Rockwell Posted October 2, 2019 Share Posted October 2, 2019 2011 CXU613, MP8 630K miles. Hey all, So here`s a weird one... This problem went away as mysteriously as it came. Yesterday, every time I set the tractor parking brake, (Yellow Air Valve on dash) the motor would race up to 1200 rpm, drop to about 1000, rev back up, then drop to idle and idle normally. Every single time I set the tractor parking brake. Other than that, truck performance was fine with all gauges reading normal, no MILS or codes at all. The problem disappeared towards the end of day and it hasn't done it again, including all day today. Any ideas? Quote Link to comment https://www.bigmacktrucks.com/topic/59247-mp8-races-only-when-parking-brake-is-set/ Share on other sites More sharing options...
MackTech Posted November 16, 2019 Share Posted November 16, 2019 Is there rollover stability on the truck? Quote Link to comment https://www.bigmacktrucks.com/topic/59247-mp8-races-only-when-parking-brake-is-set/#findComment-436039 Share on other sites More sharing options...
white_HULK Posted November 17, 2019 Share Posted November 17, 2019 @MackTech how does rollover affect the idle? Mine did that this evening when parking the truck and idling. Mines a 2013 pinnacle. Quote Link to comment https://www.bigmacktrucks.com/topic/59247-mp8-races-only-when-parking-brake-is-set/#findComment-436066 Share on other sites More sharing options...
MackTech Posted November 17, 2019 Share Posted November 17, 2019 (edited) 11 hours ago, white_HULK said: @MackTech how does rollover affect the idle? Mine did that this evening when parking the truck and idling. Mines a 2013 pinnacle. The computers in the truck are tied into acceleration when there is stability in the vehicle. Edited November 17, 2019 by MackTech Quote Link to comment https://www.bigmacktrucks.com/topic/59247-mp8-races-only-when-parking-brake-is-set/#findComment-436072 Share on other sites More sharing options...
white_HULK Posted November 17, 2019 Share Posted November 17, 2019 8 hours ago, MackTech said: The computers in the truck are tired into acceleration when there is stability in the vehicle. So is it a bad thing? Is it a problem? Something I need to have fixed or checked out? Quote Link to comment https://www.bigmacktrucks.com/topic/59247-mp8-races-only-when-parking-brake-is-set/#findComment-436082 Share on other sites More sharing options...
MackTech Posted November 17, 2019 Share Posted November 17, 2019 If you have rollover stability in the truck I would suggest you have it checked out. Quote Link to comment https://www.bigmacktrucks.com/topic/59247-mp8-races-only-when-parking-brake-is-set/#findComment-436099 Share on other sites More sharing options...
Rockwell Posted November 23, 2019 Author Share Posted November 23, 2019 MackTech, thank you for the response. I have to assume the truck does have rollover stability. Another symptom I've noticed when the idle races after setting parking brake. My exhaust temp guage Instantly pins itself far right and my HEST light comes on, as if exhaust temp instantly went thru the roof. If I release the parking brake and drive 10 feet, HEST light goes out and exhaust temp guage returns to normal instantly. The high exhaust temp reading is false, its only high electronically, not in reality. i did have the truck do this high idle with brake to me about 2 years ago. Mack installed new batteries while it was at the dealer shop. When i got it back, I got this symptom. I noticed they had hooked the ACM power wire to a different positive battery terminal than where it was with the old batteries. I put the ACM wire back to the original battery terminal and the problem went away. Now I know logically this dont make sense, and perhaps the resolution was just coincidental and had nothing at all to do with ACM wire. Two years later, the truck starts racing when I apply the parking brake. Some days it will do it all day, everytime. Other times it will disappear for days. Could it possibly be a weak battery causing this? Maybe a slight drop in voltage playing games with the computer? Whatever it is, it does not and has not affected operation of truck. Quote Link to comment https://www.bigmacktrucks.com/topic/59247-mp8-races-only-when-parking-brake-is-set/#findComment-436483 Share on other sites More sharing options...
MackTech Posted November 27, 2019 Share Posted November 27, 2019 If it's an electrical issue you sure have a lot of hunting to do, I'd check all your grounds, both chassis and engine grounds as well. Make sure there is no corrosion and you have good contact with the terminals. Quote Link to comment https://www.bigmacktrucks.com/topic/59247-mp8-races-only-when-parking-brake-is-set/#findComment-436704 Share on other sites More sharing options...
JoeH Posted November 27, 2019 Share Posted November 27, 2019 Freightliner with a CAT I drive for winter work revs up after a minute to around 1k for a minute during the warm up cycle. Only does it when started cold, maybe your truck thinks it's cold started every time you park it? Quote Link to comment https://www.bigmacktrucks.com/topic/59247-mp8-races-only-when-parking-brake-is-set/#findComment-436707 Share on other sites More sharing options...
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