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360 rev counter reading approx half actual rpm


rich_sutcliffe

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Hi all,

I have just finished my 360R build, but when the engine was started for the first time, the rev counter was reading roughly half of what it should be reading across the rev range. Does anyone have any bright ideas please?

My IVA is booked for Tuesday. There isn't time to take it to Caterham for the PBC as the final missing parts from the kit only arrived a few days ago. Eric at Millwood is in the process of checking the build for me, but if it ends up being a warranty job it will need to go to Caterham. Do you think it will pass the IVA?

Many thanks,

Rich

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Let's start with the easy bits.

  • Are there any settings switches on the back of the rev counter, and if so how are they set?
  • Check the instrument earthing point.
  • Check the engine and ECU earths.
  • Wiggle the ECU wires while it's running and see if that affects the rev counter display.

Jonathan

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Thank you, will try these things tomorrow.

I have a multimeter and I know a bit about electronics, but I don't have a circuit diagram.

The engine is running smoothly. The rev counter needle sweeps correctly from min to max and back when the ignition is switched on. All other electrics seem ok. The rev counter doesn't bounce - it just always reads about half of the expected value, which is why I hoped it might be a setting / mapping problem and therefore a quick fix.

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Thinking about the dip switches a bit more, if the rev counter was configured for 1 Pulse Per Revolution, but was actually receiving 2 PPR from the engine, then wouldn't it read double the correct value? So in my case maybe it is set to 4 PPR (dip switches 10100001) and it is only receiving half the number of pulses that it is expecting. Will find out tomorrow!

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Have seen this before with a US 360 kit build.  Dip switches where incorrectly set.  Might be able to get a picture of my dip switches at the weekend.  Also had a faulty tach on my 420 build, but that was because one of the warning lights didn't work. CC sent replacement via post.

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There isn't any sensor for the revs, Rich. It is just a feed from the ECU on Pin 34 (white wire with black stripe going to Pin 8 on the tach, if Caterham hasn't changed the wiring lately). A number of things that may cause the issue:

- change in MBE ECU programming, only fixable by Caterham with a new image

- poorly crimped ground wire or loose ground terminal post either for the tach or ECU

- need for a pull-up resistor or a faulty pull-up resistor between the ECU feed and 12V on the tach connection (this would only be needed if either an ECU or tach change in the signal requirements have been made by either Caerbont or MBE).

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Problem fixed - the ECU wasn't sending enough pulses to the rev counter. Not quite sure why Caterham couldn't configure it correctly in the first place - they knew what combination of engine, ECU and instruments they were supplying. Yet more QC issues...

In other news my IVA was canceled yesterday as the examiner has COVID :-(

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interested in this thread as since rebuild i have the same issue. Nothing has changed in the ECU so it may be poor earth or resistor or switch settings. Does the tach need to be removed to check the switch settings? or do i have to invert myself with a torch?

 

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Re #14 Rich, it sounds as though Caterham has been changing some of the ECU characteristics. The MBE ECU has a setting under "Tacho pulses per cycle" for both the number of pulses (set to 4 on my non-Caterham ECU) and "Tacho duty cycle" set to 75% on my ECU, indicating the time the pulse stays "high" (+12V).

On a locked ECU you can confirm the settings using an oscilloscope on the signal line while the engine is running and Easimap is displaying the revs to be matched to the pulses.

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