Inaccurate Tach

[Sorrento 25]

My tach seems to read a little high at times. Idle will show 1500 etc. and at other times it seems 100% accurate. Is this a bad gauge or could there be another issue??

88 Sea Ray Sorreto 25 7.4L Merc

 

[Silvertip]

Re: Inaccurate Tach

Check the back of the unit to see if the calibration dial is set to 8 cylinder. If it's set to 4 cylinder it would read double. Since you say it reads 1500 at idle -- that is about double what it should be so it may just be set wrong. If the engine actually does idle at 1500 RPM you have other issues.

 

[Sorrento 25]

Re: Inaccurate Tach

It is about 800 RPM high intermittently but it is also correctly set at 8. Sometimes it is right on and idle is about 750 or so as it should be.

 

[ziggy]

Re: Inaccurate Tach

have ya checked the rpm out with a shop tach? compared to the oem tach. i guess my thought is that it may not be the tach and checkin against a shop tach may let ya know which way it is. other problems or truely an inaccurate oem tach....

 

[Major Woods]

Re: Inaccurate Tach

I had to replace my tachometer a couple months ago. It was reading 8% to high at WOT.
That does not sound like much but put me over the recommended rpm range of the motor, which showed up on the computer readout, that's how I found out that the tach was bad.
Replaced it and ran the motor hooked up to the laptop to see both (boat and computer) tach readings. All better now.

 

[Sorrento 25]

Re: Inaccurate Tach

Whaddya know! I sniffed around on Ebay and found the exact correct Sea Ray NOS tach for my boat! Everything is really coming together nicely with this boat purchase and refurbishment thanks in no small part to the knowledge on this forum.

 

[imported_airxsteel]

Re: Inaccurate Tach

Don't want to repost here, but.......

I have a similar question out on another post.

How in the world do you even hook a tach up ???

I have heard I have to go under my control box and hook it up to the wires there. I have hear to hook it up to the motor on the coils.

I still do not have a good answer as to how to do it.

I have a 79 Johnson 140 HP.

 

[Chris1956]

Re: Inaccurate Tach

Sorento25, The tach in your boat runs off the negative side of the ignition coil. The usual reason for inaccurate readings is bad connections to the sender, battery or ground, or a bad tach. Since yours is intermittent, it is likely a bad connection.

I have an '88 19 foot Sorrento, and have the same issue. The tach will read at least 1000RPM too high at cruising speed. it has been like that for many weeks, and I was to lazy to fix it. However, last week when I left the dock, the tach read high, when I returned it read accurately. Go figure.

Airxsteel, Outboard tachometers run off the alternator pulses. I am not too familar with Johnson motors, but an owners manual or shop manual should show you where to hook up the tach. You need to set it for the correct number of pulses (look for a "J/E" setting on the back of the unit), an connect it to battery, ground and the sender (alternator wire).