Rev Counter Reading

[Beagler]

It has always seemed high to me so I thought I would check it.

Is the rev counter faulty or something  else. 
 

[Jonathan Kay]

That's interesting.

Where are you taking the signal to the Timorn instrument?

What setting do you have the on the Timorn instrument? That might be "Number of cylinders".

Does your Seven have four cylinders and does it have a wasted spark ignition system?

And which ECU?

And does that ratio between the two instruments hold across the rev range?

Jonathan

[Beagler]

Thanks Jonathan I will try and answer some of your questions.

The car is a four cyl sigma tuned fro 270s to 310s and ecu upgraded but the cat was bought from Catterham Gatwick and was their car. It had always read high from purchasing it and its obvious it doesnt Idle at 1150 rpm as indicated. Queried it with Caterham and they said it was reading a bit high but I didnt persue then.

The Timon is reading correct and the induction wire is taken from  the plug lead. 

[Beagler]

Think I'll just order a new Rev counter and see if that helps.

[Jonathan Kay]

I wouldn't do that yet.

Could you do some more comparisons at different engine speeds?

Jonathan

PS: It's often possible to repair instruments.

[BenF]

Is there a setting for the rev counter which needs to be changed depending on the engine being used?  i.e. are there dip switches on the back of the instrument which have been set wrongly?

[Jonathan Kay]

Could be... I couldn't find a neat ratio from the displayed example. I might at higher engine speed... 

Jonathan

[Iain McCallum]

What does Easimap say the revs are? That is the 'normal' way we compare readings.

[StevehS3]

I think that is normal for that engine - I don't think yours is faulty. Mine is the same! Reads 1200RPM at 950. I have been thinking about speaking to Caerbont about it. 

[StevehS3]

#8 Easimap shows the correct RPM (950 in my case whilst the tacho shows 1200).

[Richard Price]

What does it matter if the tacho over reads a little?

My (2002) tacho seems to over read by around 400rpm throughout the Rev range. (When I compare what it says compared to what my data logger and shift lights suggest at high rpm, and online to my DTA ECU at idle)

but why does it matter if the numbers are not that accurate? Surely it's there to give an idea rather than accurate numbers.

I seriously doubt a new Caterham tacho will be any different.

[John Vine]

Before you consider a replacement (at £250+ ?), I would suggest that you record readings (gauge vs measuring instrument) for several speeds (say, 2K, 3K, 4K, 5K), plot them on a graph, and send it to Caerbont Technical Support (technical@caigauge.com), along with the Caerbont and CC part numbers from the back of the gauge.  If you explain what your problem is, I'm sure they'll advise on the best course of action.

JV

[Beagler]

Thanks to for all for input, think I will take advice and live with it, have other spend priorities. 

[GulfSeven]

I remember similar when Williams had my new 310R plugged into EasyMap to reset the TPS. The rev counter read much higher. It's not at all accurate.

[SLR No.77]

This is the very reason it's impossible to work out what diff is fitted from the speed and engine rpm!

[aerobod - near CYYC]

It is also worth verifying the dip switches are correctly set, although if it is a constant offset, they probably are. Here is the Caerbont dip switch instruction sheet for recent tachos: https://www.caigauge.com/documents//product-instructions/tachometers/pil055-iss2-279.pdf

[StevehS3]

I wondered if the tacho was CAN bus or pulse. I have sent a note to Caerbont and will update on here when I hear back. 

[StevehS3]

Reply from Caerbont:

"Is it the same model of ECU as originally fitted as this gauge and software goes back to 2006 and has not been changed.

The calibration is fixed so cannot be changed. It is set to read 4000 rpm at 33.66Hz"

I will do a calibration check at other points. 

[SLR No.77]

33.66Hz is 2019.6 pulses per minute, which would relate to 4039.2 rpm (4 stroke engine) so 4000 would be fairly close, albeit slightly under reading.  
Surely this would suggest that a rev counter reading high is therefore out of calibration?

[StevehS3]

#19 Well worked out! I will check another couple of RPM points to understand a bit more. 

[John Vine]

That's all very interesting. Thanks for taking the trouble to ask Caerbont.

For info, can you tell us the Caerbont P/N?  (It's obscured in your photo, and 30D147A no longer appears on CC Parts.)

JV

[John Vine]

#3 ...its obvious it doesnt Idle at 1150 rpm as indicated.

In fact, that reading is more like 1300, which is even more inaccurate.

If indeed the calibration is fixed (as stated in #18), it looks as though your only option is to add a little (paper?) overlay around the perimeter of the gauge to show actual rpm -- apart from just living with it, of course!

Question for the electronics gurus: can the pulse frequency be modified in any way -- with a special circuit, fo example?

JV

[Richard Price]

Reply from Caerbont:

"Is it the same model of ECU as originally fitted as this gauge and software goes back to 2006 and has not been changed.

The calibration is fixed so cannot be changed. It is set to read 4000 rpm at 33.66Hz"

That's interesting....

For reference, that is different to earlier cars. My 2002 K series cars tacho is (roughly) calibrated to work with 2 pulses per rev (from the MEMS ECU), so 4000rpm is 133.33Hz. 

[Richard Price]

Question for the electronics gurus: can the pulse frequency be modified in any way -- with a special circuit, fo example?

JV

John. Yes it could, and using pretty cheap hardware. However, you'd then need to program it. My home made shift lights are based around a programmable Arduino board (about a fiver from china). My shift light code measures the frequency of the tacho signal and illuminates LEDs at preset values. The Arduino could be programmed to drive an output from a programmable signal generator having applied whatever factor you wish to apply deliver a different frequency to the input frequency. 

However, in the OPs application, why would you bother......

[John Vine]

#24:  Thanks, Richard.

However, in the OPs application, why would you bother......

Well, yes.  But some folks like their instruments to be super accurate if at all possible.  For example, I've been known to recalibrate my speedo to within tenths of a % just because I can.

JV