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Tach on boat acting wierd


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Need some mechanical advise, tach on 92 astro scx18 w/merc135 acting up. Acts like it is getting a signal part time only at higher rpm. Checked all fuses and connection that are known to me, need some advice on exactly were the signal is generated from for the tach. Also the temp water temp gauge has died with tach.

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  • BassResource.com Administrator

The tach signal comes from the Rectifier or voltage regulator..The temp sensor is in a cyl. head. Since you have problems with both I'd guess that it is a main harness issue or it's a faulty ground or power wire to gauges.. Does it have a panel (flush) mount control box or side mount control box?

If it's a side mount it has a harness that plugs into the control box and connects to the tach etc

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There is a test you can do, when the eng, running,take a volt teast on the craken batt, it schould be reading 13 volts or higher. I have a 200 hp merc and I had to disconnect the volt reg, it would short and take out the rectifer. IF you have ever shorted you power feed to the batt. it will take out the fectifier.  I would get a service manual,it will have all the steps you need to take to find the problem.

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  • 2 weeks later...

Not sure how you made out on this but I just joined the group and went through the same thing this spring. The answer was a control module that I believe is located under the fly wheel. It is a little deck of cards sized hunk of epoxy sealed electronics and when it starts to fail strange electronic things start to happen. Such as the Tach giving you some very odd readings or no reading at all. I think it ran about 150-160 to get changed but it made a large difference.

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