Transmission on my m939 series ..issues :((

Chitangi

Member
71
2
8
Location
Athens,Ga
Hey guys !
Need some input here ,I have had some issues with my truck,each time I drive it,it’s seems the Truck takes too long to engage in third gear and so the first thought was charging the filter and fluid..did that nothing or little change..second I adjusted the throttle and transmission cable to bring them in sync per video I saw on YouTube..little help ..ok then I changed the fluid to 10wt and it seems to be taking even longer to shift Into third ....I have noticed that once it runs for a while the drivability is much improved but still have concern and I’m honestly dreading the thought that my trans could be failing..Also sometimes it’s just seem to be dragging a lot taking time not much power coming out especially in second ..whiles this g to decide when to shift in third..hopefully it’s notbing to wild...every piece of advice will be appreciated thanks guys !
 

Floridianson

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Interlachen Fl.
Has the truck always shifted this way or just started. If I remember you tranmission is governer controled and lock up comes in right after second gear. Some mistake this for a gear shift. If you don't push the peddal to the floor on shifts then it should be shifting around 14/15 maybe 1600rpm. If you don't hit that rpm range the transmission will not shift. If you push the peddal to the floor all shifts should be at the governor or close to 2100rpm. Warm her up good and take it for a test run and start from a stop and peddal to the floor and see where the shift points are. Then half pedal and record shift points. A flat road is better.
 
Last edited:

sandcobra164

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I'm curious about the rpm indicated when it shifts as well. You stated that once it warms up it shifts better. My NHC-250 was getting rather weak on cold acceleration when the injection pump was going bad causing a delay in shifting. It would still start and idle perfectly. Once warmed up it would run fine as good as any other truck. I've never done this test but a quick was to judge the health of the injection pump is to check rail pressure with a 250 psi gauge T'd into the line coming from the top of the injection pump. You should have between 150 to 180 psi under full throttle depending on fuel setting and health of pump or the rest of the fuel system feeding it. How well does the truck drive once in 5th and pulling an incline?
 

Chitangi

Member
71
2
8
Location
Athens,Ga
I was able to check the governor and the filter which had a little pieces of rubber assuming its seals from somewhere..the transmission had some slug once I took the bottom cover and cleaned very well.I ran it today move a little better and stalled at times in second and almost came to a craw.I don’t know what could possibly be going on,I’m thinking maybe the second gear is not getting the right amount of pressure to engage or move to third..will keep brainstorming and will post pictures once I decide to dig in ��..but seriously every contribution brings me closer to have this thing resolved.
 
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