Tachometer Needle Jumps

[Cadwelder]

'87 Mariner 115, runs great, but when I hit the trim button (up or down) at WOT the tachometer needle jumps, falls right back to normal when the trim button is released. Is this a concern? Owed the boat for 23 years and it's never done this before. Alternator working fine and I've checked for loose wiring at the tach and rectifier all seems okay. Any suggestions?

 

[bobkyle2]

Re: Tachometer Needle Jumps

My mercury 150 does this also..

 

[Cadwelder]

Re: Tachometer Needle Jumps

Mine just started doing this, has yours always done it??

 

[Faztbullet]

Re: Tachometer Needle Jumps

This is fairly common on the OMC motors and it usually one of the following: 1) trim motor drawing to many amps(surge) 2) weak battery(at lower end of CCA spec's) 3)defective stator(one leg weak) or defective rectifier(weak output)

 

[Cadwelder]

Re: Tachometer Needle Jumps

Hmmmm, intresting, Battery is new and so is the stator, but I'll check the rectifer, and amp draw on the power trim motor. Do you know what the specs are for the trim motor? (1987 Mariner 115) I know my trim motor leaks down much worst than it used to. I have to keep trimming it up while running. Thanks for the input.

 

[bobkyle2]

Re: Tachometer Needle Jumps

well mine has did this since i bought it..

But ive taken this boat out prolly 40 times since ive boat it(everyime im off work and i can stand the weather its on the river) couple nights ago me and a buddy deicded to take a little night fishing trip on it...and we was out for a few out then decided to head back to the ramp... and the battery was dead(first time this thing has failed to start)... paddeld 5 miles to the ramp..lol.. needless to say i bought 2 new batterys.. so maybe that will fix my problem if you fix yours keep me updated

 

[pokman]

Re: Tachometer Needle Jumps

my 94 mer 125 started doing that then the tach would bounce all over at all speeds, it ended up being the rectifier.

 

[Cadwelder]

Re: Tachometer Needle Jumps

Well mine turned out to be a rectificer too. Very odd it's new, but of course that doesn't mean it's good. But the volt meter shows 15+ volts at WOT so I would have never suspected it. I didn't do any test, because I had a spare rectifier on hand and just tried it out and tachometer jumping problem went away. Still shows 15+ volts at WOT. I really don't understand, but hey it's fixed.

 

[bobkyle2]

Re: Tachometer Needle Jumps

^ thanks cad..and others.. I think i might pick up a new rectifier ...

How was the change... ive not ever changed one ... guess im going to get out my book now and start reading up

 

[Cadwelder]

Re: Tachometer Needle Jumps

Super simple change, two yellow wires on top(stator leads) a gray wire (tachometer lead) and a red wire (charging lead)

Two mounting screws attach it to the mounting plate on the side of the outboard. Took me about 10 minutes.

You'll need a phillips head screwdriver and a 3/8" nutdriver.

 

[j_martin]

Re: Tachometer Needle Jumps

Well mine turned out to be a rectificer too. Very odd it's new, but of course that doesn't mean it's good. But the volt meter shows 15+ volts at WOT so I would have never suspected it. I didn't do any test, because I had a spare rectifier on hand and just tried it out and tachometer jumping problem went away. Still shows 15+ volts at WOT. I really don't understand, but hey it's fixed.

The tachometer signal is the distorted off center waveform from one side of the bridge rectifier. If there's a noisy connection or diode in that bridge rectifier, the tach will lose it's mind.

If you put an oscilloscope on the tach lead with the bad and good rectifiers, you would easily see a difference.

Not quite perfect connections, especially the ground connection through the mounting screws, would have the same effect.

hope it helps
John

 

[Cadwelder]

Re: Tachometer Needle Jumps

Thanks for the explanation, just for curiosity I put to old recifier back on and connected the tach lead to the other stator lead and the tach is fine there. Electrican by trade so I understand the waveform and know how diodes work, but didn't know exactly how the tachometer worked, do now. Isn't it odd that it still charged ok though?