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Old 09-10-2017, 10:56 AM   #1
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What causes fast idle?

2000 Dynasty 36. ISC-350. Allison 3060

Under what "normal" conditions might the Cummins ECM call for an idle speed faster than normal? Unless I'm experiencing some "normal" condition that triggers fast idle, I have a problem.

I have owned this coach four years and this only recently began--Drive some distance and when I stop, the idle has gone from a normal of 600 RPM to speeds between 675 and 750 RPM. I could understand the ECM calling for a fast idle if it were needed for the radiator fan (side radiator, hydraulic fan), either to give additional engine cooling or to support the AC condenser. But in four years, I've been through all kinds of weather, including VERY hot, and this has never happened.

Coach is equipped with a dedicated "Fast Idle" rocker switch on the driver's side console that will adjust idle upward in about 50 RPM increments. It also has the capability to provide two faster idle speeds using the cruise control buttons on the SmartWheel.

When the fast idle happens, neither shutting off cruise control nor pulsing the dedicated fast idle rocker switch will decrease the idle. But momentarily switch off the engine, then immediately restart and idle is normal.

A couple of years ago, as routine maintenance, I unplugged/re-plugged the Cummins ECM on the side of the block. And I have recently done the same with both the speed sensors on the Allison. The electronic throttle control (accelerator pedal) was replaced just before I bought the coach.

Whatever is causing this fast idle condition is also affecting the transmission shifting pattern. It never fails to shift, but whenever the fast idle occurs, it begins shifting as if it lost Economy Mode--it downshifts prematurely.
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Old 09-10-2017, 11:19 AM   #2
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The throttle pedal MAY be the problem , with the proper shop tool a mechanic can look at the TPS ( Throttle Position Sensor ) signal to the ECM to see if it is a clean signal at idle .
Think of the TPS as the volume knob on a radio , if you turn the volume up and get a scratchy noise over the speaker you know the volume control is failing.
If the TPS is giving a scratchy ; varying signal ; to the ECM , then the ECM will set the idle speed where it thinks the TPS is calling for.
Because the ECM is still seeing a signal from the TPS , it won't trigger a CEL and related codes.
Not sure how your throttle pedal is configured , but I had issues with the hinge ( floor level in my case , in the carpet ) of the pedal rusting ( binding ) .
WD-40 on the hinge and silicone spray on the TPS roller on it's ramp, is part of my annual maintenance schedule, after shampooing the carpet.
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Old 09-10-2017, 11:29 AM   #3
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Quote:
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The throttle pedal MAY be the problem , with the proper shop tool a mechanic can look at the TPS ( Throttle Position Sensor ) signal to the ECM to see if it is a clean signal at idle .
Think of the TPS as the volume knob on a radio , if you turn the volume up and get a scratchy noise over the speaker you know the volume control is failing.
If the TPS is giving a scratchy ; varying signal ; to the ECM , then the ECM will set the idle speed where it thinks the TPS is calling for.
Because the ECM is still seeing a signal from the TPS , it won't trigger a CEL and related codes.
Not sure how your throttle pedal is configured , but I had issues with the hinge ( floor level in my case , in the carpet ) of the pedal rusting ( binding ) .
WD-40 on the hinge and silicone spray on the TPS roller on it's ramp, is part of my annual maintenance schedule, after shampooing the carpet.
Thanks, Skip. I've been casually looking into acquiring some of the Cummins software, but don't want to pay an annual fee. It's my understanding that older versions can be purchased that, if not supported with an annual fee, will still read trouble codes, but not allow parameter changes. I'm incline to think the problem is NOT the throttle control, since it was relatively recently changed.
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Old 09-10-2017, 02:19 PM   #4
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My friends coach kicks into high idle. He usually just kicks the pedal down and it stops. My coach doesnt do it, but I can get high idle with the cruise control.
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Old 09-10-2017, 02:22 PM   #5
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Our Cat 3126B will kick itself into high idle if I don't set it myself when I first start it up.
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Old 09-10-2017, 02:36 PM   #6
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DNK Diesel, but gas I would be looking for a vacuum line loose/ broken/ disconnected or a diaphram device operated by the vacuum to have a bad diaphram. 4-yo is potential for a rubber line in a hot area to be heated/ brittle and crack/ leak. Had one a few years ago, finally went into parts store buying several feet of each size and started replacing, until I found a 2-inch long line that was hidden from sight that was brittle/ cracked.
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Old 09-10-2017, 02:52 PM   #7
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Van,

I'm by no means a mechanic but relating what I've noticed with my Chevy duramax and other vehicles as well. Could it be a slight glitch in the computer program of the ECM? My Chevy would do crazy things (mover power seat forward, ding the instrument cluster continuously, move power mirrors around) until the dealer flashed the computer. It would only randomly do it, no rhyme or reason.

Just thinking maybe there's something that once a certain condition is met the ECM is thinking it needs to idle faster. As stated, I'm no mechanic just throwing this out as a possibility.

Chad
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Old 09-10-2017, 08:19 PM   #8
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I hope not!

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Van,

I'm by no means a mechanic but relating what I've noticed with my Chevy duramax and other vehicles as well. Could it be a slight glitch in the computer program of the ECM? My Chevy would do crazy things (mover power seat forward, ding the instrument cluster continuously, move power mirrors around) until the dealer flashed the computer. It would only randomly do it, no rhyme or reason.

Just thinking maybe there's something that once a certain condition is met the ECM is thinking it needs to idle faster. As stated, I'm no mechanic just throwing this out as a possibility.

Chad
Chad, I sure hope that is not the case, but it very well may be. I have never had to make a trip to a Cummins or Allison facility for ANYTHING, much less a control module problem, and I have always dreaded it. The one clue I cannot ignore is that the Allison DEFINITELY changes its shift pattern when this high-idle is occurring. I would almost take Skip's suggestion and buy a $100 accelerator module first.

I HATE to take this beast anywhere for any kind of repair. The only time I've used an outside mechanic is when my heater core needed replacing, and a young, small, wiry mechanic at the truck repair shop I frequent said he thought he could crawl in over the genny and replace the core without even disturbing the AC. He did it! $200 well spent to save this aging body from the agonizing contortions he was able to do easily.
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Old 09-11-2017, 04:55 PM   #9
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Also occurred to me that IF YOU HAVE HAD THE BATTERY DISCONNECTED, the ECM has to "relearn idle" and may take 10-15-minutes, and maybe even a drive-off.
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Old 09-12-2017, 07:54 AM   #10
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Also occurred to me that IF YOU HAVE HAD THE BATTERY DISCONNECTED, the ECM has to "relearn idle" and may take 10-15-minutes, and maybe even a drive-off.
Thanks, but battery has not been disconnected in months, and I've driven several thousand miles since then. Still lookin'...
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Old 09-12-2017, 06:50 PM   #11
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Do you have a silverleaf or ATI 1000? I'd see if the intake air temp is going super low, or maybe the coolant temp sensor is low causing the ecm to go into a fast idle mode. I'm not super familiar with how Cummins ecm programming works, but on modern cars if the coolant temp or trans Temps are too low, it won't engage lockup torque converter for example and will run a slightly higher idle and delay shifts to try to get the fluid Temps up.
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Old 09-12-2017, 07:57 PM   #12
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Do you have a silverleaf or ATI 1000? I'd see if the intake air temp is going super low, or maybe the coolant temp sensor is low causing the ecm to go into a fast idle mode. I'm not super familiar with how Cummins ecm programming works, but on modern cars if the coolant temp or trans Temps are too low, it won't engage lockup torque converter for example and will run a slightly higher idle and delay shifts to try to get the fluid Temps up.
Have the Silverleaf. Need to learn how to retrieve codes. Thanks!
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