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Helimadness 09-15-2019 10:55 PM

4TNV98 RPM meter take off
Hi All.
Not sure if got the right area.
Got a digger with a Yanmar 4TNV98 Diesel engine in it. The RPM meter is not working and want to try and get it going. Not sure where to start looking. Anyone got any ideas on where they might be getting the reading from on the motor. Cant seem to see anything obvious.

Kubota Ronin 09-16-2019 04:33 AM

Re: 4TNV98 RPM meter take off
Diesel tachs are often cable-driven. Any sign of something like that running from the gauge to the engine area?

Helimadness 09-16-2019 05:01 AM

Re: 4TNV98 RPM meter take off

Originally Posted by Kubota Ronin (Post 12344427)
Diesel tachs are often cable-driven. Any sign of something like that running from the gauge to the engine area?

Its a LCD display so all electronic. 100% sure its not cable driven.

Kubota Ronin 09-16-2019 05:36 AM

Re: 4TNV98 RPM meter take off
I see... which digger are we talking about, and is the tach OEM for the machine?

Helimadness 09-16-2019 05:59 AM

Re: 4TNV98 RPM meter take off
1 Attachment(s)
Its a Chinese Brand Sunward. Yes the display is OEM from them. See attached pic.
Yanmar motor and Rexroth Hydraulics.

I thought that Yanmar might have a standard place they get the tacho feed from. Maybe not?


tomw0 09-16-2019 04:33 PM

Re: 4TNV98 RPM meter take off

Seems to have an ECM that fiddles with the injectors, etc. Bet there's an output from that which is used to operate a tach.
Page 9, number 3, the 'speed sensor', optional...

Or the 'service manual' from here:

An electronic display would need to be wired, by individual wire conductors, to the engine to have information to display. Follow the wires back to the engine. Think on where the wires are attached, such as sitting by the crankshaft(crank position sensor), perhaps used to count revs?, or the cam position sensor, ditto. Other methods would be 'derived' numbers, such as counting injector pulses.(same as cam Hz., 1/2 crank)

Helimadness 09-16-2019 11:08 PM

Re: 4TNV98 RPM meter take off
1 Attachment(s)
Thanks for that very helpful. I am overseas at the moment but will dig deeper next time I am back.

I have a new fuse box to install as well as it had some corrosion on it. I dont think the problem.

There is some sort of ECM box behind the seat. Dont have a pic unfortunately. I did find this sensor floating around in the engine bay. It goes into that black box. The throttle is just a manual cable but so not an electronic throttle.
I shipped the digger back from China to NZ and the Cab had to be removed to fit in container. I got the transport company that picked it up to put back on. Im not 100% sure if this is not meant to be in the cab or engine bay. The wires are squashed between the chassis and cab. So I'm going to have to loosen the cab and jack it up to free. Thought it might be some sort of temp sensor maybe ?


tomw0 09-17-2019 02:38 PM

Re: 4TNV98 RPM meter take off
If I were guessing.. (I am) I would say it looks like a pressure sensor. Kind of like a bellows or capsule that expands/contracts in response to barometric pressure. It would then also be a elevation sensor to modify fuel delivered based on atmosphere density. From the photo it is difficult to tell if the disk is soft/hard, heavy or light.(hollow?)
What does the other side look like? Where do the wires go?
Have you tried to get a schematic from the 'Sun' company? Even in non-western characters, the diagrams can be informative. "This thingy connects to that thingy... one is attached to the engine where there is oil pressure from a gallery... or coolant system reservoir or jacket..." Interpretation and deduction will get you a good way even if reading is difficult/impossible. IF you can find online documentation, you may be able to submit parts of it to a 'translator' program. It would come out in Chinglish, but give some guide.

Helimadness 09-17-2019 11:26 PM

Re: 4TNV98 RPM meter take off
Unfortunately there is zero documentation for the machine itself. That round disk is hard plastic and fairly light. It plugs into the little black box that is behind the seat. I thought might have been temp sensor. But then looks like it should be inside the cab which is air con. Maybe pressure like you say. I think it just got stuck in the engine bay when the cab went back on. The people putting it on where not the ones that took it off. Unfortunately I didn't see it come off either. They left all the electrics unplugged. We plugged all back in and most works. Just the RPM.

Hopefully will be back there in a few weeks and planning to pull the body off so I can replace the muffler and starter. Should give me good access around the whole motor so can hopefully track it down.

Thanks for the ideas all very helpful. If I find the answer ill let you know.


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