How to test a tachometer

[fenwick]

I have a 1991 Evinrude (40hp) ,at WOT I'm only turning 4600 rpms.My prop is a 12.25x15.My motor has been checked out for compression and it is good(110) both cyl.It has been suggested that my tach could be not working properly.My question is,can I hook up a tach and dwell meter to test my tach to see if it reading properly?
fenwick

 

[rickdb1boat]

Re: How to test a tachometer

Start by checking the setting on the back of the Tach. There should be a dial with multiple settings. You will want to set it to "6"...

 

[Barnacle_Bill]

Re: How to test a tachometer

Just in case there are 2 6's, you want 6P and not 6C.

 

[Silvertip]

Re: How to test a tachometer

If idle RPS is in the 650 - 750 RPM range the tach is probably ok.

 

[Swampmouse]

Re: How to test a tachometer

I have a question along the same line - My tach is definitely busted. Is it as simple as removing the one in the dash and putting in another?

Sm

 

[Silvertip]

Re: How to test a tachometer

As long as the new tach is compatible with your engine it is as simple as disconnecting the wires, removing the old tach and sliding in the new one. Pay attention to which wire goes where.

 

[fenwick]

Re: How to test a tachometer

My tach is at the correct setting.Does any know if hooking up a tach/dwell meter to the #1 cyl will give me an accurate rpm reading?

fenwick

 

[fenwick]

Re: How to test a tachometer

Bill,

I looked on the back of my tach and I'm a little confused:/ I see the dial where you can adjust pole settings.The dial has a slot for a screwdriver and directly across from it is what looks like a triangle indicator.The slot points to the 6 pole setting while the triangle points to the 12 pole setting:/ (they are directly across from each other) Do I use the triangle indicator or the slot? By the way, it is a Teleflex tachometer.It is original equipment on a 1991 alumacraft.

Thanks for you input!

Fenwick

 

[ezeke]

Re: How to test a tachometer

Point the arrowhead at 6Pas bhile said

 

[Silvertip]

Re: How to test a tachometer

Go to the Teleflex web site and check their installation instructions. As for the Tach/Dwell, most of them are capacitive coupled so it won't hurt to try. Remember that you are dealing with a two cylinder two stroke which fires every rev, not a four stroke that fires every other rev. On a car, that meter is reading the pulses from a single coil whereas on your engine you are reading just one cylinder. You do the math.

 

[Texasmark]

Re: How to test a tachometer

Ok guys, got me curious again.

Here we are talking about a 2 cylinder engine. I have immediate access to a 3 cyl and a 6 cyl Merc. Both use the 6P position. I do know that the 3 cyl has a single stator and the 6 has 2 stators.

The 6 has a (one of the) stator(s) connected to a VR and the other to another VR. Each VR has a grey wire for tach drive which tells me that the 6P is counting off of 3 cylinders regardless of whether it is on my 3 cyl, or my son's 6 cyl (with 2 stators and 2 regls, and 2 grey wires)......or maybe the number of cylinders has nothing to do with it.

My alternator is (Red and ) rated at 16 amps (3 cyl). His is rated at 40 amps (2 ea 20 amp circuits I'd assume).

So how is it that the poles on a 2 banger are going to be set up the same as for a 3 banger since the tach is counting pulses of a sine wave generated as the flywheel rotates through 360 degrees? Would the 2 banger not be set for 4P?

Just asking......or does the number of cylinders have nothing to do with the number of coils on the stator?

Maybe not. If the stator is just to recharge the battery then the OEM could put as many coils as they want.

So does the Merc 8, 16, and 20 amp stators have the same number of poles? If so, then the 2 banger and 3 bangers would both have their tach's set on the same number (as you'll recommended) and all this gobbledegoop would make sense......to my feeble little mind (and anyone else reading this that has a reason to be interested).

Now if the latter is true then it makes sense that it doesn't matter how many cylinders the engine has......but now we get into another question and that is triggering the spark plugs, but that comes from the trigger coil, not the stator. d

Just asking.

Thanks,

Mark

 

[Barnacle_Bill]

Re: How to test a tachometer

Number of cylinders have nothing to do with it. The most common stator has 12 poles. 6 of them feed one side of the rectifier and 6 feed the other side. Since the tach get connected to just one side, that is why you set the tach for 6P (pulses). Hope I made sense.

 

[Texasmark]

Re: How to test a tachometer

Hey B, thanks for the hit. Stay with me on this.

To me a pole is a magnetic pole having a + and - terminal, in electrical jagon, a N and S pole in mechanical jargon; doesn't matter if it is a permanent magnet, an electromagnet, or an induction coil (form of electromagnetic).

If I have a coil of wire (cored usually) and pass a magnet by/thru it, I will induce an AC voltage and current (power) in the coil which can be routed elsewhere via interconnecting wires.

So when you say 6 feed one side of the rect and 6 the other, what I am assuming you are saying is that the stator is made up of 6 (little magnetic coils) parallel electromagnetic sources of energy (power) that feed the rectifiers that are used for battery charging, but while they're at it (the Rect-Regl Modules), they also count these pulses and use them for tach drive.

Rather than one big coil and a single cycle electromagnetic, we use 6 small ones and sum the outputs (under the flywheel) because it is physically easier to mount 6 small ones around the flywheel (they fit nicely mechanically....guessing their rationale) rather than one big one.

Then we glue all these outputs together and come out on two wires with the AC power (yellow in the case of Merc) and feed this to our Rect/Regl/Module which happens to have a tach output.

The module sends the sine waves to the tach, rectifies them into Direct Current, provides regulation to prevent over charging the battery and sends the volt-amp DC product to the battery to charge it.

Yes-No.

Appreciate your response.

Mark

 

[Texasmark]

Re: How to test a tachometer

Bump