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Topic: Cummins C8.3 Idle RPM Drop In Gear (Read 832 times) previous topic - next topic

Cummins C8.3 Idle RPM Drop In Gear

I posted awhile back about increased vibration when shifting from Neutral into Drive on the MD3060R, less so in reverse. 

Today while driving to dump my tanks, I noticed that my RPMs drop about 50-75 when shifting into Drive which is what is causing the increased vibration.  My idle in Neutral had been lowered to about 650 and would drop to right at 600 when put into gear.  Increasing the idle Neutral RPM up to the 700-725 RPM that the coach was at when I bought it, takes away the vibration.

So my question is, is this correct behavior on all mechanical 8.3s or is it something not working properly?  I never noticed it happening before.
Robert
Build # 5304
1998 34' U270 Cummins 6CTA8.3

Re: Cummins C8.3 Idle RPM Drop In Gear

Reply #1 –
Just bought mine 2 weeks ago and the idle in gear rattles the hell out of everything. A few RPM higher and it smooths out. I am going to raise it. No clue on exact RPM--my tach doesn't work :(
1998 U270 34'

Re: Cummins C8.3 Idle RPM Drop In Gear

Reply #2 –
Look up RPM specs as too high idle will overpower the service brakes on steep hills in first gear when the brakes are cold. High idle in gear with parking brake on will also cause the coach to roll if on much of a slope.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Cummins C8.3 Idle RPM Drop In Gear

Reply #3 –
If you set the idle too high you will not be able to shift out of neutral. Mark
Mark Freeman 1995 grand villa, Brimson Mn
8.3 Cummins  G95 4709 40 WTBS U280 WE21

Re: Cummins C8.3 Idle RPM Drop In Gear

Reply #4 –
Someone will certainly reply with 100% certainty, yet if I remember correctly, the old c8.3's like my 1997 model should have idle set at 700 +/- 50 and that above 800 RPM is when it will not shift out of neutral.
Jeff & Sandy
1997 American Eagle 40' Slide-less
2013 Toyota FJ Cruiser 4X4 Toad
Interested in Foretravel Coaches

Re: Cummins C8.3 Idle RPM Drop In Gear

Reply #5 –
I apologize if I may have worded my question strangely. 

I was trying to find out if everyone else had noticed that their idle RPMs dropped, and stayed down 50-75 RPMs, between Neutral and Drive?  I am trying to determine if this is normal behavior for the engine RPMs to lower when put under the load of the transmission.
Robert
Build # 5304
1998 34' U270 Cummins 6CTA8.3

Re: Cummins C8.3 Idle RPM Drop In Gear

Reply #6 –
Mine Drops, think it is normal.

Roland
1993 U280 4341
2010 Jeep Liberty
The Pied Pipers

Re: Cummins C8.3 Idle RPM Drop In Gear

Reply #7 –
When you go into drive, the torque converter sees the difference between the spinning crankshaft speed and the now stationary Allison input shaft so uses some energy working against the drag of the ATF so the RPM drops. Normal.

This power/energy robbing slippage continues while in the lower gears until the transmission goes into torque converter lock-up that some of our coaches have.

The heat generated by the slippage is also what is responsible for the necessity of a transmission cooler. So, the end result is a major malfunction and resultant cost that Roland had to endure.

Just another reason why I don't care for automatic transmissions. Modern manual bus transmissions are no harder to drive than a stick shift BMW or Mercedes.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Cummins C8.3 Idle RPM Drop In Gear

Reply #8 –
Quote from: Pierce & Gaylie Stewart –
Just another reason why I don't care for automatic transmissions. Modern manual bus transmissions are no harder to drive than a stick shift BMW or Mercedes.

Modern truck and bus transmissions that are not the standard "automatic" style are autoshift manuals...some with auto clutches as well. It's a manual style transmission with automatic shifting/rpm matching and sometimes even automatic clutches. It's good for highway driving. In reality, the allison automatic is pretty dang good.

Many car makers are now saying their cars with automatic transmissions are getting better fuel mileage than manuals! I guess due to more gears, computers, etc.?!?
1998 U270 34'

Re: Cummins C8.3 Idle RPM Drop In Gear

Reply #9 –
Quote from: Pierce & Gaylie Stewart –

Just another reason why I don't care for automatic transmissions. Modern manual bus transmissions are no harder to drive than a stick shift BMW or Mercedes.

Pierce
Lots of "Big Trucks" are coming out with automatics now, as well as those big single tires in lieu of dualies, at least the ones I saw in Hagerstown.

Roland
1993 U280 4341
2010 Jeep Liberty
The Pied Pipers
 

Re: Cummins C8.3 Idle RPM Drop In Gear

Reply #10 –
 ;D i know a guy who can do anything mechanical better than most, BUT given a clutch, gear shift and throttle, completely gets confused, totally lost as in not a clue.  Makes for laughter.
So the auto is a must.
Tried to teach him the 10 speed RTO910, not good, found just get him into high gear and you could go to sleep (with the 12V-71) did not need to change gears down to 30 mph. :o