Tachometer on a Merc 120XR not working...

[Sharkbaite]

I am working on a Rinker Rocket Jet boat that I purchase a year ago and have been replacing all the top end end electronics IE Stator, Trigger assembly due to the Magnets coming off the flywheel and eating everything.

I finally have the boat running and noticed the Tach doesn't work. I checked my connections to ensure the ign wire and ground are good, and just for giggles I put a volt Ohm meter on the Sender wire and I reach voltage there and as I rev the engine the voltage climbs, so obviously I am making it back to the ignition module, or do I have a problem with the module?

I went and bought a used meter just to see if it was mine or not, and that meter shows the same signs as the original one, IE with ignition off, the needle moves to about 2-3 on the gauge, and when I turn the ignition on the needle drops to 0. and as I start the engine you can see the needle move abit, but never reads.

 

[Sharkbaite]

If I am following my Wiring Diag correctly, it shows the Grey wire for the Tach going into the Voltage Regulator. I haven't been able to see where and if it could be anywhere else, any thoughts??

 

[Texasmark]

The grey picks up pulses from the alternator output that is fed to the rectifer/regulator and uses them to determine engine rpms. Other wires are power, purple or red with purple stripe, and ground black. Dial light would be the other wire if you have one.

 

[Sharkbaite]

The grey picks up pulses from the alternator output that is fed to the rectifer/regulator and uses them to determine engine rpms. Other wires are power, purple or red with purple stripe, and ground black. Dial light would be the other wire if you have one.

So being that I am seeing power coming from the Grey wire when I start the motor and see the voltage change when I advance the throttle would say at the least that part is working, could I be missing something from that feed, IE Frequency? or could I bypass and come right off the stator to verify?

I have tried a second gauge, but it was used so no telling right now if that is a good tach or not but it gives same indication as original one.

 

[sam am I]

or could I bypass and come right off the stator to verify?

Typically, yes.

Most tach's (F--->V) normally have/use a current liming resistor 10K-100K and clamping diodes (standard signal's, zener's, varistors, etc) arrangements of some sorts in their front ends in order for them to see/be hooked up to to several hundred volts (high dv/dt switching transients and given freq's[e.g., positive post on primary sides of ig. coils]) on the signal in wire safely.

The "raw" stator voltages waveform's, if you will, are actually less likely to damage a tach than the "flyback" transients of the ig. coil's primary that we usually run them on.

 

[CharlieB]

Most outboard tachs a basically a volt meter with a different scale. They are very sensitive to AC and will not work if there is a weak or blown diode in the rectifier.

Remove the rectifier and test it properly with an Ohm meter. Generally from what you are describing a new rectifier will cure it. Don't take my word for it PROVE the fault first before spending any money.

Search iBoats for 'rectifier testing'. You will find it spelled out.

 

[Sharkbaite]

I finally found the testing Voltage Regulator section in my book, and I'm not impressed with my results. it def out of spec's and when I test the tach lead to the box, I get nothing. and I first used a digital meter then opted for my trusted analog meter, I found that I am not even close, I have about half the readings I should. This could be due to the fact that the last time the boat ran other than my testing was when the Flywheel flew apart loosing all the magnets and it destroyed the Stator and Trigger assym...

I will be looking for another one, a local shop said they could order it for $170, but I found them cheaper online, but I also won't be buying it from this shop as the guy SWORE to me that I had ONLY 2 yellow's and 2 Grey's coming out of mine, I told him I only had 4 wires and it was 2 yellows a grey, and that I couldn't remember the other color. but he said grey, I said no, he said I am looking at it online right now.. HAHAHA

Thanks for the notes, and Info. I have some other work to do on the boat before I can afford to replace the unit. I have to pull the motor and Drive and reseat the drive as its not fully seated and is sticking out more than a 1/4 inch, and this could be part of my plaining issue, as well as I think my fuel tank has a hole in it, so I will be pulling the shell off to get at the tank to see if it can be repaired as the hole appears to be on the side of the tank VS the bottom..

 

[Sharkbaite]

[h=1]883071A1 [/h] This is the part number for the one I have, now it says 16-20 Amp systems, does this include me switching to the Red Stator, I didn't look to see what the Amps were when I replaced the stator, and nothing online say's the Amps...

 

[sam am I]

Buy this one. Don't get muddled up in wire count. This one/type is rated to 20 amps

1) The two yellows go to stator's yellows.

2) Tie the reds together where the battery positive bolts on the starter solenoid.

3) Gray to tach

4) Black to ground

Ohm readings can vary, esp as components that run HOT age.....Splitting hairs here over 10K will cause gray hairs unless its a dead short 0 ohms when it's suppose to be 1M ohm

Check charge path wires/bullet connectors (and stator charge windings). If the rec/reg isn't putting 13.5-14.5VDC on the battery after starting and running a few minutes when your running up at say 1500 RPM AND the tach isn't work.........Replace the rec/reg, it's bad!! Buy 3 at this price!! These things are designed to run hot and don't last that long.

 

[Sharkbaite]

Thank you Sam, for the price you are correct. I am trying to cross reference the part numbers to verify if they are compatible, or if this aftermarket one will still work, the red Stator is rated to 16 amps if I red the remark correctly on Crowley's page. I don't have an issue with the amount of wires, just don't wanna fry any other components due to over or undersize regulator. sometimes cheap is easy, and sometimes your better off buying top end... HAHA, I'm sure youve been there as well.

All this started from no working tach... lol

 

[sam am I]

As long as the rec/reg's is rated at or above the stator, you're good to go. Voltage reg's are voltage reg's!! Spec'd for line and load regulation with properly rated parts(inside)....Pay what you will.

 

[sam am I]

Buy 3 at this price!! These things are designed to run hot and don't last that long.

Perhaps this statement was a bit contradictory/confusing, my apologies, fishing season ya know ........

I would add (or should have said) that this design being used (OEM, aftermarket or otherwise) by its inherent properties/nature of being a "shunt" type regulator, runs "hot".

No way around it, they ALL run hot and with any manu's label slapped on it!! Heat is the killer typically with any electronics and "over time", no one is immune, OEM or otherwise, they will ALL run hot and burn out in time!!!

That is the nature of this design, has been since they started using it with a magneto's charging systems. They've water cooled it, they've heat sunk it, they've air cooled it and paralleled it in attempts to make it last longer and as long a possible.

Quality components that are spec'd properly for the heat they see and decent line and load regulation percents across the board is what ya need here to get down the road furthest and stay within a good tight reg tolerances. These do the job and I'm still running the same road with a three battery system third season now!!

But, I did buy extra's and still had extra money for a nice steak dinner

JMHO/E of course.