Tach died- any way to test signal to tach with volt meter?

[CircleHook]

Tach died- or lost signal- anyway to test for signal to tach with a volt meter or??
1998 merc 225 carb

[dompie99]

Usually a Tach gets it's informations (pulses) from the ECU so, If you have a osciloscope (Scope) or a voltmeter DVM, that has the ',frequency function on it, you would be able to find out if the pulses are getting to the Tach.
That would be my Engineer,s advice to you.

On an osciloscope, they would look like Square waves varying in quantity as RPM changes.

Or the other way around, If you have an Audio generator, plus your tach pos (+) and (-) to the 12V supply and feed the Tach's terminal (send) with the audio signal ( square waves ) in the range 200hz to 2000hz and you will see the needle moving according to the frequency change.

[pyro]

If the meter has a frequency counter, this can be done.

Take a long piece of insulated wire and coil one end around one of the spark plug wires a few times. (Some very LOW-voltage pulses will leak through the rubber insulation, trust me) then strip and and attach the other end to the red (+) lead of the voltmeter. Ground the black (-) lead to engine ground or any ground point at the dash. Read the counter and multiply by 60 to convert Hz (pulses per second) to RPM.

[dompie99]

The only risk would be to get the spark voltage to the DVM and ruin it. Most DVM won't' accept anything above 1,000 volts AC

[pyro]

Like I said... Ordinary INSULATED wire, wrapped around the middle of the INSULATED spark plug wire-- NOT near either end where a leak is most likely to happen. Very low risk.

[perfmarine1]

Volt meter set on HZ. red lead gray wire at rec/reg- black lead ground. 70HZ =700 rpm 120HZ=1200rpm ect

[dompie99]

just my 2 cents again but have you ever measured the distance that could be reached at a voltage of a CDI ignition, well above the small insulation of the spark plus wire itself.

And in adition to that, some wire could present small cracks along the sparkplug wire, increasing the risk mentionned above.

In other words, an ounce of précautions is well Worth a ton a médicine

[pyro]

Volt meter set on HZ. red lead gray wire at rec/reg- black lead ground. 70HZ =700 rpm 120HZ=1200rpm ect

That works too... The gray wire is the tach signal itself. 6 pulses per revolution.

[j_martin]

Tach died- or lost signal- anyway to test for signal to tach with a volt meter or??
1998 merc 225 carb

I have no idea what these folks think you want. The tach signal on a v6 merc is obtained from the rectifier or the rectifier section of a regulator. It should be a distorted AC wave form that would measure somewhere between 10V and 25 or so on any AC meter. If the battery is disconnected and the engine is revved high enough to endanger rods, it could hit 90V. No big deal for a multi meter.

If you have the AC reading at the tach, the tach is bad. If not, it's either wiring, or the rectifier is bad.

[dompie99]

I have no idea what these folks think you want. The tach signal on a v6 merc is obtained from the rectifier or the rectifier section of a regulator. It should be a distorted AC wave form that would measure somewhere between 10V and 25 or so on any AC meter. If the battery is disconnected and the engine is revved high enough to endanger rods, it could hit 90V. No big deal for a multi meter.

If you have the AC reading at the tach, the tach is bad. If not, it's either wiring, or the rectifier is bad.

Sorry Bud but we (or I ) was trying to help him isolate if the problem came from the tach itself. And some of the ways given were prone to end up into a defective DVM that's all..

[David - WI]

I have no idea what these folks think you want. The tach signal on a v6 merc is obtained from the rectifier or the rectifier section of a regulator. It should be a distorted AC wave form that would measure somewhere between 10V and 25 or so on any AC meter. If the battery is disconnected and the engine is revved high enough to endanger rods, it could hit 90V. No big deal for a multi meter.

If you have the AC reading at the tach, the tach is bad. If not, it's either wiring, or the rectifier is bad.

...

[woundedwarrior]

I don't know about your 225 carb motor but..A 225 Promax has two rectifiers just use the gray leade from your other rectifier if you have one..I that don't work its prolly your tac or in the wireing shorting out

[CircleHook]

Thanks Guys- sold the whole rig yesterday somebody else can figure that one-
Now I need a new project... I love new projects!!!