how to test tach signal?

[pcrussell50]

2000 Merc 90 FourStroke* (Mercaha/Yamaha powerhead)

The tach is deadsville, no reading at all... Nothing at all. Though over the last couple of years, it would spring to life for a few seconds before dying. Now it doesn't even do that. The gray wire is properly and securely attached at the back of the tach. Before I spend money on a new tach and crawl under the dash to swap it out, is there a way I can test the gray wire to make sure I'm getting a proper tach signal?

This is my 17 Whaler fish boat.

*Yes, it's the carb'd FourStroke with the bad reputation... But... It still runs great, so what can I say?

-Peter

[nitro_rat]

I've noticed some of those smaller Mercs and Yamahas looking the same lately. IDK anything about newer 4 strokes but glad to know I'm not crazy. Sorry for being of absolutely no assistance!

[pcrussell50]

I don't think this is anything specific to the FourStroke. All Mercs (even the Yamaha powerhead) as far as I can tell use the gray wire for tach signal, with one end on the regulator and the other on the tach. I just want to know if there is an instrument I can attach the gray wire to, to see if I'm even getting a tach signal at all.

Maybe the regulator is dead. I put this boat on a battery maintainer every day after use. So I might not know if the regulator is failing to charge.

-Peter

[RSWORDS]

Check for varying AC voltage on the tach wire with a DVM. Should be pretty damn stable at idle and go up with RPM.

It's not actual AC voltage but pulsed DC. Its pulsed to fast for a DVM to read but it will show as AC.

[RSWORDS]

I've noticed some of those smaller Mercs and Yamahas looking the same lately. IDK anything about newer 4 strokes but glad to know I'm not crazy. Sorry for being of absolutely no assistance!

They are both Japanese built, similar but not the same.

[pcrussell50]

They are both Japanese built, similar but not the same.

IME, the powerhead is the same other than the paint. Carb kits, sensors, etc... I buy all the powerhead parts for it from Yamaha because it seems like Merc takes it's cut over and above Yammie. When I've had it serviced, it was at the Yammie dealer. Of course the harnesses, cowl, mid and lower are all Merc.

Anyway, because the harness and wire color coding are Merc, I'm still wanting to know if I can test the gray tach wire. Anyone know? Pretend it's the "official motor of SandF": the 2.5l ProMax. How would you test the gray wire on one of those? Can you test the gray wire?

-Peter

[RSWORDS]

IME, the powerhead is the same other than the paint. Carb kits, sensors, etc... I buy all the powerhead parts for it from Yamaha because it seems like Merc takes it's cut over and above Yammie. When I've had it serviced, it was at the Yammie dealer. Of course the harnesses, cowl, mid and lower are all Merc.

Anyway, because the harness and wire color coding are Merc, I'm still wanting to know if I can test the gray tach wire. Anyone know? Pretend it's the "official motor of SandF": the 2.5l ProMax. How would you test the gray wire on one of those? Can you test the gray wire?

-Peter

I was talking about the new smaller motors in response to nitro.

Yes your powerhead is 100% yamaha.

If you look at the connection at the motor chances are it has a adaptor to plug the merc harness to the Yamaha powerhead. And I just told you how to test the tach in the post above the one you quoted. At least on the old OMCs you can. With anything else I just hook up my known good shop tach.

If its charging it's most likely the tach itself. As a matter of fact the Yamaha's have very few issues with electronics any way so ot really is most likely a woring or tach issue

[pcrussell50]

Aha! thanks mate. I missed your helpful post completely. Sorry about that.

-Peter

[H2Okie]

Haven't tried this yet but it should work . Using a DMM that can read AC Hertz you should be able to read frequency and determine RPMs . The merc V6 flywheels have 6 magnets glued inside the rim for 6 positive pulses and 6 negative pulses that alternate . The tach only uses the "Positive" pulses via the gray wire for the tach signal .
Assuming the 4 stroke is also using a total of 6 magnets that will be 6 pulses per engine revolution .
Example : if your engine is idling at 600 RPM producing 6 pulses per revolution this equates to 3600 pulses per minute . Divide this number by 60 for 1 second time period =HZ and you get 60 HZ
OR you could forget all the math and clip the DMM set on AC hertz from the gray tach wire to engine ground and start the engine and you should read something between 60-70 HZ at idle if the rectifier is working .

[RSWORDS]

Oh and one more tip/trick

Turn the switch on the back the tach a few times then back to its proper setting. Sometimes its selector gets cruddy

[H2Okie]

OK ran my 200 merc equipped with a 40 amp charging system mounted on a stand this morning using a hose and muffs . I clipped the meter set to HZ on the spare gray tach wire to ground and was reading 120- 130 HZ at idle . The gray tach wire that is connected to the engine harness from the second regulator appears to have a diode in the wiring to the tach . On that wire I read 60-70 HZ at idle.