Rockwell Posted May 24, 2018 Share Posted May 24, 2018 This is a really weird one... 2011 Mack CXU613, 445 MP8, 580K miles. Whenever I set the Parking brake (yellow knob), my idle shoots way up. Set Brake, motor will idle at 650 RPM for about 5 seconds, then you hear the turbo begin to spool up and the motor revs up to 1200 rpm, as soon as it gets to about 1200rpm, you hear a slight and rapid thumping noise (almost as if the air compressor is kickin on?). At this point the motor will then fall back to 700 and idle normal. It will not do it again unless you release the parking brake and reapply. This ONLY happens when you set the yellow parking brake. Does not happen if I use trailer valve (red knob) to park. Does not happen if i drop to idle at a stop. Only when the yellow parking brake is engaged. This condition just suddenly started out of nowhere, nothing leading up to it. Other than that, everything else about the truck is aces. Starts, runs, pulls great! All temp/pressure gauges are normal. Any ideas anyone? Thank you Quote Link to comment https://www.bigmacktrucks.com/topic/53412-erratic-idle-only-with-parking-brake-set/ Share on other sites More sharing options...
fjh Posted May 25, 2018 Share Posted May 25, 2018 How cold is your climate? Quote Link to comment https://www.bigmacktrucks.com/topic/53412-erratic-idle-only-with-parking-brake-set/#findComment-399415 Share on other sites More sharing options...
fjh Posted May 25, 2018 Share Posted May 25, 2018 (edited) Periodic heat ? Mode? We have several customer trucks that do this our climate is cool at times only once and a blu moon and have never nailed it down to one thing other than something in the programming telling the truck the incoming air needs to be heated for regening we were told by tech support that that this was periodic heat mode! could be the same instance in your case. Edited May 25, 2018 by fjh Quote Link to comment https://www.bigmacktrucks.com/topic/53412-erratic-idle-only-with-parking-brake-set/#findComment-399416 Share on other sites More sharing options...
Rockwell Posted May 25, 2018 Author Share Posted May 25, 2018 8 hours ago, fjh said: Periodic heat ? Mode? We have several customer trucks that do this our climate is cool at times only once and a blu moon and have never nailed it down to one thing other than something in the programming telling the truck the incoming air needs to be heated for regening we were told by tech support that that this was periodic heat mode! could be the same instance in your case. Thanks for that feedback. Climate is about 75F. Truck has never done this before, now it does it every time I apply the parking brake and ONLY when I apply the parking brake. As I said, truck runs absolutely perfect and has no active codes. Quote Link to comment https://www.bigmacktrucks.com/topic/53412-erratic-idle-only-with-parking-brake-set/#findComment-399418 Share on other sites More sharing options...
fjh Posted May 25, 2018 Share Posted May 25, 2018 (edited) Maybe have the truck looked at have them check ambient air temp sensor! and inlet air temp could be getting a false reading from one or more sensors! also make sure the boot is sealing on the hood to the air cleaner! Edited May 25, 2018 by fjh Quote Link to comment https://www.bigmacktrucks.com/topic/53412-erratic-idle-only-with-parking-brake-set/#findComment-399420 Share on other sites More sharing options...
Rockwell Posted May 27, 2018 Author Share Posted May 27, 2018 Many thanks for your responses and suggestions fjh. When you say ambient air temp sensor, is this the same sensor that tells the dash display the outside air temp? I know that sensor is malfunctioning because sometimes i will get lightning bolt and my outside temp will disappear and just two lines appear - -. Lightning bolt goes out and temp reappears. Is this the same sensor the ECU relies on? If so, is the location in the drivers side mirror housing as I've heard? I still don't understand the correlation between this erratic idle and the parking brake, but there is obviously a connection. Quote Link to comment https://www.bigmacktrucks.com/topic/53412-erratic-idle-only-with-parking-brake-set/#findComment-399551 Share on other sites More sharing options...
fjh Posted May 28, 2018 Share Posted May 28, 2018 YES I believe the sensor is mounted on the lower rad just behind the bumper I am not positive why with park brake on either we have trucks that do it at a stop light foot on the brake . jf may be its tied in with the brake lights switch if this is the case this would be the answer to that question! Quote Link to comment https://www.bigmacktrucks.com/topic/53412-erratic-idle-only-with-parking-brake-set/#findComment-399599 Share on other sites More sharing options...
Rockwell Posted June 10, 2018 Author Share Posted June 10, 2018 Just wanted to follow up on this post with what the remedy turned out to be for me. Before this happened I had to replace battery cables. When hooking up the positive cables, I hooked the Positive starter cable and the ACM positive cable to the same battery stud. (prior to this they were each on a different positive battery stud). As a long shot, I decided to put the cables back exactly the way they were and POOF, problem gone. How this has any effect whatsoever i have no clue since all the batteries are connected inline to one another. All I know is my problem is gone. Maybe this will help another down the line. Thank you again fjh for all your feedback! Quote Link to comment https://www.bigmacktrucks.com/topic/53412-erratic-idle-only-with-parking-brake-set/#findComment-400349 Share on other sites More sharing options...
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