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Been driving an 04 RD690S for a while now and have an issue. Truck has approximately 273K miles on E 7 385hp engine. The problem is that when you are building speed either loaded or empty at around 13 to 14 rpms the engine decelerates and it takes forever to get to 1600, then it gains power and goes on. It only does this in top 3 gears(T308 tranny) but it also does this loaded on hard pulls, pulls down to 1600 then looses power sometimes down to 1300 rpm. Local Mack dealer hasn't a clue as to the cause, hoping someone out there has had the same issue or an idea as to what will fix it. Thought it might have been accelerator pedal and change that helped somewhat but problem came back. Last week had to replace fuel pump with no luck. Any body got any ideas.

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Disconnect the torque limit switch on the trans shift cyl at the rear of the trans just above the out put yoke?

just something to try!

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might be turbo boost sensor,but that should give you a lightning bolt on the dash

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