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  Topic Review (Newest First)
06-26-2008 03:29 AM
It's the number of poles in the alternator plus...

The way that the "diesel tachs" that run off the alternator work is based primarily on the number of poles that are in the tach - usually 12. A lot - but not all - of these tachs have one or two rotary switches on the back; typically on is for selecting the number of poles since not all alternators have 12 poles. The other rotary switch is for selecting the pulley ratio. Sometimes both functions are in a single switch. Calibration is done either by having another tach (mechanical tach for example) hooked up and then trying different switch settings to get it to register close to the "mechanical" tach; alternatively use a "guesstimate" method - if your diesel idles at 800rpm according to spec, adjust the tach so it shows 800rpm, if peak speed at spec if 3200 rpm - go to WOT and see if tach is close to 3200rpm.

Learned about this the hardway installing a tach on my little Universal diesel. Good luck, I hope that this helps.

06-26-2008 01:31 AM
Omatako Hey, thanks for that. I would prefer an analogue type simply because digital stuff makes you have to look at it and sort of makes peripheral vision redundant.

But I'll follow this through as a viable alternative.

06-25-2008 09:01 AM
arf145 One possibility for mortals would be the Tiny-Tach diesel tach:

Tiny-Tach: Diesel Tachometers

I have no personal experience with it, sorry to say.
06-25-2008 06:17 AM
Omatako OK, so I finally determined that the tacho itself is toast. Now the question is where to from here. The Yanmar replacement costs around $300 and frankly, I don't need to know the RPM that badly.

What pearls of wisdom are out there regarding non-genuine rev counters? I suspect it will be real hard to find another one that runs off the same Yanmar sensor.

I recall in my youth working with gasoline engines, rev counters could be bought that ran off the back of the alternator and my thinking is that this should work irrespective of the fuel type.

My question (probably rhetorical) is how does the rev counter know what the ratio is of alt RPM relative to engine RPM? I have a very accurate analog tacho that I can run off the crank pulley to determine the real engine RPM but if it's different to the alt RPM (and I can only assume it will be), how does it get corrected?

Or is there a better idea altogether that mere mortals like me can afford?

05-27-2008 03:04 AM
Originally Posted by boydgatlin View Post
When the tach on my Yanmar 4JH3E quit, I found the problem to be nothing more serious than a bit of corrosion on the wire disconnects at the sensor. A bit of cleaning restored full functionality.
I've been down that track and no success but thanks for the tip.

The next problem is that the body of the sensor is badly rusted (one would think these things would be salt-water-resistant) and I'm not keen on trying to move if unless I know I need to.

05-26-2008 07:29 AM
boydgatlin When the tach on my Yanmar 4JH3E quit, I found the problem to be nothing more serious than a bit of corrosion on the wire disconnects at the sensor. A bit of cleaning restored full functionality.
05-26-2008 05:07 AM
Yanmar rev counter

Does anyone have any info on the resistance of a rev counter flywheel sensor on a Yanmar 4JH series engine?

My tacho stopped working and I read in a manual that the sensor's internal resisteance should check in at 1.6 kilo-ohms. Mine measures 1.2.

I just wondered how sensitive these things are. I suspect my tacho is toast but want to cover all the bases before spending unnecessary bucks on a new tacho.


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