Quicksilver tach not working w/ 84 merc 90hp

[444]

My quicksilver tach isn't working and I'm wondering if I can run a wire directly from the tach to my 1984 mercury 90hp 6cyl? The way it is setup now, the tach signal wire leads into my quicksilver controls, and from there they go back to the motor. At one point someone did some monkeying around with the wiring on this boat. It starts and runs fine so I haven't paid much attention to it until now. Can I run a wire directly from the tach to the grey wire on my motor connector? I looked at the wiring diagrams posted at maxrules.com and they show grey as being the wire used for tach signal. The diagram itself shows the grey wire going to the rectifier so I just want to be sure this is the correct wire? I would just like to try bypassing the tach signal wiring in the boat in case it's disconnected somewhere that I can't see. The tach itself is getting power/ground, so either it's the tach signal wire, the tach itself is shot or not compatable with this motor. The selector on the back of the tach is set to "12 pole alter" which I think is the correct setting for a 2-stroke?

 

[roncoop75]

Re: Quicksilver tach not working w/ 84 merc 90hp

I have the same engine (an '85) and just replaced my tach (teleflex) and yes, the gray wire on the input to the rectifier is for the tach. You should have 3 studs on the rectifier, 2 have yellow wires coming from the stator and one red wire for output is on the third. My gray wire is on the left input (yellow wire), though I don't think it matters as it's AC.

And also, yes, the 12P setting on tach is what works for mine.

Edit: Almost forgot, a bad rectifier can make the tach inoperable also. IIRC there are a few tests on CDI's website that you can do with a simple DVM that will tell if the rectifier is OK.

 

[444]

Re: Quicksilver tach not working w/ 84 merc 90hp

Thank you for your help roncoop. I checked for continuity on the tach wiring and checked to make sure the tach wiring wasn't shorting against power or ground and it was perfect. So I did a search and found the testing procedure to checking the rectifier and your suggestion was bang-on. Rectifier is blown, will pick up a new one on monday. At least it's an easy enough of a repair.

 

[444]

Re: Quicksilver tach not working w/ 84 merc 90hp

Well popped in a new quicksilver brand rectifier and still no luck with the tach. Hooked the motor up to the ear muffs and ran it a bit and the tach moved up and down by a couple hundred rpm, did so very slowly and had no bearing on the actual engine rpm. I guess the tach is shot...?

 

[roncoop75]

Re: Quicksilver tach not working w/ 84 merc 90hp

Hmmmm, that is strange.

I vaguely remember someone on here who had a partially working tach that would only act right after moving it to the other post on the rectifier. Don't know why it made a difference but it's stupid easy to try.

Beyond that, if that tach has +12v and ground and you run a different wire, bypassing everything else, straight to one of the rectifier inputs (on one of the posts with one of the yellow wires coming from the stator) it should work.

You do have some V coming out of the rectifier right? It doesn't regulate so you should see it changing just a bit when you rev slightly on muffs (don't go past 1500 rpm). I mean, what if one of the stator coils that feeds the rectifier is bad? Just trying to verify both inputs to the stator. I guess you could also look at them with a DVM on the AC setting, they should be fairly close in V while running. Duty cycle is pretty short so you may need a DVA adapter to see it, I'm not sure. I just went ahead and made the DVA adapter myself so I'm not sure what you'll see without the DVA. If you have one I'd use it to look at the stator outputs.

My guess, the tach is bad. But it's pretty easy to check the stator before spending another $90 on a tach (that's what I paid for mine at least).