #6 (permalink)
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no not really. Your truck will coast a little better once u open up the exhaust. You dont save that much MPG by shifting into neutral going down hills. The allison is just that way. It will downshift if you are going down a hill and tap the brakes....it does it the most when you put it into tow/haul.
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Old 11-10-2009, 08:15 AM   eBay Motors  #7 (permalink)
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Quote:
Originally Posted by ob_1jr View Post
no not really. Your truck will coast a little better once u open up the exhaust. You dont save that much MPG by shifting into neutral going down hills. The allison is just that way. It will downshift if you are going down a hill and tap the brakes....it does it the most when you put it into tow/haul.
Thanks, but just for the record, I don't normally shift into neutral to save on fuel, I was just experimenting with my new truck and was curious if the rpm's would drop if I put it in neutral. The last few highway runs I made I noticed the rpm's were always right around 2k and didn't drop when I let off the throttle or was coasting down hill like my gassers do!
And that didn't seem normal to me!
I'm still getting used to all the differences between this truck and my 2003 2500HD 6.0!!!
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Old 11-10-2009, 11:51 AM   eBay Motors  #8 (permalink)
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Well, even with a gasser if the TC is locked the rpms will stay the same. the allison is a different animal. If you bought a diesel just because you heard the stories of the outstanding fuel mileage...you bought it for the wrong reason.....but if you bought it to drive it and to tow..that is the reason. The LLY gets a little worse mileage than the LB7 that i have. It has emissions crap that if you got rid of would gain a little back. You are on the right track with your mods. Talk with the person you buy EFI Live from and they can help you out greatly.
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Old 11-10-2009, 11:56 AM   eBay Motors  #9 (permalink)
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mine does it too totally normal...its so when you decide to go from neutral back into gear the rev are matched....it does this till you go below like 40ish mph....do it sometime and watch closely the rpm will actually drop with speed but still be in neutral
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Old 11-10-2009, 12:24 PM   eBay Motors  #10 (permalink)
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Speaking of mpg's, the best I have gotten so far was 18.2 and that is by topping off the tank before and after, the readout said I was getting 20.5mpg's! And I was trying to get the best mpg's keeping it around 68 mph, I normally drive a little faster than that on the highway!

I'm hoping that with the exhaust and tuner I'll be able to get much better mileage, especially in town which has averaged 11-12 mpg and that sucks, I was expecting better than that!
So what do you think I'll end up getting after the above mentioned mods???

Last edited by BigBlueGMC; 11-10-2009 at 12:26 PM.
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