Tachometer testing

[skrillz78]

I have found lots of helpful tips on this forum.My question is about a Tachometer.I was rewiring the ground to my gauges and i have them lighting up.The tach does not register RPM.I have a polarity tester(red and green lights stay lit with power,and red only lights when introduced to other (+)current and green for(-)current. I am getting current (with ignition switched on) to this gauge and when I check the post marked send(using the tester)the red and green lights flicker rapidly which I assume they do because of the pulse signal coming from the send wire; this is with engine running and upon throttle.Is it possible that I have the tach pole set wrong or is my gauge most likely bad.The tach needle doesn't move at all.My engine is a 1999 Yamaha c60tlrx.The scope of my question is not only about the tach but also is the flickering on the tester an indicator that the send wire is getting a good pulse? Thanks for any help or suggestions.

 

[Markov1985]

Re: Tachometer testing

Hello

I had same issue with my RPM sensor not working. My post was http://forums.iboats.com/showthread.php?t=456513 " Where is RPM sensor located 1997 5.7 volvo penta with cobra sx sterndrive? "

Basically i found my issue inside the dash. It was a faulty resistor on the pcb, that didnt allow the input voltage from the coil to arrive at the IC chip which converts that varying volatage into an RPM reading. It was alot of mucking around, but in the end worth it, My RPM guage to me is more important then my speedo as i use boat for watersport, so RPM gives me a more steady/reliable reference.

 

[skrillz78]

Re: Tachometer testing

Ok so is the resistor in the tach gauge?I guess I'm asking because I'm getting a signal to the end of the send wire so I assume(maybe wrongly)that up until it reaches the tach gauge send terminal The signal is coming through as a pulse?sorry if I'm not getting this right.

 

[Markov1985]

Re: Tachometer testing

I tested the volatge going into the dash with engine on, and as i increased the throttle, volatge on the RPM wire on the dash connector also went up. As i slowly moved back towards nutral, the voltage went back down. Voltage always remained steady if i didnt move throttle in my case. Im not sure why you are seeing a pulse :S maybe your multimeter is measuring in AC rather then DC? not sure to be hounest.

 

[jonesg]

Re: Tachometer testing

Generally, if the battery is charging and a length of test wire doesn't reveal any breaks in the original signal wire you need a new tach.
The test proceedure in the factory service book is to just substitute with a known good tach.

I tested the tach in my volvo by putting a multimeter on the signal wire and watching the needle go up and down with rpm.
Got a used tach from ebay.

 

[skrillz78]

Re: Tachometer testing

Thank you all for your help.My tester was a polarity light thus the pulsing.It has LEDs on it.I guess i should have just used my multimeter and check voltage.I was just hoping to see power to the send terminal.The polsing could have been the leds trying to get brighter.I am getting charge to my battery so i will test with multimeter.I found a G3 tach on ebay that looks like my current one.I ordered it.Thanks for that jonesg.Even if its not a bad tach I can always keep a spare.I'll let you fine folks know how it goes.

 

[skrillz78]

Re: Tachometer testing

OK after using multimeter i see voltage @ idle is 6 volts approx.As I give it throttle it takes the send wire up past 7 volts.I guess I have a bad tach.I don't know if a tach can be taken apart and repaired.

 

[jhebert]

Re: Tachometer testing

The tachometer signal is not a DC voltage that varies with engine speed.

The tachometer signal is a pulse whose frequency varies with engine speed. The voltage range of the pulses is really rather constant. The negative peak of the pulse is near 0-Volts DC or slightly below; the positive peak is close to the battery voltage. Since the battery voltage rises slightly with engine speed due to charging current, the tachometer pulse peak positive voltage will rise slightly, too. But this voltage change is not what is measured by the tachometer.

When people try to measure this signal with a DC voltmeter they get various readings. The readings are affected by any capacitance in the voltmeter input. It is hard to predict what you might read with a random voltmeter.

The Tachometer is really a frequency measuring device. The frequency of the pulse varies directly with engine speed. The tachometer measures the frequency and converts it to a speed indication. Calibration controls are provided to adapt a tachometer to different set-ups in the engine. The relationship between tachometer frequency and engine speed is determined by the arrangement of poles and magnets in the permanent magnet alternator of the outboard engine.