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MP8 races only when parking brake is set

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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?

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  • 1 month later...
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 by MackTech
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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.

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