Intermitent Tach

[mtstringer]

I have a 1995 Merc 150hp OB and my tach all of a sudden started to drop off line to zero, then come back for a while and drop again. I have no audible drop in rpm so I know it is electical. Any idea's? Is something slipping somewhere?

 

[Texasmark]

Re: Intermitent Tach

Your local Merc dealer has Quicksilver tach's for $88. We had a similar problem and a new tach fixed it. Don't ask me what the failure mechanism was. Since it has been raining here for 2 months (about 20" already for May and another 2 has already fallen today) I may go out to the shop and dissect it just for the knowledge of it.

Folks on here have asked as to what's inside. Might just enlighten all of us.

Mark

 

[G-Daddy]

Re: Intermitent Tach

On my Johnson 100, the same symptoms exhibited themselves when the rectifier failed.

 

[mtstringer]

Re: Intermitent Tach

How hard and expensive was your rectifier to replace?

 

[G-Daddy]

Re: Intermitent Tach

Problem was it is located under the flywheel. I had to pull the flyywheel, change the rectifier with 4 (I think) small bolts, replace the flywheel and retorque to 150 ft lbs. Holding the engine from turning over when retorquing the flywheel bolt was the hardest part. 30 minutes to pull the flywheel, 5 minutes to change the rectifier, and about an hour until I could figure out how to hold the flywheel from turning when retightening.

 

[Texasmark]

Re: Intermitent Tach

Son's engine is a '95 150 Merc. It has two rectifier/regulator modules for charging the battery......gets 40 amp rating that way.

The gray wire from one is the tach drive. If you want to troubleshoot your rect/regl module, remove the tach from the gray wire where it is currently located and plug it into the gray wire on the other module. Will have your answer immediately. We did and bought the Merc Tach for $88. Grin.

HTH

Mark

 

[Texasmark]

Re: Intermitent Tach

If you don't know where they are, they are on the back (aft) of the engine in the center of the block under the plate which contains the trim switches (solenoids). They have yellow, red, gray, purple I think, and maybe a black. Can't miss em. Just find the gray wires and swap your gray tack drive wire to the other module.

No under the flywheel and very easy to see and do.

Mark

 

[mtstringer]

Re: Intermitent Tach

Ok now all, I have since taken my boat in for winterization and asked them to take a look at the tach problem. Then said that the rectifier reads out good and changed the gauge. It worked once then didn't work the next time. They thought that it was because it was a 4 stroke gauge. Well they put in a 2 stroke gauge and the same thing. I told them about what was said about switching the grey wires and they tried it with the same results. Anyone have any other idea's? Even they are having troubles thinking of something else without just throwing parts at it.
Thanks in advance.

 

[Texasmark]

Re: Intermitent Tach

Pretty simple. You have a drive signal, power and gnd........and the tach Period. If your battery is being charged adequately and you swapped gray wires with no change in performance, you can consider your signal is good.

12v and ground at the tach can be measured (just put your voltmeter across those leads at the rear of the tach.....tach studs, not terminal lugs as the terminals could be corroded causing it) easily. Either you have it or you don't.

Then there is the tach......nothing left.

Like I said we bought a newun and problem disappeared.

Mark