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I have a Thomas 1991 school bus (36K gvwr) with a CAT 3116 engine and an Allison MT 643 transmission. I purchased it quite recently and when driving it on the highway, the engine RPMs are at 4000 driving at 55mph.

I don't believe the transmission has an overdrive gear, but the rpms seem like a lot to a newbie like me. In fact, 4000 rpm is the furthest possible point on the tachometer.

  1. is this harmful to the engine?
  2. is this normal?
Joe B
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1 Answers1

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I had the mechanic look into it and he was able to determine that the rpm sensor on the engine was bad. He replaced that and a grounding wire and now the RPMs read perfectly.

I also found out from the mechanic that the max rpms for this engine (CAT 3116) are 2800 RPM.

Joe B
  • 195
  • 2
  • 11