Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

I bought a tachometer from eBay for $23 including shipping and what a total junk it was (http://cgi.ebay.com/Tachometer-Hour-meter-2-4-Stroke-Small-Engine-Spark-/120689310104?pt=LH_DefaultDomain_0&hash=item1c19a4b998  I got mine from a different seller and was able to get a full refund but what a junk - you can buy a nice bottle of wine for $20 so don't waste your money on this model.

Well here is the inside of my tach which showns the location of the white wire.Which is used as the ground wire on the 2 wire tach.

For some reason? the picture would not post.

Here is the picture !

relics,

Thanks for the picture - I can clearly see the two wires on your circuit board.  I just tried it on my Honda lawnmower and the rpm reading is reasonable.  I was getting 2950-3000 rpm, which is consistent with the manual.  However, the correct RPM reading came at the "2" setting (1 spark every revolution) instead of the expected "3" setting (1 spark every two revolutions).  So my cheap tachometer needs to be at "2" setting to read correct RPM.

2 is the correct setting.Its the same on my old tecumseh 8HP.These engines do fire everytime.As stated already its a waste spark system.So the plug fires every revolution.An for sure your tach is giveing the right rpm.I think the ground wire just makes the tach most likely work slightly better.Maybe a slightly faster refresh rate who knows? But for sure these tachs are accurate.I have used mine for over 8 hrs and its just fine.I find its a real plus to know my rpm.Also running at max rpm i can be sure i am within a safe range and not going above (3600rpm)

Just for reference;

It looks like all NM_Runner and Rlics boards are similar to mine, all HM011's but all different revisions.  Mine's from 5/10/2010.  Relics is stamped 2010, has different parts and seems newer.   Mine has GND at the open pad by the wire input lead so I added a ground there and drilled an outlet hole for the ground wire. 

It's now operating well on 2 and much more stable.   ( I have three of these).  They were not so stable before and would take a lot of time settling.  Often they read better in 3 than 2.  I had tried a few different wire leads, shielded and non shielded.  The length and type of wire would cause readings changes.  They had to be frigged and fragged with to get good readings.  Often they would read well in a small range rather then read well over the whole range of 700 to 3600 rpm.  They are noisy.  If I put an unconnected meter close to a connected meter the unconnected meter will display rpm readings. 

The upshot is there are a number of rev's of this board out there with various features.  The single wire will work but has to be messed with to get reliable and consistant readings.   The grounded version of the HM011 is the one to get. 

  

Thats great Dave now we know the ground is needed to make the tach work correctly.Did it take you long to update it with the ground wire? It also can be seen the batterys can be replaced with a little effort of removeing glue.Although i would not be surprised if there is some type of set up procedure if battery power is lost.?

   It will probably reset by itself with a bat install.  There "reset" printed on the board so worse case those pads could be messed with. It would be nice to have a schematic.  The two big chips are a 7400 series which is a simple D flip flop and probably takes the S1 and S2 inputs.  The other is an 8 bit risc chip.  The data sheet for it is online.  It should have a reset pin which usually is pulled to ground for a reset.

   The back cover comes off easily and the soldering is simple with a pencil tip soldering iron.  I put a short wire on for ground and will put a perminant wire on machines so the meter can be removed easily.   The case is not brittle so easy to drill without cracking or breaking the case.  The back cover also popped off easily with a knife. 

   Mine was advertised as "replaceable battery" and also listed as potted.  It's not potted and there's no batter holder or compartment as I expected.  There's probably a lot of ticked off people who bought the single lead version.

   On a closer look your board and my board are very different.

Why not just ground the shield of the "single" wire?

On the single wire units are a single "wire", no shield.

I did try miniature coax starting a few inches outside the case which did not help.  It would have been better to run the coax shield as the ground inside the meter and tie the other end to the engine but I did  not want to solder the shield in such a tight spot inside the meter.

RF can be very weird.   It might be useful to try an R plug.  It's possibly it might have cured the meter