Andre Wright Posted September 16, 2021 Share Posted September 16, 2021 When I'm at a stop light and take off when the truck reaches 7th gear sometimes it will not accelerate so I would let off the gas pedal then give it gas then I would hear a thunk noise and it will take off does anyone know what the problem could be? It only happens towards the end of the day. Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/ Share on other sites More sharing options...
kt_Engineer Posted September 17, 2021 Share Posted September 17, 2021 What transmission do you have? If it is 12-speed MDrive, something seems to be confusing TECU where it is trying to go to low range (from high range) or could not go to Hi-Split in main box. Maybe splitter synchronizer is going bad. Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480051 Share on other sites More sharing options...
Andre Wright Posted September 24, 2021 Author Share Posted September 24, 2021 Yes its a 12 speed automatic the mack dealer says its the transmission wiring harness but I didn't have any lights on my dash? Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480378 Share on other sites More sharing options...
Joey Mack Posted September 25, 2021 Share Posted September 25, 2021 the wire harness is a small one, and it runs over the left frame rail under the cab. I have replaced these due to them chaffing where it goes over the frame edge. its costs under $150.00 3 years ago when I did the last one... jojo Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480418 Share on other sites More sharing options...
Andre Wright Posted September 27, 2021 Author Share Posted September 27, 2021 So that would cause the transmission to slip? Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480543 Share on other sites More sharing options...
Hayseed Posted September 27, 2021 Share Posted September 27, 2021 kt_Engineer has the best Theory so far, In my Opinion. 1 Quote "Be who you are and say what you feel...Because those that matter...don't mind...And those that mind....don't matter." - Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480550 Share on other sites More sharing options...
fjh Posted September 27, 2021 Share Posted September 27, 2021 On 9/16/2021 at 6:46 PM, kt_Engineer said: What transmission do you have? If it is 12-speed MDrive, something seems to be confusing TECU where it is trying to go to low range (from high range) or could not go to Hi-Split in main box. Maybe splitter synchronizer is going bad. I agree it sounds like the synchronizer this a weak point on the Auto a fairly common Failure specially I its left with the second speed programing in reverse ! This is a theory however ! But it Seems to knock the sycro cone loose from the hub due to its design it seems to happen more frequently on those with the two speed reverse program left enabled! Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480559 Share on other sites More sharing options...
Andre Wright Posted September 28, 2021 Author Share Posted September 28, 2021 Would the transmission wiring harness cause the transmission to slip? Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480603 Share on other sites More sharing options...
fjh Posted September 28, 2021 Share Posted September 28, 2021 Highly doubt it! Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480608 Share on other sites More sharing options...
Andre Wright Posted October 2, 2021 Author Share Posted October 2, 2021 Update got my truck back yesterday and its fixed the Mack dealer was right. So apparently a bad transmission wiring harness will cause your transmission to slip Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480770 Share on other sites More sharing options...
fjh Posted October 3, 2021 Share Posted October 3, 2021 19 hours ago, Andre Wright said: Update got my truck back yesterday and its fixed the Mack dealer was right. So apparently a bad transmission wiring harness will cause your transmission to slip Good to know thanx for posting your fix! Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480793 Share on other sites More sharing options...
Joey Mack Posted October 7, 2021 Share Posted October 7, 2021 good deal, which harness? just curious, in case someone ask's again. Jojo Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-480964 Share on other sites More sharing options...
Andre Wright Posted November 1, 2021 Author Share Posted November 1, 2021 Don't know they just told me its the transmission wiring harness Quote Link to comment https://www.bigmacktrucks.com/topic/68160-transmission/#findComment-482235 Share on other sites More sharing options...
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