CAT 3516 C - CRPM increase from 1600 up to 1670

Dear all Engineers

My Main Engine is Cat 3516 C and CRPM is 1600.
But the CRPM increase gradually up to 1670 RPM.
That CRPM used for Shaft Generator for Bow Thruster power.
What is the cause that CRPM increase ?
Hopefully there is somebody knows about this matter or have ever got this kind of matter.
Need your information.

I guess my initial thought would be an issue with the droop setting/function on your governor. But there’s a couple questions.

  • I’m assuming the C in CRPM is Constant? (I’ve not seen it written that way before.)

  • Does the speed increase occur only after putting the shaft generator online?

  • Does the speed increase to 1670 and remain constant at the speed from then on? Or does it change further or reduce when the thruster power is increased/decreased?

  • What type of governor? Is there a mechanical engine mounted gov or is there also an electronic primary gov? And what brand (Woodward?)


Check the wiring and connections on your mag pickup. Possibly change it.


  • C in CRPM = Constant

  • The RPM increase occur only after putting Shaft Generator on line for Bow Thruster

  • Normal constan RPM is 1600, once we put in Constant RPM command direct to 1638, then increasing gradually in 5 hours become 1670.
    I observed when the Bow Thruster take the load, RPM increase and sometime decrease.

  • The type of the governor is electric type from Caterpillar.

I suspect this matter from electric speed control.

Anyway , thank’s a lot for your information.

If you still have information about this matter, it will be appreciated if inform me back

You may be heading in the right direction. I know you said its “electric type from Cat” but its still likely either actually made by Woodward or Heinzman. I didn’t think Cat made their own electronic gov. But either way, you should be able to put a meter on the speed setting transmitter input to the elec gov to check (depending on type) the mV or mA signal to the governor. See if this changes before/after putting the shaft generator online, and again after the couple hours.

Most generators I’ve worked with will default to a slightly higher RPM either when taken in local manual control, or when the electronic governor has failed in some way. This was intentional so that the generator would be running slightly over 60hz and ready to take load. It was also a good indicator of a problem with the elec gov.

Something else to consider is the fuel rack actuator on the engine. Again dependent on the type, but if it is in anyway hydraulic or reliant on oil, this is a component that should be inspected.

As @MickAK mentioned you should check any mag pickups you have. Usually when there is an issue with a dirty/greasy pickup or damaged tooth it results in fluctuations not slow steady speed change. However if this is the only source of your speed input you should verify it anyway as it is easy to check.


If you can determine which pot controls your constant RPM setting I would check that first. Sometimes pots can get a little dead spot in them and change their signal very slightly, enough to cause a steady slight decrease or increase. This might explain the variance under load.

Best way to check this is get a resistance measurement of the pot then attach a new pot at the terminals set to your reading and see if you can hold steady there.

Dirty and chipped tooth mag pickups will cause varying signals but I’ve seen bad wiring connections cause a steady drop or increase.

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Main engine that runs at constant speed? Is it CPP?

I agree with shipengr and you that speed control could be an issue. I don’t know about these CAT governors but definitely eliminate the speed control as a problem if possible. I had a similar situation where it ended up being some tweaked rack components.

You should be able to monitor the signal to the stepper motor or whatever controls the speed inside the governor. Usually it’s a 4-20 mA signal as shipengr stated previously. If the mA signal is creeping up in proportion to the speed that’s a dead giveaway. You can also test the MPU but it doesn’t sound like thats the problem.

On SOME hydraulic governors you can actually bring the engine up to speed, put it online and then disconnect the the plug and it will run fine. Obviously make sure you’re in a safe area.

Check the number of teeth on the flywheel. Then read the makers manual on entering data into the computer for speed control. One vessel I was on had this make and model of engine as generators. We found that someone or something had entered the wrong data. I think it had 70 teeth but only 60 on the computer. The manual will tell you how to input data. It worked for us. So often the solution is simple but getting there not so easy. Hope this helps. Good luck.

This question is from a year ago.